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I noticed NASA's demonstration engine uses an aerospike engine nozzle which is great for single stage to orbit (SSTO) attempts due to its ability to be optimized for both vacuum and sea level conditions.

Although this was likely not its purpose, I am curious if this type of higher efficiency engine could help make SSTO a reality. Is this true, or are there other drawbacks such as scalability and thrust to weight that could hold it back?

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    $\begingroup$ Why SSTO at all? $\endgroup$ Commented Jan 28, 2023 at 2:16
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    $\begingroup$ @user3528438 - It makes the overall launch system way simpler, and make reuse way easier as there are no separation events. $\endgroup$
    – David
    Commented Jan 29, 2023 at 6:19
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    $\begingroup$ @David While those aren't fundamentally incorrect, the mass ratio impact from SSTO versus not-SSTO is brutal for anything that uses chemical rockets. $\endgroup$
    – ikrase
    Commented Jan 29, 2023 at 8:30
  • $\begingroup$ Changing Starship to haul its booster to orbit and having to protect the entire thing against full orbital reentry with SSTO mass fractions would not be making things simpler or easier, and stage separation hasn't been a problem for reuse. $\endgroup$ Commented Jan 29, 2023 at 14:36
  • $\begingroup$ @ChristopherJamesHuff - I think you are neglecting some of the complexity as this is how it has always been done. If we found a super high ISP engine, stage separation would not make sense. We would not launch an airplane that goes up and separates with a smaller aircraft so it can fly longer because the cost and complexity would not makes sense. Also, I should point out the initial shuttle replacement was SSTO, but it was cancelled only due to carbon fiber tank problems. $\endgroup$
    – David
    Commented Jan 30, 2023 at 17:22

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The Łukasiewicz Research Network Institute of Aviation has done a test launch of a small rocket powered by a RDE using liquid fuels (propane and nitrous oxide), demonstrating that it's at least feasible to get T/W greater than 1. This was not a long flight, burning for 3.2 s and reaching only 450 m. It's not apparent if this was a limit due to the amount of propellant it could lift, the duration it could fire for, or if it was just kept short for test purposes.

The aerospike form factor is likely because the combustion chamber itself is toroidal, with detonation waves rotating around it. It's not obvious how it would work with a conventional nozzle. The combustion process involves a large, brief spike in pressure as each detonation wave passes, which the combustion chamber must be constructed to withstand. This will come with a mass penalty of some degree. Cooling should be easier, as the combustion in any given section is intermittent, but the same intermittency means thrust is likely much lower than for a similar-sized conventional engine.

In short, I would not expect these to have terribly high T/W ratios, making them poorly suited to SSTOs. These are probably better suited for high efficiency upper stage engines, where the relatively heavy combustion chamber and nozzle can be offset by using a smaller, lower thrust engine.

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