5 ms·

This seems to be a solid rocket booster with moving parts. Isn't that the worst of both worlds? Lower ISP than liquid fuelled rockets due to the propellant choi

This seems to be a solid rocket booster with moving parts. Isn't that the worst of both worlds? Lower ISP than liquid fuelled rockets due to the propellant choices and higher manufacturing complexity than a solid rocket booster...

1053r 8y ago
The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. We have tech with VERY high ISP (ion drives, for example), but we can't use them until our vehicles hit space because of the extremely low maximum thrust. Meanwhile the high thrust options we have are all very heavy, which means we have to carry more fuel, which means we need a bigger rocket, which means have to carry more fuel, and so on. The sum of this infinite series is finite, but it is still large. If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself.
- chroem- 8y ago
  I don't think you would even necessarily need stages if you don't have any empty fuel tanks to discard. The engines might not be as optimized for operating both in and out of atmosphere, but that seems like a fair tradeoff.
  - Symmetry 8y ago
    Part of the reason for staging is the tanks but part of it is the weight of the rocket engines themselves. The original Atlas had a single tank that started off with two large and one small engines attached to it. When it had been lofted enough the two large engines dropped off and the small engine finished pushing it into orbit.
    - outworlder 8y ago
      Yeah, tanks are not THAT heavy. But you are forced to use multiple engine types due to the bell nozzle. If we used aerospikes, then it would be a different matter.
- Maybestring 8y ago
  >The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). >If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself. ISP is depends only on exhaust velocity. Changing the mass of the rocket cannot effect it.
  - amluto 8y ago
    > The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). Only sort of true. If you naively apply the rocket equation like you're imagining, you would predict a nonzero final velocity for a hypothetical rocket that has enormous ISP but thrust less than its weight. This is true if the rocket is starting out in orbit, but it's totally wrong when you're on the ground. Starting from the ground, you also have to fight gravity, so you care more about (thrust - weight) / mass.
    - IshKebab 8y ago
      Sure, but "the rocket equation" is 100% talking about the classic ideal rocket equation, which I just discovered is also called the Tsiolkovsky rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation If you want to be more specific, e.g. include gravity then you can't say "the rocket equation".
      - robotrout 8y ago
        > If you want to be more specific, e.g. include gravity > then you can't say "the rocket equation". You may not be able to say "the rocket equation" in that context, but NASA does. [1] [1] https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...
        Maybestring 8y ago
- progval 8y ago
  > The sum of this infinite series is finite, but it is still large. Out of curiosity, if g was much greater, could it be infinite? What order of magnitude would g need to be for that to happen?
  - Avshalom 8y ago
    The ability to launch from the surface is proof of it's finiteness. As a corralary it should be finite as long as you're not inside a blackhole. It should be asymptotic up to that point. Mathematically infinite at the horizon and then completely unbounded inside it (in the sense that it converges to infinity versus not converging at all ( see a "flat" universe versus hyperbolic)).
  - a1369209993 8y ago
    Yes; if the acceleration from gravity exceeds the rocket's thrust-to-mass ratio, the rocket cannot make any upward progress against gravity. The best engine listed on the Wikipedia article was the Merlin 1D with 180.1 gravities, so g ≥ 1767 meters per square second would suffice to keep it on the ground, less if you account for the fuel tanks and such.
    - darkmighty 8y ago
      You can always make a mass driver system (which is not limited by the rocket equation), but given how insanely hard it is at our own gravity, it would be more insanely difficult at a few times great g. So there's definitely a quite low maximum gravity allowing practical space access.
vertexFarm 8y ago
It's a niche application for sure. But niche applications count. I know one place it could be handy: getting ISRU products off the moon and into lunar orbit or over to a Lagrangian point. The moon is practically made of alumina-oxide solid fuel propellant. Along with its lack of atmosphere and low gravity, it would be preferable to use the literal dirt that is everywhere on the ground to loft precious refined or manufactured products into orbit instead of more efficient yet much more valuable cryofuels. It also would have a very good shelf life, and a brutally simple machine can have the kind of reliability people would kill for in space. For most missions you'd need something with better ISP, but for some applications it could be just the ticket. Granted it's not currently as simple as conventional solid rocket motors, but that will probably improve as they work out the kinks. The fact that it eliminates structural mass continuously is almost like a CVT for spacecraft; the staging doesn't happen in big, clumsy chunks, it gradually sheds mass all the way down to the payload.
azernik 8y ago
See the paper's description: "Autophage Engines: Towards a Throttleable Solid Motor". i.e. a small amount of extra complexity for a big new feature.

