
The Tyranny of the Rocket Equation (2012) - rfreytag
https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html
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brohee
Funny he didn't mention beaming the energy from the ground. At the time of
writing (2012) NASA was experimenting with microwave transmission, but it
seems they ran out of fund before succeeding...

[https://en.wikipedia.org/wiki/Beam-
powered_propulsion](https://en.wikipedia.org/wiki/Beam-powered_propulsion)

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WalterBright
Another possible solution is to use an air-breathing rocket until it gets out
of the atmosphere. This saves the weight of the oxidizer.

There have been some experiments in this direction, notably Richard Branson's.

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chipsa
Air breathing saves wet-mass, but increases dry mass. So far, the dry mass
increase has been enough to make it not worthwhile. Especially since the
change in trajectory required results in much higher drag losses, and more
worse thermal performance (rocket gets hotter)

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Retric
With the notable exception of anti satellite weapons which have often been
fired from military aircraft. It’s cheaper because an individual flight on an
existing high altitude high velocity aircraft is cheap. Unfortunately, it
would require an enormous number of space flights for this first stage to be
worthwhile otherwise.

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dang
A thread from 2018:
[https://news.ycombinator.com/item?id=17425156](https://news.ycombinator.com/item?id=17425156)

From 2014:
[https://news.ycombinator.com/item?id=8537390](https://news.ycombinator.com/item?id=8537390)

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asah
Idea: bot that auto posts previous HN conversations? (could be useful for
Reddit as well...)

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tomp
There's a _past_ link at the top, below the link to the source article.

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adrianN
It's time we start manufacturing rockets a bit higher up in the gravity well.

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Nodraak
It's not only about altitude ( _going_ to space is easy), it's also about
horizontal velocity ( _staying_ in space is hard: 8 km/sec is a lot).

I dont recall the exact number, but I think that altitude is 50% of the
energy, the other 50% is horizontal velocity. So building a rocket on top of
the Everest would save us maybe 5% (10 km vs 100 km altitude), that's not much

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adrianN
I was thinking at least LEO, not Everest.

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marcosdumay
LEO is as much a speed as it is a height.

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adrianN
Obviously? When you want to manufacture things you probably don't want to fall
back down right away.

~~~
marcosdumay
Well, I misunderstood your gravity wheel comment for a static position too.

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JanSolo
Don Pettit is my favourite astronaut. He's easily the nerdiest guy who ever
made it up to the International Space Station.

The reason he's so cool is that he can present scientific topics in an easy-
to-understand way that really captures the imagination. He conducts a whole
bunch of personal experiments in orbit and then uploads them to Youtube. Using
static electricity to spin water droplets around a knitting needle. Using
surface tension to improve coffee usability. Real science, but presented with
a wide-eyed innocence that shows how stoked he is to be up there doing all
this cool stuff.

A great asset for NASA in my opinion. They should fly him all the time!

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Robotbeat
An interesting aspect of this that is not mentioned here is that,
fundamentally, although the amount of propellants is large compared to the
payload, the vast majority of the propellant (and thus the lift-off mass) is
oxygen. Liquid oxygen is extremely cheap as it just requires compressing the
air (and, of course, airliners also must compress the air to burn with fuel,
it's just that they compress it in-situ).

Only getting 4% of your lift-off mass to orbit sounds like a terrible deal,
but it's really not so bad when you consider the vast majority of your lift-
off mass costs just 5 cents per pound, and almost all the rest costs about
10-30 cents per pound. Near the limit of rocket performance would be the 2016
SpaceX ITS (now Starship) proposal which had an expendable capability of about
550 tons of payload for about 8500 tons of propellant (of which about 20% was
fuel). That's a fuel to payload ratio of 3:1. (unfortunately, reusable
performance is nearly half as bad, so about 5 or 6:1).
[https://www.spacex.com/sites/spacex/files/making_life_multip...](https://www.spacex.com/sites/spacex/files/making_life_multiplanetary_2016.pdf)

For long-haul aircraft carrying cargo, the situation isn't that much better.
Near the edge of their range, cargo airliners have about 2-4:1 fuel to payload
capability. And considering that rockets are now starting to use LNG for fuel
(often significantly cheaper than jet fuel), the fuel cost for Starship (or
similar vehicles) could actually be _less_ than that for a long-haul
airfreighter on very long routes.

In fact, for single-trip flights half-way across the world (i.e. at the edge
of the capability of modern aircraft), the fuel:payload ratio for ITS
(Starship) or an airliner would be about the same, might be even better for
Starship/ITS. In part that's because launch vehicles stage, which makes them
extremely efficient.

And Don is a fantastic guy, but I also think he exaggerates slightly the
engineering in rocketry vs other fields. The cost per unit dry mass of an
airliner and a launch vehicle is approximately the same. The factors of safety
are also similar (although usually rockets don't have high cycle
requirements...). And in some ways, rockets are simpler structurally as their
typical load case is pressurization and axial loading (which is often in the
same direction). That means they can use relatively inexpensive thin-gauge
stainless steel construction in ways that the more complexly-loaded aircraft
cannot.

The main issue is we just throw away rockets for the most part. Fix that, and
we don't need exotic nuclear propulsion or anything to have low costs to
achieve orbit.

There is one figure of merit that rockets tend to do much better at:

Final dry mass to payload.

Because rockets stage off the vast majority of their mass early in flight,
only a small part of the rocket dry mass has to go through the majority of the
delta-v. A rocket upper stage may be a fourth to a fifth the mass of the
payload. With airplanes, the dry mass of the airplane essentially always is
greater than the payload mass, and on longer flights this might be 2:1 or
greater.

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sunkenvicar
Seems there are typos in table 2 rows 4 and 5.

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HNLurker2
Did the slashdot effect just happen or it is just me?

