4 ms·

I think you just invented ullage rockets (if you meant that the thrust of these small rockets should help to settle the fuel at the bottom of the tanks). https

I think you just invented ullage rockets (if you meant that the thrust of these small rockets should help to settle the fuel at the bottom of the tanks). https://en.m.wikipedia.org/wiki/Ullage_motor https://en.m.wikipedia.org/wiki/Ullage_motor

numpad0 2y ago
I mean, firing all the way down. Ullage motors are for ignitions only, my proposal is dedicated descent engines. It'll improve control authority by a lot too.
- quartesixte 2y ago
  complexity + weight probably not worth the trade off. If you can hit it with just main engine gimallinga and grid fins, why bother? also, combustion devices are always one more point of complex and catastrophic mihaps. better keep it as simple as possible and don't miss.
  - numpad0 2y ago
    But there's flame coming from lower rocket body in the video, that suggests partial RUD around thrust structure, which has to be looked into and worked a lot. There's already fuel line going into the fins as a ghetto hydraulics, so it should be more of "just" adding the oxidizer line rather than completely redoing that area for something that hasn't been done. Plus it could make the entire stack more flush and sleek.
    - simonh 2y ago
      The engines combust fuel to run their turbopumps, the exhaust will be the flames coming from the side of the rocket body. You can see it on the ascent as well. On Falcon 9 too.
      - _Microft 2y ago
        Raptor is a full flow staged combustion engine [0], that means that the preburner exhaust is fed into the main combustion chamber. This is more efficient than dumping it separately like it happens on the Falcon 9 engines [1]. The black, sooty smoke on the first image of the Wikipedia article [1] is the (kerosene) fuel-rich preburner exhaust of the Merlin engine. [0] https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor [1] https://en.wikipedia.org/wiki/SpaceX_Merlin https://en.wikipedia.org/wiki/SpaceX_Merlin
- XorNot 2y ago
  It's inefficient: the rocket engines they have have a specific impulse which they get for their design and size. Generally smaller rocket engines have worse Isp. In both cases the issue is that (1) you need the engine to light up and (2) you need it to last through the entire burn. And all of this would be more weight and complexity. The other problem is fuel: while the stage would be getting lighter burning fuel the whole way down, it's also slowing down so you spend more time in the descent stage, which in turn is increasing your engine durability and cooling requirements and increasing your fuel use etc. And after all that, you're either (1) coming to zero velocity on the descent engines alone or (2) still need the Raptors to relight so you can actually land. In the case of (1) you're hefting around a bunch of engines you're not using most of the flight, and expending more fuel etc. on the take off and landing. In the case of (2) well, why not just get the Raptors to be super-reliable and do the braking burn as-is? As it is, we've now seen the Superheavy can do a landing burn with an engine failure to boot - improving from there takes us from those initial Falcon 9 landings to where are today (so common it's not news).
  - numpad0 2y ago
    Yeah so like, a pair of "Merlin Methalox" on sides. Flying machines need at least four independent degrees of freedom to fly, but not less, so authority wise two XY gimbaling Merlins should do. The current Raptor setup seems to be working pretty well for boostback and entry burns while the vehicle is in total vacuum, but it seem to get sketchier as propellants are expended and TWR increases, while altitude coincidentally reduces and aerodynamic stress increases. Almost every materials are stronger against tensile stresses than compressive stresses, and stainless steel is no exception, so an array of mighty Raptors trying to compress the booster lengthwise from the bottom, could be, just could be, less ideal than a pair of tip-jets pulling upwards and straightening the shell. As for them being dead weight on ascent, I don't know, but as I understand it, every bit of thrust buys a bit of payloads at very moment of launch, then thrust requirement rapidly decreases and effect of Isp becomes more dominant. So I think there could be potentially some net positive effect in "test firing" the proposed side-jets at launch for few moments, depending on various parameters. I'm way too much of a layperson on actual rocketry to be even remotely sure, though. I mean, picture Superheavy coming in to launch site, then the side facing retro-rockets light up as high as 100m above the ground, suspending the booster under it and sliding it gently onto the chopsticks. The retro-rockets are slightly angled, like Dragon 2 thrusters or slightly more angled. Isn't that going to look even cooler than what it is already?

