I remember reading an interview with Elon Musk where he said that SpaceX had looked into air launching. I really wish I remember where that was that I saw it! However, If I remember correctly he stated that early on SpaceX had studied it and found in terms of cost that air launch was more expensive for what they wanted to do. That's cost though not performance.
I recall the statement, too. He was focused – wrongly – on pure performance enhancement. While there are undeniable performance improvements, the real value of air-launching comes from operational flexibility and regulatory advantages. Over-ocean launches at sufficient remove from the coastline will be treated like SeaLaunch (no destruct required, only engine shutdown). And one can hit a first orbit rendezvous on any day, from anywhere.
He wants to go to Mars
I have a question about scaling effects...I know that you can't scale a rocket or rocket engine without effect (that is, a rocket twice as big in every respect won't behave the same as a smaller rocket). That's partly because the Earth's gravity doesn't scale with the rocket, and partly because of fluid effects (viscosity changes with scale).But I was wondering what the smallest possible orbital rocket might be. Is it conceivable to build a small rocket with staging, cryogenic engines, etc. that could take a small payload to orbit? What are the limits on this? Large hobbyist rockets are obviously limited by solid-fuel ISP's, but is there any way you could build a small rocket that could reach orbit?
If a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.
Quote from: Danderman on 04/23/2010 04:46 pmIf a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.I get a 22% reduction in first stage mass, while keeping the 1010kg payload of the Falcon 1e. Had to make some assumptions/tweaks, but I got a reasonable set of assumptions (for pmf, and total dV) that gives a gross liftoff weight of 35,200 kg, which is pretty close to the published 35,180. Then, I held second stage values constant, dropped the dV by 290 m/s, and ramped the Isp on the Merlin to 310 s (from an averaged value of 285). This reoptimization gave a new gross liftoff weight of 29,300 kg. This is only a step above BOTE, but probably not too bad an estimate.
Quote from: notsorandom on 04/25/2010 08:13 pmI remember reading an interview with Elon Musk where he said that SpaceX had looked into air launching. I really wish I remember where that was that I saw it! However, If I remember correctly he stated that early on SpaceX had studied it and found in terms of cost that air launch was more expensive for what they wanted to do. That's cost though not performance.I recall the statement, too. He was focused – wrongly – on pure performance enhancement. While there are undeniable performance improvements, the real value of air-launching comes from operational flexibility and regulatory advantages. Over-ocean launches at sufficient remove from the coastline will be treated like SeaLaunch (no destruct required, only engine shutdown). And one can hit a first orbit rendezvous on any day, from anywhere.
Quote from: strangequark on 04/26/2010 06:15 pmQuote from: Danderman on 04/23/2010 04:46 pmIf a Falcon 1 were carried aloft by a Lockheed 1011 (or comparable aircraft), and air dropped at .85 Mach and 40,000 feet, how much smaller could the first stage be to take advantage of the altitude gain and velocity increase from air launch, and still meet the nominal Falcon 1 payload capability?I am thinking that the first stage could be reduced by close to 50% and still do the job. If so, the claims in my first paragraph may be true, but misleading.I get a 22% reduction in first stage mass, while keeping the 1010kg payload of the Falcon 1e. Had to make some assumptions/tweaks, but I got a reasonable set of assumptions (for pmf, and total dV) that gives a gross liftoff weight of 35,200 kg, which is pretty close to the published 35,180. Then, I held second stage values constant, dropped the dV by 290 m/s, and ramped the Isp on the Merlin to 310 s (from an averaged value of 285). This reoptimization gave a new gross liftoff weight of 29,300 kg. This is only a step above BOTE, but probably not too bad an estimate.Is dropping the dV by 290 m/s purely from the velocity gained by the aircraft, or does that take into account the additional altitude from the airlaunch, as well? Or for that matter, the gravity losses that were NOT incurred on a rocket propelled climb to 40,000 feet?
Also, what kind of Isp would you get for B2H6/LOX? Use just enough LOX to burn the B (about 1.73 to 1 O/F). Assume 20MPa chamber pressure and altitude compensating nozzle. Engine start @ 40,000 ft.
A lot depends on the flight path angle you release the rocket into. Done correctly, I've seen analyses that indicate that you can avoid as much as 1000m/s of delta-V between altitude, velocity at staging, and the drag and gravity losses you've avoided. Admittedly that was for a maneuver where you light the rocket engine before dropping the stage (and then using lift from the carrier plane to do a zoom climb of sorts), which is...kind of sporty. But if you can make that work, that's a huge performance benefit, on top of all the other benefits of air launch.~Jon
If only LiF was not toxic. Imagine 542 sec Isp!IMO Li, Be, and F2 are all far too toxic to be practical (except in deep space, but I suspect that's a job for an NTR), but boron oxides are far less toxic, and the theoretical Isps are incredible. What about adding a small amount of B to LH2 and burning it with LOX?Even if you get no Isp improvement, you DO get a density improvement, and we all know that that must help. In fact, you would need to have a worse Isp than hydrolox to not have a performance boost.
Quote from: tnphysics on 05/01/2010 02:44 pmAlso, what kind of Isp would you get for B2H6/LOX? Use just enough LOX to burn the B (about 1.73 to 1 O/F). Assume 20MPa chamber pressure and altitude compensating nozzle. Engine start @ 40,000 ft.Boronated fuels have, at least from what I've heard, always been a big disappointment compared to theoretical calculations, and a real pain in the neck.~Jon
Why is it so difficult? What are the problems?