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Paul Allen's new rocket-launching plane

arstechnica.com

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Re: Paul Allen's new rocket-launching plane

#31

Post by ThrustVectoring »

vlasev wrote: post #20
ThrustVectoring wrote: post #15

Earlier quoted context omitted.

From wikipedia [1]: >Atmospheric and gravity drag associated with launch typically adds 1.3–1.8 km/s to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h). So it'll help. Also significant is the ability to base out of an airport and launch closer to the equator. [1] https://en.wikipedia.org/wiki/Low_Earth_orbit

Combine that with the tyranny of the rocket equation and it becomes sensible.

Right. Reducing the delta-v by a constant amount reduces the fuel requirements by a specific percentage. The correct thing to measure delta-v savings to isn't the launch requirements, but rocket exhaust speed. That is, if your rocket exhaust speed is 1 km/s, saving a km/s of delta-v will reduce your fuel requirements by a factor of 2.718

Re: Paul Allen's new rocket-launching plane

#32

Post by mikeash »

Analemma_ wrote: post #9
tejtm wrote: post #7

Earlier quoted context omitted.

The less air you are pushing the cheaper it is to achieve that delta-V

Is it really that dramatic a difference? I suppose it must be, if they're building it, but it's strange that no one has done this before if it really helps.

It is a pretty big difference. In vacuum you want a huge nozzle to extract as much energy as possible from the engine's exhaust. In atmosphere, there's a limit to how large the nozzle can be before the exhaust flow separates from the sides and you get asymmetric thrust or worse. Since the nozzle can only be optimized for one particular altitude, that means that rockets which launch from sea level spend a good portion of their ascent with a smaller nozzle than they'd like to have. By launching from high altitude they can be significantly more efficient.

As others noted, this has been done before, but it's not too common. I think that's mainly because rockets need to be really, really big to get a decent-sized payload to orbit. Launching from an aircraft would let them be a bit smaller, but they still need to be huge. Pegasus (the one currently in use) is a small rocket and can only orbit a small payload. This plane is designed to carry about 500,000lbs of rocket. For comparison, a fully loaded Falcon 9 is about 1.2 million pounds, and that's only a medium-sized rocket when it comes to orbital launchers.

Amusing aside: in the 1970s, the US Air Force tested the feasibility of air-launching ICBMs. The test was carried out by loading a Minuteman ICBM into a C-5 Galaxy transport, flying it over the ocean, and then essentially shoving the missile out the back door and lighting it up. In this case, the advantage wasn't efficiency or payload, but rather the ability to move ICBMs around to make it more difficult to destroy them in a first strike. They decided it wasn't worth all the trouble in the end.

Re: Paul Allen's new rocket-launching plane

#33

Post by guscost »

mLuby wrote: post #27

IANARS but this plane should give the remaining stage(s) four advantages: 1. +230m/s orbital velocity [1] 2. +9km launch altitude [1] 3. can launch in more weather conditions 4. 1st stage can abort mid-launch without loss of payload. Of these, the most important is the increase in launch altitude, because that means stage 2 launches in air of greatly decreased density, leading to the following efficiencies: A. a vacu…

Neat - I kind of intuitively knew about B. from playing KSP, but the other two are interesting.

Re: Paul Allen's new rocket-launching plane

#34

Post by mLuby »

ThrustVectoring wrote: post #15
Analemma_ wrote: post #3

I'm confused as to what this thing can accomplish. It can probably fly very high, but I assume it can't fly very fast (relative to orbital speed), and I thought the overwhelming majority of the delta-V to get something into orbit was for forward momentum and that just launching from higher up wouldn't help much. Clearly there's something I'm missing here (or they wouldn't have built it), can anyone fill me in?

From wikipedia [1]: >Atmospheric and gravity drag associated with launch typically adds 1.3–1.8 km/s to the launch vehicle delta-v required to reach normal LEO orbital velocity of around 7.8 km/s (28,080 km/h). So it'll help. Also significant is the ability to base out of an airport and launch closer to the equator. [1] https://en.wikipedia.org/wiki/Low_Earth_orbit

Looks like mostly gravity losses [1], which I understand to be essentially "how long does it take to reach orbit".

[1] https://forum.nasaspaceflight.com/index.php?topic=9959.msg18...

Re: Paul Allen's new rocket-launching plane

#36

Post by mikeash »

fdej wrote: post #24
Analemma_ wrote: post #3

I'm confused as to what this thing can accomplish. It can probably fly very high, but I assume it can't fly very fast (relative to orbital speed), and I thought the overwhelming majority of the delta-V to get something into orbit was for forward momentum and that just launching from higher up wouldn't help much. Clearly there's something I'm missing here (or they wouldn't have built it), can anyone fill me in?

Look at this analysis of the Falcon 9 flight profile, in particular the graph at the bottom right: https://www.reddit.com/r/spacex/comments/6303ko/falcon_9_ful... The majority of the horizontal delta-V is produced in the second stage. The job of the first stage is to a large extent just to lift the second stage out of the densest part of the atmosphere, to minimize drag for the second stage. A plane can do the same t…

Keep in mind that it's not just a matter of getting the second stage up high, but giving it enough vertical velocity so that it can reach orbital speed before it falls back into the atmosphere. If you could magically drop the second stage off at its separation altitude but with zero vertical speed, it wouldn't be able to reach orbit. An airplane won't provide much vertical speed, if any, so it's not really doing the same job. Still, just letting the rocket start up high is a big advantage.

Re: Paul Allen's new rocket-launching plane

#37

Post by mLuby »

jeffdavis wrote: post #35

Dumb question: why don't they launch rockets from the tops of mountains?

Not a dumb question—Saddam Hussein was trying to do this. https://www.wikiwand.com/en/Project_Babylon

It's prohibitively expensive and logistically challenging to build on a mountain, and mountains on Earth don't go nearly as high as planes can.

Re: Paul Allen's new rocket-launching plane

#39

Post by mikeash »

jeffdavis wrote: post #35

Dumb question: why don't they launch rockets from the tops of mountains?

I think it's mainly that the difficulty of building infrastructure up there, and hauling rockets and payloads up every time you launch, isn't worth the benefits. If you could magick a launch pad and rocket to the top of Mount Everest and launch it from there, you'd get a small but decent performance improvement.

Re: Paul Allen's new rocket-launching plane

#40

Post by adt2bt »

jeffdavis wrote: post #35

Dumb question: why don't they launch rockets from the tops of mountains?

I imagine it's a mix of mountainous terrain making it hard to transport launch vehicles to the top and most mountains aren't next to the sea (so the payload would have to travel over land, raising the stakes of an unplanned early failure).
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