
In California, giant Stratolaunch jet flies for first time - lelf
https://techxplore.com/news/2019-04-california-giant-stratolaunch-jet-flies.html
======
avmich
There are several advantages for airplanes as first stages. Less air drag on
the higher altitude and more efficient engines, as they are optimized for
smaller external air pressure. The speed of aircraft adds to the speed of the
rocket. Another plus is that with aircraft you don't need to build a launch
pad, the rocket is just released in mid-air. Yet another benefit is that
aircraft can fly to a more preferable point for the launch - for example, the
point where it is convenient to drop the first stage of the rocket, or the
point closer to the equator so the satellite won't need to spend as much fuel
to change the orbital plane. The airplane first stage can potentially fly to
very different launch point, in many different countries, which can make
logistics simpler. Those advantages all add up economically.

~~~
ThenAsNow
I've a detailed comment that addresses several of the points you're making at
[https://news.ycombinator.com/item?id=19660163](https://news.ycombinator.com/item?id=19660163)

Is it true, strictly speaking, that you may use less propellant in the
expendable stage due to air-drop? Sure. Although to be clear when talking
about engine efficiency, it's really about being able to run a larger area
ratio nozzle and getting a higher mission-averaged Isp. But ultimately when
you consider all the "puts and takes" in launch system economics, those
considerations are small potatoes (price out bulk LO2, and your choice of
fuel). People (even university professors make this mistake) get so focused on
minimizing propellant use that they fail to realize that complexity and dry
weight are orders of magnitude more important.

The only serious advantages to air-launch are the logistical ones you noted.
Also note that for U.S.-based systems, the idea that you will be able to fly
down to equatorial latitudes requires you have the range to carry the laden
launch system (and top-off to contend with LOX boil-off) pretty far. Strato
certainly doesn't have these kinds of legs. Virgin will probably be able to
get more out of their system, but it's not going to scale significantly
higher.

~~~
nbadg
I agree that for traditional launch systems it doesn't make that much sense --
particularly in light of the recent development of reusable (or at least,
partially reusable) rockets.

That being said, I think the math changes somewhat for smaller launchers. The
smaller the rocket, the worse the losses to the atmosphere get, and the more
advantageous it is to escape atmospheric resistance in the soupy low
atmosphere. So I think where air-launch systems could really shine, is by
enabling low-cost launch of very small payloads.

I am, however, not entirely convinced that a sufficient market for those kinds
of _dedicated_ launch services will emerge (as opposed to hitching a ride in
spare fairing space of a separate launch). I could see this going either way;
as much as I'd like to see easier space access for eg. university-funded
nanosats, I'm not sure the timing will be right for something like
Stratolaunch, because the new private aerospace (SpaceX, Blue Origin, etc) is
focused so intensely on pushing down the cost of larger-scale rocketry, to a
point where that same market would be better served by bulk launches using
some kind of standardized dispenser.

~~~
ThenAsNow
> That being said, I think the math changes somewhat for smaller launchers.

Do you have any analysis you can point to to substantiate this? I suspect any
such analysis is heavily predicated on assumptions, such as whether or not the
small launcher is designed around existing engines/motors.

I don't think there is a compelling case to be made that operating an air-
launch carrier aircraft in addition to the costs associated with the remaining
rocket stages works out economically^.

Orbital was not able to get compelling economics out of Pegasus. To be fair,
they were doubly-hamstrung by their use of a carrier aircraft airframe
operated by vanishingly few (L-1011) as well as the Pegasus using expensive
solid rocket motors (here's another non-intuitive reality of rockets - solid
motors are very expensive unless you buy in bulk).

^The most compelling test of economic viability of air-launch for small
systems will come with Virgin Orbit/VOX space that is using a carrier aircraft
airframe which is still in significant commercial use (quite a few 747 air
freighters) as well as modern-yet-relatively low complexity LOX/RP liquid
rocket propulsion. And if they want to say double their payload capability,
they will not be able to do that with the 747. Maybe at the extremely low-end,
GOLauncher will also be similar.

~~~
nbadg
> Do you have any analysis you can point to to substantiate this?

Unfortunately I don't, no. I thought about running such an analysis as part of
my grad studies (this was quite a while ago; I've switched careers since then)
but I didn't get much farther than filling up a napkin before I decided I was
already stretched too thin in my coursework.

Ultimately the premise is based on cubed-vs-squared relationships in rocketry
(both in terms of aerodynamic forces and structural ones, though through a
neat trick of math the _mass_ of your fuel tanks actually scales linearly with
their volume [1]). Like most other things in engineering, there are also
economies of scale at play (for example, avionics mass consumes a smaller mass
fraction of larger rockets), but my hunch -- and this is, as you say, fairly
unsubstantiated -- is that the aerodynamic effects alone are sufficient.
Cubed-vs-squared in aerodynamics is really just incredibly punishing. At 35kft
(Stratolaunch's altitude) it's not as good as at 100kft or so (like you might
expect with a weather balloon), but it's still a pretty big difference [2] --
basically allowing you to halve the radius of your rocket compared to an
equivalent aero loss at sea level.

