there's been some mod drama in the past year but hopefully they've figured it out. i don't know if biased moderation was ever a big problem, but OTOH you wouldn't know unless you were constantly watching. comments have been consistently high quality either way.
People often lump different subreddits together too much. /r/gaming and /r/games are both ostensibly about video games but their content and culture are very different from each other.
The two best organised subs that I use are '/r/AskHistorians/' and '/r/syriancivilwar/'.
Historians is very well moderated (strict) and has users have expert tags.
The civilwar sub always has the newest information tagged with the needed facts about the sources. Lots of different people and sometimes pretty high level discussions. They are literally producing their own magazin now.
The problem I have with AskHistorians, what keeps me from subscribing, is that Reddit's algorithm isn't designed for content that gets popular and then starts receiving replies, it's weighted towards content that gets popular because of the replies. When an AH post hits the front page, there's never any answers, and by the time there are answers it's gone.
I don't subscribe, but I browse "best of last week" occasionally.
Yes, half a dozen people in this thread have praised niche subreddits in the abstract, but none have actually given particular examples. Yes, r/askHistorians isn't bad, but this isn't particularly niche. It's constantly on the reddit front page. I'd like to learn about something new.
EDIT: Happily, in the last hour some people have made at least 3 specific recommendations. (Anime doesn't personally interest me, and I think /r/TrueReddit is highly overrated, but /r/syriancivilwar looks very interesting.)
The deeper, the better. For instance, post-2011 magical girl anime subreddits like https://reddit.com/r/madokamagica have nothing on post-2011 magical girl anime's far-future transhumanist scifi fanfiction subreddits like https://reddit.com/r/tothestars (though the traffic levels suffer)
What are you using to judge the quality of the sub?
I can name many subs that attract some of the top members in the world in that field. If you are fascinated by rocketry, then yes /r/spacex is going to really impress you.
There are knitting subs attended by the top knitters in the world that write the knitting books everyone else uses. But I think it less likely to impress you if you don't value knitting as a skill, like you value rocketships.
I judge the quality of the subreddit by the signal-to-noise ratio for posts, by the insight and intelligence of the comments, and by the extent to which is provides content that cannot be found through simple googling. Most knitting subreddits do not have this. Again, do you have specific examples?
I don't know why you think my opinion of r/SpaceX is driven by some obsession with rocketry. I am much less interested in SpaceX than in a dozen other hobby topics, but that subreddit stands out in the objective quality of the moderation and community. Likewise, HackerNews is one of the higher quality places for discussion on the internet even though I do very little programing.
I look at r/askscience from time to time, but it's not reliable. Often has wrong answers, people ask the same questions again and again, mods allow lots of obnoxious/caustic/sarcastic answers. Substantially worse than r/spacex.
Counter-anecdote, I unsubbed from truereddit a while back (probably a couple years now) because the discussion didn't seem very productive, though I don't remember details.
(I do remember that the mod had a very hands-off attitude that IMO didn't work very well, with clickbait headlines to bad articles frequently getting upvoted.)
/r/spacex has definitely above average content and discussion.
I find /r/teslamotors mediocre, most posts are uninteresting and the comments can be ridiculously naive and optimistic. Many people there are convinced that Tesla has the most advanced self-driving tech and hugely exaggerate the importance of crowdsourced fleet learning.
Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle?
I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title.
I'm also kindof curious why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home like the shuttle did. Is that a weight-concern? Aerodynamics, mabye, but couldn't the wings be articulated in the same way that the landing legs are right now?
(I will admit some ignorance in this field. I'm definitely a fan, but I'm definitely not a historian or a rocket scientist)
As I understand it, the main difference is that the Falcon rockets are (or are eventually meant to be) much less dependent on refurbishment between launches. The Space Shuttle required extensive and expensive work between launches to the extent that many critics[1] claimed that it wasn't truly "reusable". (IMHO, while the shuttle program definitely didn't achieve its goals, it seems like calling it "reusable" is fair).
As for gliding back: the Falcon booster does not actually achieve orbit-- when the main engine cuts off, it is on a ballistic trajectory back to the ocean. While it may be possible to design some sort of gliding apparatus to "save" a booster on a sub-orbital trajectory, it is (again, as I understand it), much simpler to simply adjust that trajectory via a boostback burn that reverses or slows that trajectory, and to then perform a suicide burn[2] to recover the stage, either on a barge or (on lower orbit missions) back on a land-based pad.
I am not a rocket scientist, nor do I have experience in the space industry-- these thoughts are just based on what I've read following the SpaceX reusability program as closely as I can for the past few years.
Ideally they reach zero velocity the instant the legs touch the ground. There was only one hard landing (that didn't result in a fireball) so far, and the legs are designed to take the remaining speed (they are replaced anyway and not reused).
They also have no other option. Even one engine at lowest throttle (70 %) is powerful enough to lift the almost empty booster again, so they cannot hover (which would make things a bit easier, at the expense of needing more fuel).
I can't find the reference, but I remember reading that the landing is complicated by the fact that even at minimum thrust, with the tanks nearly empty, the ship is so light that even with a single engine, it would accelerate back up after hitting zero velocity. So if the engines are restarted too early etc. it could "miss" the ground!
(If someone can confirm/deny this? I'm seeing approx 28T dry mass, thrust per engine of 66T and min thrust of 70%, but that dated guess work from http://space.stackexchange.com/q/4466/ )
> Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle?
The Space Shuttle's engines were extremely complex beasts that begat the most advanced alloys at the time – very much bleeding edge, with the tightest tolerances possible, to squeeze out performance that wasn't thought possible before, using highly aggressive propellants (hydrogen embrittlement was a huge problem). As a result, they only had a tenth of their planned life expectancy (average of 5-6 instead of 55 flights), and needed an 80% refurbishment between every flight to replace cracked turbine blades etc. pp.
