Comments by "kazedcat" (@kazedcat) on "How NASA Reinvented the Rocket Engine" video.

 @benzzoy What efficiency? Combustion efficiency is not the same as Propulsion efficiency.
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 @mintoc8853 unless you are a rocket engineer and the new engine is giving you 5% more combustion efficiency when what your rocket design need is 5% more propulsion efficiency.
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@slyseal2091 This is wrong and the video is wrong. Higher combustion efficiency gives you better thrust but it does not mean you need less propellants. To reduce propellants what is needed is higher propulsion efficiency. A significant portion of the fuel in a rocket is not burned and they are mainly use as propellant with their chemical energy untapped. RDE burns fuel more efficiently but it does not throw propellant with higher efficiency.
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@anullhandle The rocket use propulsion efficiency (ISP) not combustion efficiency so this engine does not actually apply. RDE burns better but it has no say on the tyranny of the rocket equation.
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@mintoc8853 No it does not work like that combustion does not increase mass. Even if the new engine provides 5% more energy when you calculate the added impulse you only get a square root of that 5%. So it is very misleading to calculate the 5% additional combustion efficiency as linearly equivalent to a 5% reduction in propellants.
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@mintoc8853 Yes the square root is related to kinetic energy. But we are assuming all the energy gets converted to kinetic energy. Some of them is wasted into sound energy and infrared energy which does not increase the propellants exit velocity which is what ISP is it is the propellants exit velocity. Detonation means boom supersonic boom. RDE could mean just a louder engine. I have not seen anyone calculating the actual ISP improvement of RDE.
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@mintoc8853 NASA sometimes study impossible technology hoping to get lucky. I believe there is an ISP gain with the new engine but the fact that they are hiding it makes me believe that the ISP gain is very small.
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@mintoc8853 My guess is it will become the new holy grail engine that is very fun to talk about but no one actually uses. The previous holy grail engine was the full flow stage combustion engine which have very high combustion efficiency but also very complicated. But SpaceX is bringing that engine into production so we need a new holy grail engine to inspire new engineer.
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@slyseal2091 The tyranny of the rocket equation could not be easily defeated. The easiest solution is to just build a bigger rocket. Rocketlabs for example is now building a bigger rocket using a very simple engine architecture. Better engine is the sexy solution but better propellant tanks are more practical.
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You are confusing combustion efficiency with propulsion efficiency (ISP). They are not the same thing. Higher combustion efficiency does not mean higher ISP. Higher ISP is what is necessary to reduce the needed propellants for a fix ΔV.
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@MrKentaroMotoPI Actually no. If everyone goes for highest combustion efficiency then everyone would be using full flow close combustion engine. But real rockets often use less efficient open combustion that is like 30% less efficient. Even the Saturn V uses open combustion. Only SpaceX actually uses the most combustion efficient engine architecture that is not RDE. So a better engine than what SpaceX is using is not actually very practical outside of research.
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@MrKentaroMotoPI Full flow stage combustion has higher ISP. It is you that needs to read the text books.
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@MrKentaroMotoPI Tell me how to design an open cycle engine that could match the ISP of close cycle engine. Give me an example of open cycle engine with similar ISP to a close cycle engine.
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@MrKentaroMotoPI Let me school you on rocket engine 101. ISP is driven by chamber pressure. The higher the chamber pressure the higher the ISP. In order the reach high chamber pressure you need the intake to be at a much higher pressure than the combustion chamber to allow propellant to flow in. To do this you need a compressor. To power the compressor you need a preburner or a gas generator. This where lies the problem the power that can be delivered into the compressor is dictated by the mass flow and pressure drop on the preburner. You don't want high pressure drop because that will just eat up the high pressure generated by the compressor. So the solution to reaching the highest chamber pressure is to use two preburner and use the combine mass flow of the oxidizer and the fuel to generate the power needed to drive the compressor. This is why Full Flow Stage Combustion provides the highest ISP of all stage combustion engine because it is the only cycle using all propellant mass to drive the compressor. Now class is over go read your text book because your rocketry knowledge shows to be deeply flawed.
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