To take this to a really absurd level, I can imagine a 100kg rocket -- large
by amateur model rocket standards, but beyond tiny compared to consumer
rocketry -- with sufficient mass ratio to make it to orbit in a vacuum, but I
can't possibly imagine the same rocket making it from the earth's surface.

[1]
[https://en.wikipedia.org/wiki/Pressure_vessel#Design](https://en.wikipedia.org/wiki/Pressure_vessel#Design)

[2] [https://www.engineeringtoolbox.com/standard-atmosphere-
d_604...](https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html)

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sfifs
I'm puzzled by what added benefit being 10km up in the air and going at
300-400kmph gives over regular rocket launches since speed and altitude are a
very small fraction of what's required to orbit.

The article says it will be less affected by weather windows but (1) would a
normal LEO commercial launch be so highly time bound to find this of value?
(2) if weather was bad for a rocket launch on surface, would you fly a heavily
laden giant plane into it?

Leads me to wonder if the real customers of this is the military who do value
very short turnaround launches.

~~~
jcims
Pegasus launches occur at closer to 900km/h.

It takes a Falcon 9 about a minute and roughly 150,000kg of propellant to get
to 10km altitude, at which point it's going to be travelling closer to
1200km/h. If you can move that part of the mission to a carrier, you can
substantially reduce the size and fuel capacity of the stage or maybe even
entirely skip a staging operation (edit: not skip, carrier becomes first
stage, thanks DenisM!).

Back to the Falcon 9 at mid-first stage, from 10km to stage separation you're
running on motors that had to be built to operate at sea level. This means
that they are 'underexpanded' for operating at 10km altitude and are therefore
not as efficient as they could be. If you know your engines will always start
off at 10km, you can build bigger expansion nozzles and improve the overall
efficiency.

~~~
DenisM
You’re not skipping staging, your carrier is your (reuseable) first stage.

~~~
SubiculumCode
And, I presume, much much more efficient

~~~
umeshunni
Why is it much more efficient? Doesn't it still have to carry the same mass up
to the same altitude and velocity?

~~~
newnewpdro
If you accelerate your car 0-60MPH in 3 seconds vs. 30 seconds, which uses
less energy?

~~~
bacon_waffle
Assuming you're talking about energy transferred through the drive wheels, I'd
think the 3 second case would use less energy since less goes in to moving
air.

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dweekly
Scott Manley walks through why this isn't helpful yet since they have nothing
useful to launch in delightful detail (complete with Kerbal simulations):
[https://youtu.be/yw84qJIGZeo](https://youtu.be/yw84qJIGZeo)

~~~
jessriedel
That's not a good summary. Yes, the plane is not very valuable in the short
term (~4 years) because the only viable existing air-launched rocket is the
pegasus, which is old and can be more cheaply launched on the smaller planes
for which it was designed. But as Manley emphasizes, the whole stratolaunch
plan is predicated on developing a new air-launched rocket, and we just don't
know what the specs on that will be.

~~~
ThenAsNow
> the whole stratolaunch plan is predicated on developing a new air-launched
> rocket, and we just don't know what the specs on that will be

Neither will Stratolaunch, any time soon:

[https://spacenews.com/stratolaunch-abandons-launch-
vehicle-p...](https://spacenews.com/stratolaunch-abandons-launch-vehicle-
program/)

~~~
floil
Can confirm. The whole group working on that effort has been laid off.

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eismcc
How did they prevent the two fuselages from torquing the wing apart? Seems
like keeping both “sub-planes” from ripping the middle wing apart would be
challenging.

~~~
hliyan
This was my first thought as well. But then I remembered that modern twin-
engine commercial jets can fly with one engine down in an emergency. Not an
aircraft engineer, but I suspect the structure is strong enough to handle
normal torques during turning.

~~~
base698
The complications with flying a twin engine during a single engine failure
have more to do with flight characteristics than structural.

Ie, does the rudder have enough authority to stop a spin or does the engine
provide enough thrust to maintain level flight.

~~~
eismcc
The stratolaunch looks to have 2 rudders making things even more exciting.

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gaze
Isn't it amazing how you can tell a Burt Rutan design just by looking at it?

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yonkshi
This is such a cool concept, I really wish these guys would succeed. Though
last time I heard they gave up developing their own rocket (as a second stage
to space). Anyone who's in the loop, does Stratolaunch currently have any
potential rockets lined up?

~~~
exochrono
Article states that they're going to use Northrop Grumman's Pegasus XL.

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adventured
I know there's a lot of skepticism about whether the plane has any likely
long-term use and whether it can be successful commercially at all. I find it
spellbinding to watch it fly. I'm so glad Paul Allen funded its creation.

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dredmorbius
I get the distinct impression Stratolaunch wasn't happy with the post-landing
roll:

[https://youtube.com/watch?v=ZBj3FCdYS80](https://youtube.com/watch?v=ZBj3FCdYS80)

(At 3m20s.)