Falcon 9's Merlin engines, meanwhile, are best described as "boring": They're firmly a 60s design made cheaper than dirt thanks to modern manufacturing technology, and highly reliable thanks to a simple design and safe fuels (kerosene). SpaceX already demonstrated on the ground that Merlins can perform multiple missions without needing refurbishment.
Or, in car analogies: The Space Shuttle is a Jaguar, Falcon 9 is a Ford. While you can, on the paper, drive a Jaguar twice without seeing a workshop, nobody managed it yet.
> I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title.
Kinda, sorta, in theory: While the orbiters did refly a lot of times, the engines (the most expensive part of the Shuttle) barely did, only in the "Ship of Theseus" sense. Same for boosters (fishing steel tubes out of the ocean and hammering out the kinks was a pure work creation measure), and the huge external tanks were made anew each time.
> I'm also kindof curious why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home like the shuttle did. Is that a weight-concern?
Precision and weight: Wings are dead weight for a vertically launched rocket, and increase drag (especially supersonic) further than folded-up landing legs do. Hoverslam point landings need very little added mass on the rocket, and you expose a far smaller surface to re-entry heating etc.
The engines have minor differences, namely that the space shuttle used fully closed cycle engines while the falcon 9 does not. All this means though is that the efficiency is slightly lower because some of the propellant is burned and the combustion product dumped overboard to power the turbo pumps that push the rest of the propellant into the main combustion chamber. While this reduces efficiency, this makes the engine somewhat easier to design and I believe the next gen spacex engines will have this feature.
The vertical landing design is actually really smart because it allows a relatively un-aerodynamic object to get back to the ground without adding heavy wings and fairings. Rockets don't actually like having fins when they go up since the dynamic pressures get to be extremely high. Rather, they opt for gimbaled engines to steer (this also helps when there's no atmosphere to steer with). The only downside to this approach is that some of the fuel has to be retained for the return trip, though this is a relatively small fraction. The other huge issue you have to deal with when gliding back is having some sort of landing gear in addition to the control surfaces. You can put wheels on it but that's a lot of weight and if you land it horizontally, without the pressure of the propellant keeping the body rigid, it will most likely crumple.
The SSME was a staged combustion but not full-staged combustion as the next SpaceX motor will be (Raptor).
The SSME are special and specially efficent because they burn fuel-rich and have multible pre-burners. They were very highly optimized but that made them complex.
Full-staged combustion has a better max performance for a given fuel and promises to be simpler in design.
The landing gear would also need to be retractable, a big increase in weight and complexity. The complexity will add a lot of failure-critical reliability problems.
F9 FT can launch ~ 5.3 metric tons to GTO with landing, or ~ 8 metric tons without. I'm not sure if I'd call that a relatively small penalty. It does allow SpaceX to launch most GTO satellites. I expect Block 5 will do a bit better.
I think both views are essentially correct. It is a relatively small fraction of the fuel load, but thanks to the impact of the rocket equation, that relatively small fraction translates (as you are pointing out) to a sizable difference in payload to a given orbit.
Space shuttle's main engine was the "boosters". A huge liquid (very cold hydrogen and oxygen) tank which supplied the fuel to the actual shuttle during launch, and two smaller solid fuel boosters. The tank and boosters were all destroyed on launch.
SpaceX rockets use a much simpler engines, that burn oxygen and kerosene. The main cost is in the first stage of the engine - think two rockets on top of each other, the first stage is the bottom rocket that first first. When the first rocket is done, it falls away. This first stage accounts for the majority of the cost of the launch. Think of it was being equivalent to the main tank and 2 boosters of the shuttle.
What SpaceX manages, which no-one has done before, is to retrieve that big + expensive first stage for reuse.
The boosters were actually parachuted back down to earth and reused. Though the general consensus is that it didn't save any money, and may have been more expensive than if they had just been disposable.
These seem like problems with that design, not something fundamental.
Could one conceivably use different materials (that maybe didn't exist back then) resistant to aquatic stress and salt, to have made the old plan much more cost effective?
Following this line of thought I think you wind up with something close to what SpaceX does. Because the next largest save-able cost would be the boat crew to go recover it. So attach a propeller to the rocket to have it come back on its own. Then you want to save that cost and complexity so have it come to your boat with no crew and touch down gently.
The solid rocket boosters (SRBs) for the Space Shuttle provided 83% of the thrust at takeoff. The main engines were the other 17%, and continued to fire after the SRBs dropped off and parachuted into the sea. The main engines perform the function of part of SpaceX's booster stage and also SpaceX's upper stage.
>why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home
Basically, the physical and aerodynamic stresses of decelerating vertically are fairly similar to those of accelerating vertically. Whereas designing something which can work as a rocket and a plane would be a lot more difficult, and probably more expensive to produce.
It's more difficult to control vertically, but if you can get it right you save a lot of complexity in the physical engineering.
Winged stage will not have enough kinetic energy at cutoff to allow it turn around and fly back to launch site, and landing it at the ocean takes building and operating a whole aircraft carrier - too expensive (and probably harder to safely automatically land than the way they do it now). So, that winged stage would have to redirect back to launch site the way SpaceX does it now - using fuel. And, it will have to always do it, not having the option of ASDS launch, severely cutting the payload on GTO and other high energy trajectory missions.
For RTLS launches, it would do approximately as good as their current approach, the fuel for landing burn+legs will approximately weight as much as wings and wheels. But would definitely cost more to develop.
A winged stage also having a jet engine, allowing it to fly back as a powered airplane, not a glider, would solve both issues, and will whole lot more efficient than current SpaceX approach for RTLS, and about as good as ASDS, but a lot, a lot more expensive to develop. They would have to become an aircraft company in addition to being a rocket company.
