4 ms·

What is the flight profile and purpose of the vehicle? If it's hypersonic atmospheric cruise, then it would be for weapons development as there's no civilian u

What is the flight profile and purpose of the vehicle? If it's hypersonic atmospheric cruise, then it would be for weapons development as there's no civilian use for that technology. Satellites are launched first up and then horizontally outside the atmosphere.

lmm 4y ago
Most rocket launches start accelerating horizontally either as soon as they've cleared the thickest part of the atmosphere, or right from the start. If you can energy you can impart to a satellite with an air-breathing stage, you can save an exponential amount of fuel on the final rocket stage.
- trhway 4y ago
  So far nothing could beat just increasing the size of the first stage. Simplicity and weight as any air breathing hardware brings in weight and complexity. Even the simplest and my favorite approach - air augmentation - could not be made so far (beside a small USSR surface-to-air missile). Also with the size of the rocket the proportional air breathing machinery power scales between linear and square while rocket mass cubically, so a useful air breathing on say a Starship is hard to imagine.
- xcskier56 4y ago
  This is a common theory, but if you look at the math of the rocket equation, a very large percentage is for pure horizontal velocity, with a small percentage of gravity losses for getting from the surface to the orbital altitude, an even smaller percentage are losses due to aerodynamic drag. I haven't done the math, but in general, the benefit of something like stratolaunch is much less than you'd expect Here's a great (but complicated) reddit post on this topic: https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_do_i_calculate_delta_v_losses_due_to_drag/dg6peqy/?utm_source=reddit&utm_medium=web2x&context=3 https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_d...
  - dan_hawkins 4y ago
    "But getting to space is easy. The problem is staying there." - that's been an real eye opening read for me from fantastic "What If": https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
  - lmm 4y ago
    The point isn't reduced drag. The point is to be able to do more horizontal acceleration with less propellant, because you're getting your oxidizer for free.
    - jhgb 4y ago
      Getting something "for free" is hardly an issue since liquid oxygen is quite cheap as a percentage of launch costs (for example in case of Falcon 9, it's less than $100k out of ~$28M of launch costs for a reused Falcon 9). Now you might argue that the resulting vehicle that needs to be carrying needs to be bigger, but then again, in practice we found it less complicated than hypersonic air-breathing propulsion, so whether the vehicle capable of usefully consuming it would actually be cheaper is pure speculation. So far nobody has been able to build even just one, much less mass-produce it.
- mandevil 4y ago
  The reason that there are plenty of rocket first stages and basically no air-breathing first stages that go to orbit is that, essentially, this isn't true. OSC Pegasus and Virgin LauncherOne are the only exceptions, and both are only good for very tiny payloads and accordingly have very limited flight numbers- compare the less than 50 total across both launchers to the 150 for just the already retired Delta II rocket (which entered service at basically the same time as Pegasus, is why I chose it). The only real use case that I can see for air launch is that they can be fully launch azimuth independent for launches from within US territory, so there can be some sort of responsive launch capability (read: military wants to launch something immediately). Other than that one use case, it's hard to see what air-breathing engines bring to the table for space-flight. Payload size is clearly a massive problem because airplanes are so volume limited. But there is another issue here: heating. In order for an air-breathing engine to provide useful thrust, you have to have enough air for it to breathe. That pretty much requires you to now have to deal with aerodynamic heating, which is a thing that most rockets try very very hard to ignore on launch. The most prominent air-breathing to orbit attempt going right now, Skylon, seems to be viewed generally pretty skeptically by most people in the space community, from what I can tell. And even then, the log-line is different: instead of separate jet engines and rocket engines, the idea is to have a rocket engine that can make a mediocre jet engine for some portion of the flight. Most people outside RE don't think this idea will work very well (I recall people who ran the numbers pointing out that HOTOL- the program that Skylon grew out of- would actually have significantly more payload if it had just rocket engines) but Bond and co. continue on, with very small budgets, the sort of budgets suitable for long-shot technology development. Thus, it appears that the people who control the purse strings generally agree that this has a low chance of working out, even if it is worth trying.
jccooper 4y ago
It's a hypersonic testbed. Which basically means DoD, though I suppose if you came along with a civilian usecase they wouldn't hang up the phone.
dotnet00 4y ago
The primary motivation is probably weapons, but experience with hypersonic flight should also help with reusable rocketry, which I think may be relevant since they mention that the next iteration of the vehicle is intended to be fully reusable. If only intended for weapons, making the vehicle fully reusable doesn't seem too relevant.
- mandevil 4y ago
  I don't think that necessarily tracks: having a test-bed be reusable can mean you get more flights, can carry more payloads, and study many different things. Which for exploring something as complex as hypersonic flight, has obvious attractions. The research vehicle that has done the most study of hypersonic flight to-date is almost certainly the North American X-15, which did 199 flights over 9 years, and it was fully reusable and not related to a weapon in any way. This presumes, of course, that the cost of turn-around for reflight is <<< the cost of building a new one. This is obviously true for normal airplanes but was probably only barely true for the Space Shuttle. So there is a continuum here, a point where the between-flight inspection and cleaning costs do loom large enough to not be worth it.
belter 4y ago
"Air Force secretary hints at military space applications for Stratolaunch super-plane" (2017) https://www.geekwire.com/2017/air-force-secretary-highlights-military-space-applications-stratolaunch-super-airplane/ https://www.geekwire.com/2017/air-force-secretary-highlights...
credit_guy 4y ago
I'll take a stab. The lift-to-drag ratio decreases with speed, but the limit is not zero, but rather 4. https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic More precisely the L/D ratio as a function of the Mach number M is (4M+3)/M. Of course, that's the ideal maximum L/D ratio. But let's go with it. So, let's see what happens if you travel at Mach 2, Mach 5 (lower limit of the hypersonic regime) and Mach 10 (which is around the reported speed of the Russian Zircon missile, which they tested a few days ago, on 28May2022). The L/D ratios would be 5.5, 4.6 and 4.3. Let's say we envision a flight path of 1000 km (same as the said Zircon test). At Mach 10 that would take about 300 seconds = 5 min, and at the slower speeds of Mach 5 and Mach 2 it would take 10 and 25 minutes. Of course, you'd like the missile to get to the target sooner rather than later, if you could help it. The paradoxical thing is that the faster the missile is, the less fuel it needs to burn. You read that right. And this has nothing to do with air-breathing scarmjet engines. It's just old fashion rocket engines. Here's how this works, in our example. At Mach 2, our drag is 1/5.5 = 18.2% of the lift, so your engines need to push with 0.18g to keep you at constant speed. They need to do that for 1500 seconds, so that translates into a delta-v of 0.18 x 10 x 1500 = 2.7 km/s. A typical kerosene-oxygen rocket engine has an exhaust velocity around the same, so the rocket equation tells us the ratio of dry mass over total mass (dry mass + fuel) is about exp(-1)= 37%. So, 63% of your missile has to be fuel. At Mach 5 the drag is 1/4.6 = 0.22g. Delta-v becomes 0.22 x 10 x 600 = 1.3 km/s, and the ratio of dry mass to total mass completely reverses: now only 62% of the vehicle mass is dry, and 38% is fuel. At Mach 10, delta-v is 0.23 x 10 x 300 = 0.7 km/s and the fuel mass is only 23%. For comparison, the phenomenal Tomahawk cruise missile has a fuel weight of about 450 kg and a total mass of 1300 kg, so a fuel ratio of about 35%. A Mach 10 hypersonic missile could have a comparable fuel ratio as a Tomahawk subsonic cruise missile. Of course, the Mach 10 missile needs a booster to bring it at altitude and cruising speed. But that booster can be a fairly dumb (and inexpensive) rocket. And in the era of SpaceX, who knows, that booster might even be reusable.