~~~
jdsully
I think its just fighting a bit of crosswind.

~~~
dredmorbius
The edit cut away immediately, to onlooker shots. It's an interesting flight
phase, and pretty clearly one Stratolaunch did not care to feature.

Several other onlookers did catch the roll, and the aircraft slewed markedly,
with several cameramen commenting on this.

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ourmandave
If they're launching Pegasus XLs I wonder if they're on Northup Grumman's
future acquisitions radar.

Their competition, Virgin Orbit has it's own subsidiary Vox Space that makes
the launch vehicle.

~~~
tyingq
It looks like intended rocket has changed a few times:

[https://en.wikipedia.org/wiki/Stratolaunch_Systems#Launch_ro...](https://en.wikipedia.org/wiki/Stratolaunch_Systems#Launch_rocket)

But did eventually settle to the Pegasus XL:
[https://spacenews.com/stratolaunch-abandons-launch-
vehicle-p...](https://spacenews.com/stratolaunch-abandons-launch-vehicle-
program/)

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netman21
Is anyone looking at catapults yet? Seems that would be a great technology to
pour billions into. Maybe that is why Musk is proposing Hyperloop? Get the
engineering done for high speed trains, then scale it up. Way up.

The Startram project estimated $60 billion for a catapult that could launch
people. [http://www.startram.com/](http://www.startram.com/)

~~~
DenisM
How much acceleration can you endure? 3g is typical for a rocket launch, 9g is
almost deadly. Accelerating to 6000mps at 3g will require 6000^2/(2 _3_ g) =
600 kilometers of track, and it will take 200 seconds. That's a huge track,
which has to be exactly linear or the payload will destroy it.

Then there is also the fact that the payload will be exiting the catapult at
the sea-level, thus facing the full atmosphere at 6000mps. Which is to say the
payload will be instantly incinerated unless some kind of magic is invented
akin to supercavitation. By contrast rockets do not even reach supersonic
300mps until 10k altitude, thus never encounter this sort of problem. Maybe
you could build this 600km track thing in the Himalaya, but that much and that
high in the mountains isn't going to be cheap.

You can tell I though a lot about the same thing that you did. :)

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m3at
As cool as it looks, this airplane seems promised to go the way of the
Hercules H4: fly once then stay in a museum forever. This recent podcast from
mainenginecutoff speaks about Stratolaunch [1]

[1]
[https://mainenginecutoff.com/podcast/108](https://mainenginecutoff.com/podcast/108)

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theseadroid
I wonder why this type of specific purpose airplane needs human in the
cockpit? Why can't it be remote controlled?

~~~
craftyguy
I suspect this huge, complex aircraft with it's very expensive, highly
explosive cargo isn't as straight forward as flying a drone.

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tyingq
I'm curious what the emergency landing options are for this thing. It looks
like there would be limited places it could land with that wingspan and
landing gear spread.

Edit: Curious, not critical. Maybe it can land in a lot of places, just
reacting to the size.

~~~
degobah
Since its purpose is to launch rockets, I assume it lands where it takes off,
and might never stray far away.

~~~
tyingq
That's the sort of thing I'm interested in. Like, for example...how far away
are you once you get this huge thing high enough to launch.

I'm not trying to criticize it, I'm just curious how dangerous a typical
mission will be.

~~~
cmurf
It doesn't have to fly in one direction, it can do s-turns while climbing, or
circle.

I think clearly payload launch is the risky part. What are the kinds of launch
failures? Rocket engine failure can be uncontained and spectacular.

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bacon_waffle
I hope they take inspiration from these folks and do a simultaneous takeoff
with a smaller plane, in opposite directions:

[https://youtu.be/R5P70XtI4zQ?t=39](https://youtu.be/R5P70XtI4zQ?t=39)

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lstodd
I wonder how fast it would disintegrate in a case of asymmetric stabilizer
runoff.

~~~
orbital-decay
I guess this thing has a pretty sophisticated control system to keep it
stable, similar to modern combat aircraft. I also wonder if they have any fuel
dumping procedures for an aborted launch; landing with a non-empty launch
vehicle still on board seems quite scary.

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blunte
I hope that center wrong is unbelievably strong to handle the tortional forces
that two separate horizontal stabilizers on lengthy lever arms might exhibit
in acute turbulence.

~~~
godson_drafty
That flimsy tail design bothered me as well. Even if the center wing and wing
roots are incredibly strong, there's little reason to place them under
stresses that could be alleviated with a single-stabilizer tail design, as
other commenters have pointed out. The fact that they did not do this suggests
this aircraft is designed to launch millions in contributions into the
designers' pockets, and not so much rockets into space.

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klhugo
Does anyone have a cost comparison between different launch systems?

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rdiddly
Kind of a threadbare article. Not a word on why dual fuselages are desirable?
Why it needs to have the world's longest wingspan? Like I'm supposed to just
know all that from my aeronautics background? What if I flunked that course?

~~~
asteli
You need a large space open to attach the rocket to, and it has to be at the
center of lift. You can't have one fuselage and put the rocket to the side,
because then the balance would change drastically when you add the rocket, and
you can't put the rocket under/in the fuselage because it's very large.

~~~
rdiddly
Thanks... it's the simple things!