Do you have any references at all for a winged stage being feasible in any way? Considering that there are plenty of rocket stages out there that can't even be moved horizontally unless they're pressurised I doubt anything vaguely resembling the Falcon 9 could ever be retrofitted in the manner you describe. What form would these, presumably retractable, transonic capable, necessarily light wings even take? How would they ever be as light as the remaining fuel load in an ASDS landing, especially when wheels are added?
Russians played with exactly such a stage, that had wheels from Mig-29 and engines from Yak-40. It was called Baikal and was feasible, but eventually never flew.
https://en.wikipedia.org/wiki/Baikal_(rocket_booster) Here is a Wiki article on it. Reverse: it was supposed to use engine from Mig-29 and wheels from Yak-40 :) Anyway, it never went anywhere beyond mockup stage.
You also have to remember that these rockets are going to land on Mars, and we're not going to be able to set up a runway before they arrive, so landing vertically is a necessity.
Well, this one won't land on Mars unless it launches from there (which is unlikely to happen). But they're gathering experience and knowledge about how to do propulsive landings and the technique as such doesn't care much about an atmosphere. Dragon v2 and the ITS spaceship will both use propulsive landing and those will end up on Mars.
Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle?
Shuttle boosters are solid-fuel, so they can't restart, can't land, and were ditched in the ocean and refurbed. In contrast the 1st stage of spacex rockets can land itself.
I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title.
For certain values of re-usable. It was pretty much stripped and refurbed for each mission, at enormous cost. The costs of spacex are orders of magnitude smaller.
I'm also kindof curious why they decided to go with a vertical-landing-design,
> Could anybody here comment on the differences between spacex's engine, and the engines on the space shuttle?
It's sort of apples vs oranges to compare the Shuttle main engines vs the Falcon 9 engines, because the Shuttle main engines were on the the crewed upper stage of the Shuttle stack and the Falcon 9 booster is the boost stage of the Falcon 9 stack.
The direct comparison is comparing the Falcon 9 booster (first stage) with the solid rocket boosters of the Shuttle stack, and comparing the Shuttle with the Dragon. Dragon v1 has a heat shield (like Shuttle) and then parachutes (unlike Shuttle). Dragon 2 will have reusable engines to land propulsively, which means it will land on land like the Shuttle.
The Falcon 9 booster is much, much more sophisticated than the Shuttle boosters. The Shuttle boosters were solid fueled; once lit they could not be turned off or throttled. They just burned at maximum thrust until they ran out of fuel, which is why they needed parachutes to land in the ocean.
Unfortunately NASA learned that salt water is terrible for engines, even if they land gently, and the cost of refurbishing the boosters after each launch (and the amount of time it took) was much higher than anticipated. Falcon 9's Block 5 booster should (hopefully) reach a point where it can land, get refueled, and immediately take off again, like a 747.
> I was under the impression that SpaceX was trying to make the first re-usable rocket stage, but I've recently found out that that isn't true. The space shuttle already holds that title.
For the reasons I have above, you were right the first time. SpaceX is trying to make the first reusable booster that achieves orbit. Both Shuttle and Blue Origin's New Shepard are reusable, but neither of them can reach orbit by themselves.
> I'm also kindof curious why they decided to go with a vertical-landing-design, instead of putting some wings on it and having it glide home like the shuttle did.
Scaling laws. Vertical take-off, vertical landing (VTVL) rockets scale to any size. Wings don't. The Falcon 9, the Falcon Heavy, and future mega-rockets will all be able to use the same basic technology. Wings don't scale well at all.
> Vertical take-off, vertical landing (VTVL) rockets scale to any size. Wings don't. The Falcon 9, the Falcon Heavy, and future mega-rockets will all be able to use the same basic technology. Wings don't scale well at all.
You are correct about the disadvantages of wings, but wrong about why SpaceX isn't using them.
SpaceX has always had the goal of landing on Mars and returning home. All their rockets have been developed with this in mind. They tested their first rocket, and got it to work. They sold it to Nasa and the like (and even developed it with some of their requirements in mind), but the goal was always to learn how to make bigger rockets that can get to Mars, that Nasa is willing to pay for the smaller ones is a side benefit. Mars does not have enough atmosphere for wings to work, therefore SpaceX is not interested in Wings.
Even if reuse of the rocket was clearly more expensive that building a new one each time (it isn't) SpaceX would be working on reusable rockets now because when they get to mars they need to land the rocket and latter reuse it to return back to earth.
On reason, of many, is structural. A rocket's first stage is basically a giant coke can with a big rock (the engines) at one end. It cannot just lay down on the ground horizontally. That type of landing would require all sorts of internal structure, not to mention dedicated landing gear. It probably wouldn't be round any more. Warp a rocket by even a single degree and it will never be good to fly again. On the other hand, the big coke can is perfectly happy to stand vertically.
The Shuttle Orbiter (the reusable part) was not a complete stage as it used an expendable external tank. It did reuse the engines, which one would imagine would be a significant cost savings, but the Shuttle system was such a complex and compromised beast that it ended up being one of the most expensive launch vehicles of all time.
Vertical landing for the Falcon 9 made sense because it meant they only had to add a small amount of extra weight and new systems (landing legs, grid fins, etc.) in order to achieve powered precision landings. In contrast, adding wings, landing gear, other aerodynamic components, and potentially other engines (in order to achieve powered landings) adds a lot of extra complexity and weight. Additionally, the stage is already designed to handle vertical flight and forces, making it strong enough to handle strong aerodynamic forces while flying horizontally is no small change. And vertical landing enables barge landings out at sea as well as dramatically reducing the size of the landing facilities (a small pad versus a landing strip). All of which saves costs, complexity, and enables a higher percentage of the vehicle's performance to be used by the customer versus for recovery.
