Reusability Considerations (11/9/21)

Reusability Considerations

I made up my mind on what my next project will be. It will be a lunar lander style vehicle which I will develop into a delivery vehicle. Yes, a rocket powered delivery vehicle. I suspect the market is not exactly there for it and so it may not be developed into a profitable business but it will be very interesting and is one of the most challenging types of liquid rocket builds, combining some serious software and debugging prowess necessary with some very rigorous and intense development and iteration practices. Basically it's rocket science and it’s really hard. It’s a lot harder than what I’m working on at the moment. The reason this current rocket took so long was because I needed to get myself situated with basic rocket engineering as well as manufacturing and logistics as well as doing what I could with little to no knowledge or money, just a lot of time. That is exactly what it took, a lot of time. Just to be clear this current rocket is not going to be abandoned, far from it. It's made astonishing progress in the past month and things are finally rolling into place. The puzzle is approaching completion. The reason that the lander project is more difficult is because the software needs to be tuned a lot, and needs to be pretty much written from scratch which places some stringent requirements on the propulsion system and the entire vehicle, not to mention extremely well-built hardware.

What I’m proposing is a vehicle that can hover for at least 90 seconds and once capable of hovering it should be able to accelerate and fly in any direction I choose and land at any point I choose without damaging the ground it lands on and be able to be refueled in 30 minutes. This demands some changes slightly more strenuous than the NGLLC challenge but I think I can pull it off eventually. For the fundamentals I’ve got to get the following: An engine that can throttle and restart any which way I want without damaging anything and ideally something with a short flame. This engine will be mounted above the tanks. There will be 3 tanks minimum, same setup as current feed system wise. I somehow need to get a way of throttling it without raising the pressure of the fluid. I need some way of GNC - Guidance, Navigation and Control, to which there is a whole in depth science. I need some sort of landing gear which I am preliminarily thinking of using pneumatic cylinders as shock absorbers. John Carmack is of the opinion that rubber blocks worked just fine and I guess they did. I might do that instead. 

To get all of this to work I’m almost guaranteed to blow up some vehicles and break some hardware during testing. This means that I need to have a rapid iteration, build and test cycle, which means I need my own machine shop and tools. I won’t get to this till I finish this current rocket and that will probably not happen till I graduate. Therefore this is probably going to be my project during my freshman year in college and I should have access to whatever tools I need at my university. It’s going to be design-> build -> test->redesign till works and then test until things work. I’m hoping to get the cycles down to 48 hours from design to new design. The testing will occur in separate phases, the first of which being the engine followed by the control system and then finally the landing gear. 

Some of the issues that Paul and Armadillo had were in valves. So I’m thinking of using fast-acting pneumatic cylinders connected to a poppet cavitation venturi valve which is the smallest possible valve as calculated by orifice size for any given mass flow, thereby requiring the least force to actuate. I’ll somehow have to figure out how to cool the engine with the varying mass flows because when mass flow is cut by half, that does not mean the chamber temps will suddenly drop by half as well, and they typically stay within 90%.

I’ve got a cheesy solution to that which is to have a second fuel tank which would be connected in series to the chamber and act as a buffer. But I think I’m going to figure out how to minimize the necessary amount of time I need to throttle down. Engines like running at full throttle because everything is mostly optimized for full throttle. Lower throttles are typically just used for takeoff and landing.

It’s highly critical that I get my testing setup to run very quickly. The testing cycles are going to be a limiting factor for a project like this. If I have to drive out to the middle of nowhere and setup that is going to take up the better part of a day and likely decrease the amount of testing days I have available down to two days a week. I think I’m going to spend a decent amount of time optimizing my test stand. My next interview is with Paul Breed, the guy who wrote the blog about his project similar to this and was one of the competitors in the NGLLC. Very cool guy!

Ismail

Sources:

NASA SP-194 (1972). Liquid Propellant Rocket Combustion Instability 

Harrje. D.T

RPE George Sutton 1984.

Unreasonable Rocket Blog. (2009) Paul Breed