Thoughts on a Gas Generator

While an expander cycle is an attractive option, mainly because of the simplicity it brings to system plumbing, it comes with a few disadvantages. 

1. Pump can't be operated independent of the thrust chamber (regen required for operation)

2. Spin up is more difficult (requires some preliminary heating element for the cryogenic fuel)

3. Limits design to a few cryogenic fuels (LH2/LCH4)

4. Uncommon/not as much research

Further research has revealed that an open cycle gas generator is much more common and likely more attainable. NASA's Fastrac program, in partnership with Barber-Nicols, yielded the development of an RP-1/LOX engine that paved the way for the development of SpaceX's Merlin engines, an open cycle gas generator (GG) design. Using pumping affinity and similarity laws and the vast amount of data on this engine, it becomes much easier to develop our own turbopump.

Additionally, to simplify the configuration of our turbopump, a single shaft design or parallel dual shaft system, may simplify sealing and gearbox challenges (at the caveat of more complex plumbing to route gas generator exhaust to two turbines). Additionally, a gas generator requires a much more robust alloy or super alloy for the turbine blades. Such alloys include nimonic 90 and other titanium alloys, which can be difficult to obtain and machine. 

Figure 1: Pump Configurations

Furthermore, turbopumps of these kinds have a rich history, with the Redstone missile using a gas generator powered by the decomposition or hydrogen peroxide to pump alcohol and liquid oxygen. Additionally, online databases have a large number of photos and diagrams providing valuable insight as to how components fit together and operate (Heroicrelics has some incredible photos of the F1 Engine turbine and pumps). 

Figure 2: Redstone Turbopump

A trade study is underway to determine the best option for our application. We'll evaluate different pump configurations and engine cycles mainly on how relatively easy they are to build and how well they can be produced for a reasonable price. 

Figure 1: MODELING AND SIMULATION OF LAUNCH VEHICLES USING OBJECT-ORIENTED PROGRAMMING - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Turbopump-configurations-SOURCE-SP-8107-1974_fig7_322924066 [accessed 3 Oct, 2021]

Figure 2: http://heroicrelics.org/info/redstone/a-7-turbopump.html

-Zach