The large overriding reason the shuttle had wings is so that it could land "cross range", i.e. divert to land either left or right of the flight path. This way if there was a transatlantic abort landing (TAL) required the Space Shuttle could land in Spain, Portugal, France, Diego Garcia (and in varying years Gambia, Nigeria and a few others). This was a requirement if the shuttle was launching with a classified payload (i.e. NRO satellite).[0]
The Air Force also wanted to be able to launch the Shuttle on a polar orbit out of Vandenberg, do a single orbit, and land back at Vandenberg on the next pass. The idea was that if there was a critical mission for the Shuttle (capturing or disabling a Soviet satellite, for example) and the Soviets were in a shootin' mood, this would minimize the window of risk to the mission.
Of course, the Earth rotates underneath the Shuttle while it's in orbit, so when the Shuttle comes back around in a polar orbit it's quite some distance to the west of the launch site, so it needs to be able to make significant maneuvers for this to work.
Vandenberg never ended up hosting Shuttle launches, of course. It was nearly ready, with mission planned and everything, when Challenger exploded. This caused a reevaluation of the whole idea, and it ended up being scrapped. But the design consequences remained part of the vehicle until the end.
And of course, there was John Glenn, monitored inside and
out, blood tested, urine sampled, entire organism analyzed
for signs of accelerated aging. Close observation of the
Senator suggested that there might not be any medical
obstacles to launching the entire legislative branch into
space, possibly the most encouraging scientific result of
the mission.
Ha ha I wouldn't say skipped so much as cherry picked. There are plenty more golden nuggets in that article, the one I quoted just happened to be the one that had me laughing out loud.
There is so much to like about that essay, even the citations are good. Here is my favorite:
> The Soviet Shuttle, the Buran (snowstorm) was an aerodynamic clone of the American orbiter, but incorporated many original features that had been considered and rejected for the American program, such as all-liquid rocket boosters, jet engines, ejection seats and an unmanned flight capability. You know you're in trouble when the Russians are adding safety features to your design.
I think that the space shuttle is really a re-usable second stage, albeit one which is firing its engines for the entire ascent. The aerodynamic demands on a reusable second stage are much greater, so comparing the space shuttle to the Falcon 1st stage isn't exactly fair. The Falcon 1st stage is more analogous to the Solid Rocket Boosters, in that it's a very high-thrust, high-density launch vehicle.
The Solid Rocket Boosters are strictly speaking doing exactly what the Falcon 9 booster is doing: being recoverable. However, the Falcon 9 boosters have much greater capability to be re-used without refurbishment, since they don't encounter salt water.
Context here for those who don't know: SpaceX' declared goal is to put humans on Mars. Mars has enough atmosphere to cause trouble (burning up in free fall), but too little for any kind of coming to a stop with parachutes. Cf. how Curiosity landed, which wouldn't have been necessary on Earth. In any case, SpaceX doesn't like to be constrained by things that only work on Earth, and while parachutes were considered for recovering the first stage they found it wouldn't really work (and dropping things into the ocean tends to not do them good). So propulsive landing (despite sounding more complicated) is actually a better option than others here.
wings, retractable landing gear and aerodynamic control surfaces are WAY heavier and more complicated than 4 legs and a sensor/software based vertical balancing system that controls already-needed engine gimbals.
I'm a programmer and I love rockets. I don't work in Aerospace and all of this is from my own knowledge and learning. Take that as you will.
The SSMEs (Space Shuttle Main Engines - Aerojet Rocketdyne RS-25), required an almost complete rebuild after every launch. It was nominally reusable in that the parts could be reused, but it took a long time to refurbish and re-test the engines. Also, the first stage of the Space shuttle included the solid rocket boosters and the hydrogen fuel tank. The boosters were absolutely reusable (I seem to remember reading that of all the SRBs, all but four were reused - two from the first launch were never recovered, and two from the Challenger disaster that couldn't be recovered). But the Space Shuttle wasn't fully reusable like the Falcon first stage.
To me, the difference between the Space Shuttle and the Falcon 9 is kind of like the difference between a doctor of mechanical engineering and a mechanical engineer. The Space Shuttle was an amazing (and complex) piece of tech - the engines used liquid hydrogen and liquid oxygen, which is incredibly efficient but notoriously difficult to work with (cooling hydrogen to a liquid sate and keeping it cooled is not trivial). It was a long (~10 years) process to develop the Space Shuttle from the existing tech we already had. The result was a highly tuned and complex engine that was incredibly efficient. It makes you feel a bit warm and fuzzy at how efficient and well engineered the engines are.
SpaceX went the other way - they said we'll use a slightly less efficient set up (RP-1 (refined kerosene) and liquid oxygen) as our fuel, but we'll make up for it by making each individual launch much much cheaper. So while they may not have the fuel efficiency of the Space Shuttle, for the cost of a single Space Shuttle launch (~$400M per launch according to wikipedia), SpaceX could launch 6 or 7 Falcon 9s (~$60M per launch). The SSMEs are very efficient engines. The Falcon 9 is a very efficient platform. It's kind of like a big picture vs micro optimization sort of thing. (There is no judgement here - the Space Shuttle was developed by a government with much different concerns and constraints than the private business developing the Falcon 9).
Having a fixed wing aircraft come down from space really doesn't make that much sense when we get down to it. It's a really great and interesting concept, but the realities of it make it not worth the trouble. Sure the Space Shuttle glided, but even a brick going 17,000 miles an hour will glide for a long while. This video (https://www.youtube.com/watch?v=B3JZtY7bcbM) gives a great overview into the complexities of a plane-shaped spacecraft hitting the atmosphere at 7 km/s. When a capsule (like the Apollo Command Module) hits the atmosphere, you only need to protect one side of it, and you just let the thing fall like a stone then slow it down with parachutes. The Space Shuttle Orbiter had to have different materials with different heat shielding properties all over its body because it would glide through the air at different angles. The bottom of the orbiter had an enormous surface area, and it needed to be thermally insulated. But heat shields are very heavy, and that amount of weight would make the launches impractical and landing impossible. So NASA had to develop new, light materials that were incredibly strong and also heat resistant.
