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Pressure-fed liquid fuel rocket engines use pressurized tanks to deliver propellant to the combustion chamber, rather than pumps. This eliminates the mass, cost and complexity of the gas generator and turbopumps. Pressure-feed rockets have been used successfully by:

  • SpaceX Kestrel engine, upper stage Falcon 1
  • Apollo Lunar Module descent engine
  • Space Shuttle Orbital Maneuvering engines

Pressure feed can potentially reduce mass, cost and complexity. However, this design approach has fundamental challenges:

  1. Tank pressure must be higher than combustion chamber pressure. A Falcon 9 Merlin turbopump engine has combustion chamber pressure of 1,410 psi. https://en.wikipedia.org/wiki/SpaceX_Merlin . Trying to attain this pressure with pressure-feed means heavier tanks.
  2. If the tank is pressurized with a fixed quantity of gas, the ullage volume is “wasted” tank volume. In OTRAG rockets, 1/3 of tank volume was ullage.
  3. As the ullage space expands, tank pressure drops. Example: OTRAG tanks dropped from 600psi as ullage ezpanded 3:1 during a burn
  4. Cryogenic propellants boil off during a burn, lowering the temperature of the ullage gas and further lowering tank pressure

Some of these disadvantages can be overcome with autogenous pressurization such as helium pressurization systems, at the expense of, well, expense. And complexity. The Kestrel engine required a heat exchanger in the combustion chamber to heat the helium. The engine achieved an impressive specific impulse of 317sec.

enter image description here

https://erik-engheim.medium.com/geek-guide-to-rocket-engines-70ea91bf5971

As an alternative autogenous pressurization system, why not combust propellants inside the tanks? A small burner jet feeding oxidizer into the fuel tank (and fuel into the oxidized tank) could keep pre-launch ullage space small, but maintain constant ullage pressure during main engine burn. Feed pressure of the oxidizer (or fuel) to this burner could be maintained just above the desired ullage pressure, so the burn would be largely self-regulating and fail-safe.

enter image description here

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    $\begingroup$ @Woody - steam sprayed into a cryogenic tank will turn to ice rather than doing much pressuring, ditto things that go into solution and then foam up in the injectors. $\endgroup$ Jan 2 at 5:50
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    $\begingroup$ @OrganicMarble ... no turtles required. You are right that the propellants must be pressurized above combustion pressure so they can be fed into the ullage gas generators. But this is a small (by mass) engineering problem compared with pumping all the engine propellant to the same pressure. Electric pumps would do. $\endgroup$
    – Woody
    Jan 2 at 16:55
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    $\begingroup$ How are the two small spherical tanks are pressurized? You don't need the spheres, each of the cylindrical tanks may be pressurized by the other. But how do you ignite the combustion in the tanks? $\endgroup$
    – Uwe
    Jan 2 at 17:17
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    $\begingroup$ @OrganicMarble .... Correct about complexity. An alternative to pumps is to have the small tanks very high pressure (i.e: 3000psi) so that their ullage pressure always remains well above main engine combustion chamber pressure. Overwrapped pressure vessels would do nicely. Or recycle those helium vessels you no linger need. No turtles were harmed creating this design. $\endgroup$
    – Woody
    Jan 2 at 17:26
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    $\begingroup$ @CharlesStaats ... you don't need to ensure "100% combustion" since very fuel-rich (or oxidizer-rich) mixtures are not even flammable, let alone explosive. Even pure hydrogen/oxygen mixtures have limits on flammability (I recall 4%-95%). $\endgroup$
    – Woody
    Jan 2 at 20:20

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You have a big problem with hydrolox or metholox--the combustion products condense out at far above the temperature of the liquid in the tank. Thus you get no pressure.

Even with Aerozine/nitrogen tetroxide you partially have this problem as the water condenses out. There's also the problem that Aerozine contains hydrazine which is capable of energetic decomposition--pure hydrazine is sometimes used as a monopropellant where the simpler rocket is worth the lower ISP. Somehow, subjecting a tank containing hydrazine to combustion doesn't sound safe.

I can't see a safety issue with pressurizing a kerosene tank this way, but kerosene is usually only used in big rockets where such a pressure tank would be heavy. And note that the primary combustion product is water that condenses out, you would get some pressure from the CO2.

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