lmm 4y ago

Most rocket launches start accelerating horizontally either as soon as they've cleared the thickest part of the atmosphere, or right from the start. If you can energy you can impart to a satellite with an air-breathing stage, you can save an exponential amount of fuel on the final rocket stage.

trhway 4y ago
So far nothing could beat just increasing the size of the first stage. Simplicity and weight as any air breathing hardware brings in weight and complexity. Even the simplest and my favorite approach - air augmentation - could not be made so far (beside a small USSR surface-to-air missile). Also with the size of the rocket the proportional air breathing machinery power scales between linear and square while rocket mass cubically, so a useful air breathing on say a Starship is hard to imagine.
xcskier56 4y ago
This is a common theory, but if you look at the math of the rocket equation, a very large percentage is for pure horizontal velocity, with a small percentage of gravity losses for getting from the surface to the orbital altitude, an even smaller percentage are losses due to aerodynamic drag. I haven't done the math, but in general, the benefit of something like stratolaunch is much less than you'd expect Here's a great (but complicated) reddit post on this topic: https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_do_i_calculate_delta_v_losses_due_to_drag/dg6peqy/?utm_source=reddit&utm_medium=web2x&context=3 https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_d...
- dan_hawkins 4y ago
  "But getting to space is easy. The problem is staying there." - that's been an real eye opening read for me from fantastic "What If": https://what-if.xkcd.com/58/ https://what-if.xkcd.com/58/
- lmm 4y ago
  The point isn't reduced drag. The point is to be able to do more horizontal acceleration with less propellant, because you're getting your oxidizer for free.
  - jhgb 4y ago
    Getting something "for free" is hardly an issue since liquid oxygen is quite cheap as a percentage of launch costs (for example in case of Falcon 9, it's less than $100k out of ~$28M of launch costs for a reused Falcon 9). Now you might argue that the resulting vehicle that needs to be carrying needs to be bigger, but then again, in practice we found it less complicated than hypersonic air-breathing propulsion, so whether the vehicle capable of usefully consuming it would actually be cheaper is pure speculation. So far nobody has been able to build even just one, much less mass-produce it.
mandevil 4y ago
The reason that there are plenty of rocket first stages and basically no air-breathing first stages that go to orbit is that, essentially, this isn't true. OSC Pegasus and Virgin LauncherOne are the only exceptions, and both are only good for very tiny payloads and accordingly have very limited flight numbers- compare the less than 50 total across both launchers to the 150 for just the already retired Delta II rocket (which entered service at basically the same time as Pegasus, is why I chose it). The only real use case that I can see for air launch is that they can be fully launch azimuth independent for launches from within US territory, so there can be some sort of responsive launch capability (read: military wants to launch something immediately). Other than that one use case, it's hard to see what air-breathing engines bring to the table for space-flight. Payload size is clearly a massive problem because airplanes are so volume limited. But there is another issue here: heating. In order for an air-breathing engine to provide useful thrust, you have to have enough air for it to breathe. That pretty much requires you to now have to deal with aerodynamic heating, which is a thing that most rockets try very very hard to ignore on launch. The most prominent air-breathing to orbit attempt going right now, Skylon, seems to be viewed generally pretty skeptically by most people in the space community, from what I can tell. And even then, the log-line is different: instead of separate jet engines and rocket engines, the idea is to have a rocket engine that can make a mediocre jet engine for some portion of the flight. Most people outside RE don't think this idea will work very well (I recall people who ran the numbers pointing out that HOTOL- the program that Skylon grew out of- would actually have significantly more payload if it had just rocket engines) but Bond and co. continue on, with very small budgets, the sort of budgets suitable for long-shot technology development. Thus, it appears that the people who control the purse strings generally agree that this has a low chance of working out, even if it is worth trying.

