3 ms·

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, partial

by nbadg 8y ago

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 8y ago
> 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 8y ago
  > 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.html https://www.engineeringtoolbox.com/standard-atmosphere-d_604...
otabdeveloper1 8y ago
> reusable (or at least, partially reusable) rockets No such thing. A "reusable" rocket means you waste capacity carrying fuel for the return trip, and so raise the costs. It's not like you can refuel the rocket once you get where you're going. This is the reason why nobody uses "reusable" rockets today, not tech. (The tech was available back in the '60s.)

by nbadg 8y ago

ThenAsNow 8y ago

> 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 8y ago
> 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.html https://www.engineeringtoolbox.com/standard-atmosphere-d_604...

otabdeveloper1 8y ago

> reusable (or at least, partially reusable) rockets No such thing. A "reusable" rocket means you waste capacity carrying fuel for the return trip, and so raise the costs. It's not like you can refuel the rocket once you get where you're going. This is the reason why nobody uses "reusable" rockets today, not tech. (The tech was available back in the '60s.)