The Falcon 9's first stage is not going nearly as fast as the Space Shuttle Orbiter when it hits the atmosphere, so it's not really appropriate to make a direct comparison about the complexities there. But this is to me another part of the brilliance of SpaceX looking at the big picture - it's much easier to reuse the first stage (which is typically the most expensive part of a launch vehicle) which doesn't reach orbital velocity and capture as big a chunk as the savings as possible rather than being totally 100% efficient.
I would not be surprised if NASA was 50 or 60 years ahead of its time with the Space Shuttle. It seems like private industry is catching up to where NASA was with the Saturn V in the 60s just now (we're still not close to a rocket as powerful as the Saturn V). We may see that in 20 or 30 years, the industry uses the materials and knowledge gained from the Space Shuttle to develop new and novel spacecraft.
Again, I'm just a programmer who loves rocketry and space - I apologize if there are any technically inaccurate bits in this post.
It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion. So if you want propulsion why not use the main engines, and since you are going to use those formthe soft controlled landing why not use them for the descent too.
At this point you have traded several tons of parachutes and many more points of failure which could cause loss of mission (parachutes are complex mechanisms) for a smaller weight of fuel and exactly the same points of failure as a rocket always has. Then SpaceX added grid fins, since purely propulsive landing was not going to be anywhere near accurate enough.
Classic example of working to your strengths, where the greatest strength of a rocket is acceleration along its primary axis :D
The ULA are now planning a reusable rocket to counter SpaceX, and they plan heat-shield then deploy parachute and finally capture by helicopter. This seems much simpler than SpaceX's landing approach as helicopters and large aircraft have been snagging things parachuting back from space since the beginning.
>It turned out that parachutes are great for soft uncontrolled landings, but if you want a soft controlled landing you need some form of propulsion.
They could do something like the Russians do with Soyuz - small solid rocket boosters that fire right at landing. But yeah, that's a lot more things that have to go right.
Also solid rocket motors are a lot less weight efficient than liquid fuelled rockets and are not throttleable and are awkward to gimble. Also where would you put them without increasing air resistance on the way up?
Also even with the rocket motors Soyuz hits the ground pretty hard.
As I understand it, the F9 first stage can't actually hover because a single engine at the lowest setting gives the overall stage a TWR > 1. Thus the "hoverslam" - they calculate the landing burn such that the stage reaches 0 velocity and 0 altitude (from the pad) at the same instant. But this means the only margin for error is in the landing legs, and they've had some hard landings.
I was originally thinking about solid rockets in context of a parachute system, but the more I think about the more I believe truly gentle landings aren't really a feature of the current design.
I'm not sure how much weight it would add, but it seems like having the ability to fine-tune the thrust at the very end would allow them to land much more gently and feel better about not introducing metal fatigue or other damage. Of course the addition of a smaller LF engine probably makes the most sense.
The kinds of parachutes you'd need for a first stage are surprisingly heavy. I doubt they would save any weight, since you have to use the rocket anyway to slow down to the parachute's operational range.
In addition you are more mission-flexible with the weight expended for fuel. It can be used to either soft land the stage, or give extra boost to a GEO mission or a heavier payload.
I didn't find many of the comments enlightening, but the general argument seems to be this: expert consensus (by both SpaceX and the insurance company) is that the risk of failure is simply not that much higher than a normal launch. So it doesn't make sense to waste a probably-safe trip to LEO; you might as well get paid. Even in the extreme limit where there was a 50-50 chance of failure, it's clear that there would be some payload that would be worth attaching (e.g., a standard satellite design with a low marginal construction cost, or even a high school satellite project).
The static fire yesterday was enough proof for SpaceX that everything is working as expected. After that the major undetected flaws will show up some time from launch through MECO :D
SpaceX will no doubt discover new failure modes that nobody has thought of up till now (and thus were not looking for, and thus did not detect).
Vastly different, but the most likely point of failure is the moving parts, as tested in the static fire.
Other parts that failed had been successfully used in flight so a "dummy launch" would have provided no certainty other than "everything worked this time."
The point of the static fire is to see if the engines work, and make sure that the rocket fueling sequence works. It does not stress the rocket with any aerodynamic forces.
Well, those are flaws that don't matter much, though, as the primary mission is getting the satellite to orbit. Sure, landing the booster a second time would be nice and awesome, but usually they tend to learn much more from failures than from successes.
> “We got a discount,” Halliwell said. “I can’t go into the specific pricing, but we did get a discount for being an early adopter of the technology.”
> In response to a question whether the discount was closer to potential figures publicly disclosed by SpaceX’s Shotwell and SES chief executive Karim Sabbagh — who discussed possible reusability discounts of 30 percent and 50 percent, respectively — Halliwell said: “It certainly came out closer to Gwynne than to Karim.”
> The figures, while stated as unofficial and somewhat arbitrary, illustrated the behind-the-scenes negotiation on the real price of a reused Falcon 9.
> Halliwell did not confirm whether the ultimate price for the SES 10 launch fell between the 30 percent and 50 percent discount figures.
> “We did receive a good discount on the baseline price,” he said. “It’s not as crazy or spectacular as you might think, and that makes sense. SpaceX has to supply an awful lot of equipment, and they have to do an awful lot of refurbishment.
> “It is an interesting price, and it’s something which we believe will be meaningful going forward, and will be meaningful for the industry in the longer term,” he said.
> SES says there was “no material change” to the insurance coverage or the company’s premium payment after cinching the agreement to launch SES 10 on a “flight-proven” rocket.
> SpaceX has to supply an awful lot of equipment, and they have to do an awful lot of refurbishment.