1053r 8y ago

The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. We have tech with VERY high ISP (ion drives, for example), but we can't use them until our vehicles hit space because of the extremely low maximum thrust. Meanwhile the high thrust options we have are all very heavy, which means we have to carry more fuel, which means we need a bigger rocket, which means have to carry more fuel, and so on. The sum of this infinite series is finite, but it is still large. If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself.

chroem- 8y ago
I don't think you would even necessarily need stages if you don't have any empty fuel tanks to discard. The engines might not be as optimized for operating both in and out of atmosphere, but that seems like a fair tradeoff.
- Symmetry 8y ago
  Part of the reason for staging is the tanks but part of it is the weight of the rocket engines themselves. The original Atlas had a single tank that started off with two large and one small engines attached to it. When it had been lofted enough the two large engines dropped off and the small engine finished pushing it into orbit.
  - outworlder 8y ago
    Yeah, tanks are not THAT heavy. But you are forced to use multiple engine types due to the bell nozzle. If we used aerospikes, then it would be a different matter.
Maybestring 8y ago
>The number one thing you worry about with a launch vehicle is not ISP, it's the tyranny of the rocket equation. The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). >If this tech lowers the weight of the first stage, it might actually RAISE the ISP of the rocket overall, even if it lowers the ISP of the engine itself. ISP is depends only on exhaust velocity. Changing the mass of the rocket cannot effect it.
- amluto 8y ago
  > The rocket equation doesn't account for thrust. It's terms are mass and ISP (or exhaust velocity). Only sort of true. If you naively apply the rocket equation like you're imagining, you would predict a nonzero final velocity for a hypothetical rocket that has enormous ISP but thrust less than its weight. This is true if the rocket is starting out in orbit, but it's totally wrong when you're on the ground. Starting from the ground, you also have to fight gravity, so you care more about (thrust - weight) / mass.
  - IshKebab 8y ago
    Sure, but "the rocket equation" is 100% talking about the classic ideal rocket equation, which I just discovered is also called the Tsiolkovsky rocket equation. https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation If you want to be more specific, e.g. include gravity then you can't say "the rocket equation".
    - robotrout 8y ago
      > If you want to be more specific, e.g. include gravity > then you can't say "the rocket equation". You may not be able to say "the rocket equation" in that context, but NASA does. [1] [1] https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html https://www.nasa.gov/mission_pages/station/expeditions/exped...
      - Maybestring 8y ago
progval 8y ago
> The sum of this infinite series is finite, but it is still large. Out of curiosity, if g was much greater, could it be infinite? What order of magnitude would g need to be for that to happen?
- Avshalom 8y ago
  The ability to launch from the surface is proof of it's finiteness. As a corralary it should be finite as long as you're not inside a blackhole. It should be asymptotic up to that point. Mathematically infinite at the horizon and then completely unbounded inside it (in the sense that it converges to infinity versus not converging at all ( see a "flat" universe versus hyperbolic)).
- a1369209993 8y ago
  Yes; if the acceleration from gravity exceeds the rocket's thrust-to-mass ratio, the rocket cannot make any upward progress against gravity. The best engine listed on the Wikipedia article was the Merlin 1D with 180.1 gravities, so g ≥ 1767 meters per square second would suffice to keep it on the ground, less if you account for the fuel tanks and such.
  - darkmighty 8y ago
    You can always make a mass driver system (which is not limited by the rocket equation), but given how insanely hard it is at our own gravity, it would be more insanely difficult at a few times great g. So there's definitely a quite low maximum gravity allowing practical space access.

vertexFarm 8y ago

It's a niche application for sure. But niche applications count. I know one place it could be handy: getting ISRU products off the moon and into lunar orbit or over to a Lagrangian point. The moon is practically made of alumina-oxide solid fuel propellant. Along with its lack of atmosphere and low gravity, it would be preferable to use the literal dirt that is everywhere on the ground to loft precious refined or manufactured products into orbit instead of more efficient yet much more valuable cryofuels. It also would have a very good shelf life, and a brutally simple machine can have the kind of reliability people would kill for in space. For most missions you'd need something with better ISP, but for some applications it could be just the ticket. Granted it's not currently as simple as conventional solid rocket motors, but that will probably improve as they work out the kinks. The fact that it eliminates structural mass continuously is almost like a CVT for spacecraft; the staging doesn't happen in big, clumsy chunks, it gradually sheds mass all the way down to the payload.

azernik 8y ago

See the paper's description: "Autophage Engines: Towards a Throttleable Solid Motor". i.e. a small amount of extra complexity for a big new feature.