jccooper 4y ago

It's a hypersonic testbed. Which basically means DoD, though I suppose if you came along with a civilian usecase they wouldn't hang up the phone.

dotnet00 4y ago

The primary motivation is probably weapons, but experience with hypersonic flight should also help with reusable rocketry, which I think may be relevant since they mention that the next iteration of the vehicle is intended to be fully reusable. If only intended for weapons, making the vehicle fully reusable doesn't seem too relevant.

mandevil 4y ago
I don't think that necessarily tracks: having a test-bed be reusable can mean you get more flights, can carry more payloads, and study many different things. Which for exploring something as complex as hypersonic flight, has obvious attractions. The research vehicle that has done the most study of hypersonic flight to-date is almost certainly the North American X-15, which did 199 flights over 9 years, and it was fully reusable and not related to a weapon in any way. This presumes, of course, that the cost of turn-around for reflight is <<< the cost of building a new one. This is obviously true for normal airplanes but was probably only barely true for the Space Shuttle. So there is a continuum here, a point where the between-flight inspection and cleaning costs do loom large enough to not be worth it.

belter 4y ago

"Air Force secretary hints at military space applications for Stratolaunch super-plane" (2017) https://www.geekwire.com/2017/air-force-secretary-highlights-military-space-applications-stratolaunch-super-airplane/ https://www.geekwire.com/2017/air-force-secretary-highlights...

credit_guy 4y ago

I'll take a stab. The lift-to-drag ratio decreases with speed, but the limit is not zero, but rather 4. https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Supersonic More precisely the L/D ratio as a function of the Mach number M is (4M+3)/M. Of course, that's the ideal maximum L/D ratio. But let's go with it. So, let's see what happens if you travel at Mach 2, Mach 5 (lower limit of the hypersonic regime) and Mach 10 (which is around the reported speed of the Russian Zircon missile, which they tested a few days ago, on 28May2022). The L/D ratios would be 5.5, 4.6 and 4.3. Let's say we envision a flight path of 1000 km (same as the said Zircon test). At Mach 10 that would take about 300 seconds = 5 min, and at the slower speeds of Mach 5 and Mach 2 it would take 10 and 25 minutes. Of course, you'd like the missile to get to the target sooner rather than later, if you could help it. The paradoxical thing is that the faster the missile is, the less fuel it needs to burn. You read that right. And this has nothing to do with air-breathing scarmjet engines. It's just old fashion rocket engines. Here's how this works, in our example. At Mach 2, our drag is 1/5.5 = 18.2% of the lift, so your engines need to push with 0.18g to keep you at constant speed. They need to do that for 1500 seconds, so that translates into a delta-v of 0.18 x 10 x 1500 = 2.7 km/s. A typical kerosene-oxygen rocket engine has an exhaust velocity around the same, so the rocket equation tells us the ratio of dry mass over total mass (dry mass + fuel) is about exp(-1)= 37%. So, 63% of your missile has to be fuel. At Mach 5 the drag is 1/4.6 = 0.22g. Delta-v becomes 0.22 x 10 x 600 = 1.3 km/s, and the ratio of dry mass to total mass completely reverses: now only 62% of the vehicle mass is dry, and 38% is fuel. At Mach 10, delta-v is 0.23 x 10 x 300 = 0.7 km/s and the fuel mass is only 23%. For comparison, the phenomenal Tomahawk cruise missile has a fuel weight of about 450 kg and a total mass of 1300 kg, so a fuel ratio of about 35%. A Mach 10 hypersonic missile could have a comparable fuel ratio as a Tomahawk subsonic cruise missile. Of course, the Mach 10 missile needs a booster to bring it at altitude and cruising speed. But that booster can be a fairly dumb (and inexpensive) rocket. And in the era of SpaceX, who knows, that booster might even be reusable.