Right, they've already stated that with the current rocket there's no economical way to recover the second stage, which aren't cheap, and so far nobody knows how many launches the cores will be good for. Also the infrastructure for supporting the recovery efforts, including the drone ships and their operations, can't be cheap. I expect the SpaceX re-use project will probably be in the red economically for years before it starts paying off it's setup costs.
The second stage is about a quarter of the total cost of the rocket, so the best case, if reusing the first stage cost nothing, is a 75% cost savings.
I don't think refurbishment is going to cost all that much. They fired their "max damage" core eight times without any refurbishment and it did alright. The upcoming Block 5 version of the rocket is supposed to incorporate lots of changes to help with reusability.
Lessons from the Shuttle are commonly brought up, but may not apply very much. Aside from being very different craft (with the Shuttle running on much thinner margins), a huge difference is that SpaceX is able to rapidly and strongly iterate as they learn. The Shuttle that first launched in 1981 was not all that different from the one that last launched in 2011. The design was not entirely set in stone when it started flying, but it was pretty close. That's what happens when your vehicles cost multiple billions of dollars each. SpaceX is continuing to build new rockets and that makes it a lot easier to make changes to react to experiences with older designs.
I doubt the ships are all that expensive in the grand scheme of things. I'm sure the cost is more than what you or I could afford, but I don't think it's a particularly large fraction of the booster's multi-dozen-million-dollar cost.
In any case, it will be great to see how it works out, rather than just talking about how it might work out!
Oh absolutely, it's a revolution in the making. In fact the limiting factor on their launch cadence in a few years might just be how quickly they can manufacture second stages. That would be a great problem to have.
The way they see it, it's already had a test flight.
Another flight would just show there are unlikely to be failures on a second launch, but wouldn't say anything about a 3rd launch. So would you do a 3rd test launch to check that? When do you stop testing and just launch something worthwhile?
Just for laughs here is a scheme: get a non mission-critical satellite. Insure it. Attach it to Falcon. Explode it during SpaceX "launch". Collect insurance.
> SES says there was “no material change” to the insurance coverage or the company’s premium payment after cinching the agreement to launch SES 10 on a “flight-proven” rocket.
They have done a number of static test stand firings of individual first stage engines, simulating up to six or eight launches of the same first stage... Undoubtedly the data gathered from the ground tests plays a big part in their confidence that this will work.
SpaceX has test-fired this stage at least 8 times in Texas already (more since this article was written). It is a recovered stage, on one of the most brutal reentry trajectories that they've done so far:
https://www.nasaspaceflight.com/2016/07/spacex-returned-falc...
So it's not just the engines that have been tested. Also, they had to put some sort of beanie cap on top to simulate flight loads.
They get bragging rights. It's also very much in their interest to support development of reusable launchers, since once the technology is proven they'll save on every launch going forward.
Fingers crossed. In theory it shouldn't be any more dangerous than a first-time rocket, given that they've examined every inch of it and deemed it flawless. If it weren't flawless, obviously they wouldn't be trying to fly it. So we're just hoping there isn't some unanticipated source of entropy, so to speak. As always.
Not to be pedantic, but it's not possible to inspect an entire rocket for all defects because so many of them are microscopic and internal (not on the surface of a solid part). These can be analyzed and predicted with fatigue models in aggregate with varying levels of certainty. This comprises the fascinating field of fatigue analysis.
While you're correct, you're not pedantic enough. Using X-rays to validate the most critical engine parts has been done for literally years and is a relatively boring and old part of "rocket science". Don't expect SpaceX to spend some serious cash on this and techniques such as this (MRI and sonic / ultrasound tech comes to mind).
Source: a good friend of mine is a structural engineer for Aerospace Corporation (a rocket scientist if you will) and I'm an avid r/SpaceX reader.
X-ray takes care of macro internal defects. It doesn't reveal defects at the grain (nanometer) scale, outside of research labs. I think it'd be time prohibitive to scan at that resolution for every component of large systems.
There are pretty sophisticated machines out there for non destructive machine inspections. Many based on X-ray or MRI technology depending on the materials.
Similar to the machines used to inspect shipping containers. I would not be surprised if SpaceX had at least considered if not obtained equipment for the ability to X-ray the entire assembled rocket for any internal defects.
Anyone curious can take a look at the equipment from Yxlon (http://www.yxlon.com) for examples of COTS X-ray based inspection that's available. And Yxlon and most others in this industry do custom work, making a robotic full sized rocket inspection setup, quite a realistic concept when the costs of failure are considered.
Yeah, we used X-ray inspections to get away with smaller safety margins at my last employer. Plastic deformations start at the grain level (nanometers) though, and I don't think there's any X-ray microscopy to do this en masse.
You're right, of course. This is what I meant by "unanticipated source of entropy," though -- some form of breakage which is neither visible nor predictable with the available information.
It's true that there is fatigue but shouldn't most of the fatigue be mitigated by the fact that they slow the rocket down significantly upon reentry? I don't see why a falcon 9 should suffer much more fatigue than any jet that goes faster than 2x the speed of sound
It might not matter after you do the math, but on landing this rocket is plunging back into the atmosphere at supersonic velocity moving through its own rocket exhaust. This is a pretty visceral image that does sound like something causing fatigue.
Yes, mitigated compared to a full-speed descent. It's not really possible to compare it to an entirely different system though like a fighter jet. Fatigue is analyzed at a part level, like a single strut or a patch of aluminum reinforcing a small area, and is dependent on the type of loading, materials, magnitude/dir of stress, temps in some cases, frequency of cycles, and so on. Fatigue life in aerospace is a tradeoff between designing heavier, beefier, longer lasting parts and skimpier, lighter parts that lead to a lighter vehicle overall.
Any part can undergo fatigue damage for a given loading if it is designed to be weak enough.
It probably isn't there yet, but once they get it figured out, used rockets should be safer. Just think: how comfortable would you be flying on an airliner that has literally never flown before? The SpaceX fan community has taken to calling these rockets "flight proven," and while that might be a bit ambitious today, I think that's a good way to think about where it should end up.
