Matthew's Eclectic Engineering

Storable Propellents

One of the most critical features for propellants on many missions is that they can handle very long-duration storage on a small spacecraft. Because a small tank has a large surface-to-volume ratio, unlike Starship or Blue Moon, storing cryogenic propellants long term without boiloff would be too heavy. Whatever you use has to be liquid at reasonable pressures across a wide temperature range.

The classic answer for bi-propellants has been Hydrazine and Nitrogen Tetroxide, which are dense, liquid in a wide range around room temperature, have an ISP of 310-330s, and Hydrazine works as a monopropellant for the RCS.

Hydrazine and Nitrogen Tetroxide are extremely toxic and difficult to work with. When spacecraft and launch costs are high, it’s just a rounding error, and the reliability of hypergolics is more than worth the trouble. As satellites have gotten smaller and launch costs have gone down, the cost of working with hydrazine and Nitrogen Tetroxide isn’t always just a rounding error anymore. Especially for rideshare missions, where the potential for leaks to damage other spacecraft drives up the insurance costs. That has driven the search for good alternatives that can be safely worked with in regular lab gear.

A newer storable propellant combination is Nitrous Oxide (laughing gas) and Propane from Dawn Aerospace. It’s less dense and only gets 270-290s ISP, but it has a usefully higher vapor pressure, which means it doesn’t need a separate pressurization system, and it can get colder without freezing. Nitrous oxide is also a usable monopropellant. The next step is likely upgrading to Nitrus Oxide Ethane for slightly higher ISP around 300s and higher fuel pressure.

The last practical storable oxidizer is Hydrogen Peroxide, assuming high enough purity and a well-passivated tank so it doesn’t break down slowly. Like Nitrus Oxide and Hydrzaine, Hydrogen Peroxide is also a good monopropellant. Burned with Keroscene it gives about the same 310-330s ISP as Hydrazine Nitrogen Tetroxide, and slightly better with lighter hydrocarbons.

Pressure Fed Vs Pump Fed Engines

Pressure-fed engines are very simple, using a small tank of very high-pressure helium to push the propellants into the engine. That keeps everything simple and cheap, but also makes the fuel tanks heavy because they have to hold all the pressure pushing into the engine.

Large rockets use turbopumps of some description because they let you get much higher-pressure engines with lower-pressure tanks. Giving you higher thrust, higher efficiency, and lighter weight. The problem for small engines is that conventional turbopumps just don’t scale down very well. The first problem is that as the pump gets smaller, the mostly fixed gap between the rotor and the housing becomes a larger portion of the total diameter, so it leaks a lot more. The second problem is that as the surface-to-volume ratio goes up, pre-burners or gas generators start to lose a lot of heat to the walls instead of getting it to the turbine. Those challenges mean that really small engines have always been either solid-fueled or pressure-fed because small turbopumps just don’t work very well.

More recently, Rocket Lab has pioneered the use of electric pump-fed rockets on their Electron launch vehicle and Photon satellite bus. The incredibly power-dense motors and energy-dense batteries developed for electric cars now allow the pump to be driven electrically. By skipping the turbine and the gas generator/preburner, the electric pump can scale down effectively to very small engines. That lets you get most of the weight and performance benefits of a turbopump on small rockets.

The more expensive pumps aren’t necessary for low delta-v missions, but electric pump-fed engines can bring huge performance benefits for small rockets when it matters. A good example would be Mars Sample Return, where the NASA plan calls for a solid-fuel MAV that needs a lot of supporting hardware. By taking advantage of electric pump-fed engines, Rocket Lab could propose a much simpler alternative using conventional storable propellants at much lower cost, and with a single stage instead of two.

For an electric pump-fed engine, Hydrogen Peroxide is the best storable oxidizer. It has reasonable thermal control requirements and, unlike nitrous oxide, has a low vapor pressure that works better with a pump-fed engine. It gets the same ISP as Hydrazine-Nitrogen Tetroxide when burned with Kerosene without all the propellant handling issues, and slightly better ISP with shorter hydrocarbons. I think the ideal fuel choice is butane because it’s the shortest hydrocarbon, and therefore the highest ISP, that doesn’t have excessive vapor pressure for a pump-fed engine at room temperature.

Hydrogen Peroxide-Butane gets you a storable propellant option for electric pump-fed engines that’s non-toxic and has a ~10s ISP advantage over Hydrazine-Nitrogen Tetroxide. Until your rocket gets big enough to store cryogens with zero boiloff, it’s hard to beat that for chemical delta-V.