3 ms·

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

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?

modeless 9y ago
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/ https://what-if.xkcd.com/58/
deleted 9y ago
[deleted]
bamboozled 9y ago
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).
- deleted 9y ago
  [deleted]
- grkvlt 9y ago
  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.
tejtm 9y ago
The less air you are pushing the cheaper it is to achieve that delta-V
- Analemma_ 9y ago
  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.
  - jacquesm 9y ago
    https://en.wikipedia.org/wiki/Stargazer_(aircraft) https://en.wikipedia.org/wiki/Stargazer_(aircraft)
  - tejtm 9y ago
    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
  - desdiv 9y ago
    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 https://en.wikipedia.org/wiki/Air_launch_to_orbit [1] https://en.wikipedia.org/wiki/Pegasus_(rocket)#Launch_history https://en.wikipedia.org/wiki/Pegasus_(rocket)#Launch_histor...
  - muricula 9y ago
    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_Knight_Two https://en.m.wikipedia.org/wiki/Scaled_Composites_White_Knig...
    - hinkley 9y ago
      There's a reason they are very similar: http://www.scaled.com/projects/stratolaunch http://www.scaled.com/projects/stratolaunch
  - mikeash 9y ago
ThrustVectoring 9y ago
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 https://en.wikipedia.org/wiki/Low_Earth_orbit
- vlasev 9y ago
  Combine that with the tyranny of the rocket equation and it becomes sensible.
  - ThrustVectoring 9y ago
    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
- mLuby 9y ago
  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.msg189860#msg189860 https://forum.nasaspaceflight.com/index.php?topic=9959.msg18...
fdej 9y ago
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_full_thrust_flight_analysis/ 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).
- mikeash 9y ago
  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.

modeless 9y ago

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/ https://what-if.xkcd.com/58/

deleted 9y ago

[deleted]

bamboozled 9y ago

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).

deleted 9y ago
[deleted]
grkvlt 9y ago
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.

tejtm 9y ago

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

Analemma_ 9y ago
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.
- jacquesm 9y ago
  https://en.wikipedia.org/wiki/Stargazer_(aircraft) https://en.wikipedia.org/wiki/Stargazer_(aircraft)
- tejtm 9y ago
  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
- desdiv 9y ago
  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 https://en.wikipedia.org/wiki/Air_launch_to_orbit [1] https://en.wikipedia.org/wiki/Pegasus_(rocket)#Launch_history https://en.wikipedia.org/wiki/Pegasus_(rocket)#Launch_histor...
- muricula 9y ago
  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_Knight_Two https://en.m.wikipedia.org/wiki/Scaled_Composites_White_Knig...
  - hinkley 9y ago
    There's a reason they are very similar: http://www.scaled.com/projects/stratolaunch http://www.scaled.com/projects/stratolaunch
- mikeash 9y ago

ThrustVectoring 9y ago

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 https://en.wikipedia.org/wiki/Low_Earth_orbit

vlasev 9y ago
Combine that with the tyranny of the rocket equation and it becomes sensible.
- ThrustVectoring 9y ago
  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
mLuby 9y ago
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.msg189860#msg189860 https://forum.nasaspaceflight.com/index.php?topic=9959.msg18...

fdej 9y ago

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_full_thrust_flight_analysis/ 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).

mikeash 9y ago
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.