I'm pretty stoked we are staying right next to Jeti park on spring break and are planning on watching this launch tonight from the beach. We've got to see one other Delta IV launch, but really looking forward to seeing this one. We just missed OCSILY heading out to landing zone, but I got a good view of the parking spot and unloading area.
The BBC article doesn't use the word "today" anywhere in the text, nor does it provide a date. Does "X set to do Y later" in journalism-speak mean "later today" instead of "later somewhen"? Is this another idiosyncrasy like using commas instead of "and"?
I feel like it's a little different when you speaking to someone versus in a news article. I could read the article tomorrow and be very confused, but I won't hear you say those words tomorrow unless you actually mean tomorrow.
It's pretty common for people to make a hard problem out to be much more difficult than it actually is if they have a track record of failing to tackle it. The reality is that the problem is not that terribly difficult, all things considered. That's not to diminish SpaceX's achievement, what they've done is genius. But their genius is primarily in avoiding falling into the common trap of optimizing an expendable rocket to the detriment of any hope of easy and beneficial reuse of part of it. And indeed the most logical optimizations of expendable rockets tend to push the design in precisely that direction, so it takes a considerable amount of sticktoitiveness to avoid that pitfall. Additionally, they figured out how to do reusable R&D very effectively and cheaply (by using commercial launches to subsidize most of the costs).
Together this has led them to success, but any space-faring nation or organization with a few billion dollars sitting around could have achieved the same thing, they just didn't have the courage or foresight to do so. Indeed, Blue Origin will probably follow along and reproduce the same achievement in orbital rocketry within the next 5 or 10 years or so. And I expect others to follow as well, once a model that can be easily copied has become established and its advantages proven.
I guess the fact that it hasn't been done so far is in part due to lack of technology (bot materials and processing power) and the lack of necessity. Space programs have been purely national affairs for a long time and cost for the rather few launches was not really an issue. Even more so when the launcher was developed off ICBMs where development costs were already paid for with a military budget.
And, well, as long as no cheaper competitor comes along it doesn't matter if launch prices are high since whoever needs to launch something just has to pay them.
I disagree, I'd say the main reason has been pride. Otherwise we could have built reusable rockets in a similar fashion as early as the 1970s, maybe earlier.
There are a few reasons why this is "hard" but few of them are actually underlying engineering issues, most of them are at higher levels. A big problem is that, as I alluded to, the natural and traditional optimizations for expendable rockets lead you away from reusability. Those optimizations tend to push you toward lowering the number of engines on each stage and also concentrating more cost on the upper stages. Incremental rocket development tends to be easiest by improving the upper stage, because that takes lower upfront costs and is sometimes less complex (lower R&D costs, smaller capital investment, able to leverage existing launch and rocket processing infrastructure). But the result of those optimizations tends to be rockets (like the Delta IV, Atlas V, Ariane 5, etc.) that are difficult and costly to reuse (due to an inability to throttle the first stage thrust enough, among other reasons) and where reuse of the first stage, the easiest stage to reuse, is not advantageous because that's not where the costs are concentrated. Indeed, if you look at the Shuttle system, that involves reuse of the upper stage, at extraordinary cost and with extraordinary complexity. It takes intentional choice to design a rocket, like the Falcon 9, which is both highly amenable to first stage reuse and also where that reuse has the most benefit (due to that stage being most of the cost of the launcher). It's even more difficult to achieve that without being vertically integrated the way SpaceX is.
The Falcon 9 is fundamentally based on a 1950s era rocket design, and the main design goals aren't high performance or technical achievement but low costs. Using well-proven basic designs (LOX/Kerosene open cycle engines, two stage rockets) meant that SpaceX could concentrate a significant amount of their development efforts on working on reusability alone. That's actually surprisingly difficult for traditional rocket development orgs to do. For one, using a "simpler" basic design would seem like "going backwards". For another, doing the basic engineering to develop reusability would seem rudimentary. More importantly, any perceived failure would be very, very difficult for those orgs to accept. Experimental rockets are hard to get right, and often anytime things don't work exactly as expected on the first flight can easily be perceived by the public as a "failure". This was true even for SpaceX when they took several attempts to achieve first stage landings even though those stages already achieved their main purpose and normally they'd just get thrown away. NASA and traditional rocket makers are enormously prideful and sensitive about perceptions of failure.
This pushes those orgs to avoid "messy" development projects and to also tend to focus on "beyond the state of the art" developments. This protects those orgs from injuries to their pride because it helps avoid messy failures but also gives them an excuse if things don't work out, because they were working on something innately challenging. This is how we got the Shuttle, for example, which was a typical "beyond the state of the art" development effort. It's also how we got the X-33/VentureStar which relied on several key beyond the state of the art technologies.
It's also why we turned away from the DC-X design, which demonstrated VTVL sub-orbital flight in the 1990s. But something as simple and clunky as a 2-stage launcher using old rocket engine technology and lower stage reusability wasn't sexy enough for NASA et al back then, they had to have something like an RLV SSTO that was sexy enough to justify any risk of program failure.
If you could go back in time to the 1960s/'70s and tell them what to build you could absolutely produce a system similar to SpaceX's. It wouldn't have the same performance margins, it wouldn't have the same capabilities, it probably wouldn't be able to do barge landings, but you could absolutely build a reusable rocket using contemporary technology. And we'll see in the near future how many others copy the design once it's proven. The hard part isn't really the technology, it's the risk taking and the dogged pragmatism.
Right! Disclaimer: I'm a bit of an Elon Musk fanboy.
I think this is really the key thing. Many of the big leaps forward are about challenging deeply rooted or held assumptions. Elon Musk seems to be very good at that.
