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Comment by Analemma_ | Hacker News Reader

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?

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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

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...

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 thing, and possibly more cheaply in certain scenarios (well, that has to be Allen's plan anyway).

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.

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.

https://en.wikipedia.org/wiki/Stargazer_(aircraft)

They have been doing it all along, just in reverse. Those heat shields glowing red hot and ablating away is pretty dramatic. That is pushing very thin air at (sub?) orbital delta-V

Orbital ATK has been doing air launch to orbit[0] since 1990 with their Pegasus rocket. They've had three dozen successful launches since then[1].

[0] https://en.wikipedia.org/wiki/Air_launch_to_orbit

[1] https://en.wikipedia.org/wiki/Pegasus_(rocket)#Launch_histor...

Virgin Galactic's White Knight is a very similar aircraft. It was used to launch SpaceshipOne which one the x prize. The article doesn't mention it but this new plane is the logical extension of the White Knight.

https://en.m.wikipedia.org/wiki/Scaled_Composites_White_Knig...

There's a reason they are very similar:

http://www.scaled.com/projects/stratolaunch

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.

Cool, I was totally unaware of this "prior art". Thanks!

Here's a remarkable official USAF film from 1974 about the air-launched Minuteman. Really amazing. https://www.youtube.com/watch?v=8b8LLcdBaQc

There was also the cancelled Skybolt ALBM from the early 1960s:

https://en.wikipedia.org/wiki/GAM-87_Skybolt

Checkout the photo gallery, it seems to launch a rocket from between the two hulls while flying very high in the atmosphere (less drag and gravity).

Not really that much less gravity, certainly not enough to notice, let alone have a difference to a rocket launch. Gravity varies as 1/R^2 where R in this case is distance from the earth, so it's the difference between that at ~6380 km and ~6400 km (for a 20 km altitude) which is not much - you have to be well into space, at over 1000 km up, before gravity drops to less than 7 km/s^2 for instance.

For anyone unclear on why launching a rocket from a plane isn't as great of an idea as it sounds like it would be, this is an excellent explanation: https://what-if.xkcd.com/58/

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