3 ms·

> 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

> 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 7y 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...

nbadg 7y 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...