I don't have the details, but in his presentation of the Tesla solar roof tiles, he mentioned, a couple of times, how inefficient the whole supply chain was for roofing today. His body language implied that it was shockingly inefficient.
So when he made the claim that solar roof tiles will be about the same price as old school roof tiles, maybe that's why. Challenging a bunch of assumptions, and even looking in places that few others thought to look.
PS: I know this is a simplifications, and I'm sure others have looked into this space before, so I'm not sure what caused others not to push forward.
There were some detractors who have been in the industry for quite some time; can't remember the exact names but folks from either Lockheed or Boeing as per Ashlee Vance's Elon biography.
It's hard to say but easily tens of millions of dollars. Note that there's little incentive to offer a significantly lower price to customers without any competition applying downward price pressure in that range so it's basically impossible to figure out what the internal cost difference is just by looking at price changes.
In general they've said that launching used rockets will cost customers about 30 percent less than regular price (so ~43 million instead of 62). For this specific case I wouldn't be surprised if they charged SES less than that because of the risk involved in being the first flight.
No one knows for now, as refurbishment costs are a big unknown (and I don't think SpaceX will release those numbers at this point). The cost of the first stage is a significant part of the overall rocket cost, though, considering it's the largest part, and it has 90 % of the engines (each of which costs about a million already).
Since they took four months to refurbish this stage it may have cost quite a bit to do so for now. But they're learning and Block 5 to debut later this year will have lots of changes based on what they learned.
In fact optimizing the Falcon 9 for first stage re-use has lead it down a very different development path from most modern launchers.
On an expendable architecture the priority is saving weight on the second stage, because every kilogram saved there saves many kilograms for the first stage. So pound for pound the second stage usually costs more than the first stage. Also the cheapest way to build a first stage is with one or just a few big engines. Both these optimization criteria are reversed for a re-usable design. In the Falcon 9 the first stage is re-used while the second stage isn’t, so that’s the stage you want to spend your money on. Also a few big engines can’t be throttled low enough or gimbaled precisely enough for a landing burn. Even the using just one of the relatively small engines in the Falcon 9 can’t be throttled low enough to hover. Hence the Falcon 9 has many smaller engines to give it a much greater range of thrust settings and better attitude control.
reusability is the key to commodity rockets. look at the car and how depreciation works, people will own a rockets after 5 owners for a ridiculously cheap price. at that point fuel will be the number one cost issue and the fuel source will charge.
methane fuel is what we desperately need now, methane is the number one danger to our atmosphere and burning it is the most ecofriendly way of disposing of it.
All the next-generation large American commercial rockets will use methane.
United Launch Alliance's Vulcan (15-30 tons payload to low orbit, depending on if solid rocket boosters are added) will, using two of Blue Origin's BE-4 methane engines.
Blue Origin's reusable (initially partially, eventually fully reusable) New Glenn (40 tons payload to low orbit) will be using the same engines but 7 of them for the first stage and I think one of them for the second stage.
SpaceX's fully reusable ITS (300 tons payload to orbit reusable, 500 tons expendable) will use 42 Raptor engines, full-flow staged combustion engines at ridiculously high chamber pressure. On just the first stage. 9 more Raptor engines on the spaceship portion (which is like a spacecraft/second-stage combination).
Additionally, Masten Space Systems is developing a small-sat (1 ton or maybe 300kg, depending on if they're picked under DARPA's XS-1 program) launcher that would be highly reusable called Xepthyr that will use their Broadsword methane engine.
BTW, Masten Space Systems actually inspired SpaceX to pursue vertical landing recovery years ago (SpaceX had previously tried using parachutes on Falcon 1 and the Falcon 9 first stages, without success) with this video that Elon Musk reportedly emailed to the whole company:
(And Masten also has been pursuing using a launch cradle instead of legs for landing, similar to what SpaceX pursued for ITS. And I think SpaceX got that idea from Masten, too.)
I'm pretty sure any methane fuel they use will be manufactured for the purpose rather than extracted from the atmosphere. That means there's bound to be some leakage.
When you launch single-use rockets every next one is more advanced and reliable than the previous one. It includes some of the error corrections that were found during previous launches. This is an often ignored point.
I highly doubt that reusable rockets would be economically viable at our current state of material technology. We need better materials for the engines and the tanks to make them reusable. Even airplanes have to be retired when they still look new just because they've been through multiple pressurization/depressurization cycles and accumulated microfissures. Of course they are not even close to the levels of stress sustained by rockets.
I think your point is weakened a bit by the fact that SpaceX is both the only company flying a rocket designed for re-usability, as well as the company that is iterating their design far faster than any of their competitors.
SpaceX has had trouble getting approved for Air Force launches in the past because they "tweak" the design of Falcon 9 too often. They had to commit to NASA to fly seven launches of a "fixed" design (what SpaceX is calling "Block 5") before they will be allowed to fly crewed missions (for NASA).
> We need better materials for the engines and the tanks to make them reusable.
Better than which target? Yes there is a lot of stress, but what make you say with such certainty that the stress levels are not suitable for 2, 3, 10 or 50 launches? Just reusing a rocket a single time would be a huge cost saver. I don't see how this is not "economically viable".
> When you launch single-use rockets every next one is more advanced and reliable than the previous one
Let's not forget that we have been using russian engines build in 1960 and stored in a hangar for decades in siberia on the Atlas and Antares...
It's likely that reusable Falcon 9 boosters will fly perhaps a dozen times before being retired. That's not a particularly long lifespan, and won't greatly interfere with making design improvements. Even flying a dozen times will make launches much cheaper than flying once.
Typical airplane lifespans are measured in tens of thousands of flights. It's not really comparable.
https://www.reddit.com/r/spacex/comments/62aqi7/rspacex_ses1...
https://www.reddit.com/r/spacex/
Here's the SpaceX press kit:
http://www.spacex.com/sites/spacex/files/finalses10presskit....