numpad0 2y ago

I mean, firing all the way down. Ullage motors are for ignitions only, my proposal is dedicated descent engines. It'll improve control authority by a lot too.

quartesixte 2y ago
complexity + weight probably not worth the trade off. If you can hit it with just main engine gimallinga and grid fins, why bother? also, combustion devices are always one more point of complex and catastrophic mihaps. better keep it as simple as possible and don't miss.
- numpad0 2y ago
  But there's flame coming from lower rocket body in the video, that suggests partial RUD around thrust structure, which has to be looked into and worked a lot. There's already fuel line going into the fins as a ghetto hydraulics, so it should be more of "just" adding the oxidizer line rather than completely redoing that area for something that hasn't been done. Plus it could make the entire stack more flush and sleek.
  - simonh 2y ago
    The engines combust fuel to run their turbopumps, the exhaust will be the flames coming from the side of the rocket body. You can see it on the ascent as well. On Falcon 9 too.
    - _Microft 2y ago
      Raptor is a full flow staged combustion engine [0], that means that the preburner exhaust is fed into the main combustion chamber. This is more efficient than dumping it separately like it happens on the Falcon 9 engines [1]. The black, sooty smoke on the first image of the Wikipedia article [1] is the (kerosene) fuel-rich preburner exhaust of the Merlin engine. [0] https://en.wikipedia.org/wiki/SpaceX_Raptor https://en.wikipedia.org/wiki/SpaceX_Raptor [1] https://en.wikipedia.org/wiki/SpaceX_Merlin https://en.wikipedia.org/wiki/SpaceX_Merlin
XorNot 2y ago
It's inefficient: the rocket engines they have have a specific impulse which they get for their design and size. Generally smaller rocket engines have worse Isp. In both cases the issue is that (1) you need the engine to light up and (2) you need it to last through the entire burn. And all of this would be more weight and complexity. The other problem is fuel: while the stage would be getting lighter burning fuel the whole way down, it's also slowing down so you spend more time in the descent stage, which in turn is increasing your engine durability and cooling requirements and increasing your fuel use etc. And after all that, you're either (1) coming to zero velocity on the descent engines alone or (2) still need the Raptors to relight so you can actually land. In the case of (1) you're hefting around a bunch of engines you're not using most of the flight, and expending more fuel etc. on the take off and landing. In the case of (2) well, why not just get the Raptors to be super-reliable and do the braking burn as-is? As it is, we've now seen the Superheavy can do a landing burn with an engine failure to boot - improving from there takes us from those initial Falcon 9 landings to where are today (so common it's not news).
- numpad0 2y ago
  Yeah so like, a pair of "Merlin Methalox" on sides. Flying machines need at least four independent degrees of freedom to fly, but not less, so authority wise two XY gimbaling Merlins should do. The current Raptor setup seems to be working pretty well for boostback and entry burns while the vehicle is in total vacuum, but it seem to get sketchier as propellants are expended and TWR increases, while altitude coincidentally reduces and aerodynamic stress increases. Almost every materials are stronger against tensile stresses than compressive stresses, and stainless steel is no exception, so an array of mighty Raptors trying to compress the booster lengthwise from the bottom, could be, just could be, less ideal than a pair of tip-jets pulling upwards and straightening the shell. As for them being dead weight on ascent, I don't know, but as I understand it, every bit of thrust buys a bit of payloads at very moment of launch, then thrust requirement rapidly decreases and effect of Isp becomes more dominant. So I think there could be potentially some net positive effect in "test firing" the proposed side-jets at launch for few moments, depending on various parameters. I'm way too much of a layperson on actual rocketry to be even remotely sure, though. I mean, picture Superheavy coming in to launch site, then the side facing retro-rockets light up as high as 100m above the ground, suspending the booster under it and sliding it gently onto the chopsticks. The retro-rockets are slightly angled, like Dragon 2 thrusters or slightly more angled. Isn't that going to look even cooler than what it is already?