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Pressure-fed liquid fuel rocket engines use pressurized propellant 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. Propellant tank pressure must be higher than combustion chamber pressure to ensure propellant flow. A Falcon 9 Merlin turbopump engine has combustion chamber pressure of 1,410 psi. https://en.wikipedia.org/wiki/SpaceX_Merlin . Attaining this pressure with pressure-feed means heavy 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 to 200psi as ullage expanded 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. The 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. The size of the feed orifice would set a fail-safe upper limit on burn rate.

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. $\endgroup$ Commented Jan 2, 2023 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
    Commented Jan 2, 2023 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
    Commented Jan 2, 2023 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
    Commented Jan 2, 2023 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
    Commented Jan 2, 2023 at 20:20

2 Answers 2

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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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In addition to the issue in Loren's answer of the combustion products not necessarily being good pressurants some of the combustion products are also likely to be contaminants to the propellants.

For example a lot of propellants produce water when burnt. With cryogenic propellants this water will form ice and may well be more dense than the propellants leading to a build up of ice at the bottom of the tank probably fairly rapidly leading to blocking the tank outlet. With above freezing propellants the water is still likely denser and will again accumulate at the bottom of the tank, adding water to your combustion chamber is unlikely to lead to an efficient and stable burn.

With cryogenic propellants carbon dioxide will present similar problems to water with dry ice causing blockages.

One of the reasons for using helium for a pressurant is that it's low density should keep it at the top of the tanks and avoid it getting pulled into the engine. The combustion products are likely to be more dense than the propellants so don't have this key property.

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