Since the very beginning, solid-propellants have been the cornerstone of amateur rocketry. From the little Estes rocket picked up from the toy store, to vehicles like the University of Southern California’s Traveler IV that (probably) crossed the Kármán line in 2019, a rapidly burning chunk of solid propellant is responsible for pushing them skyward. That’s not to say that amateur rockets powered by liquid propellants are completely unheard of … it’s just that getting them right is so ridiculously difficult that comparatively few have been built.
But thanks to [Half Cat Rocketry], we may start to see more hobbyists and students taking on the challenge. Their Mojave Sphinx liquid-fueled rocket is not only designed to be as easy and cheap to build as possible, but it’s been released as open source so that others can replicate it. All of the 2D and 3D CAD files have been made available under the GPLv3 license, and if you’re in the mood for a little light reading, there’s a nearly 370 page guidebook you can download that covers building and launching the rocket.
Now of course we’re still talking about literal rocket science here, so while we don’t doubt a sufficiently motivated individual could put one of these together on their own, you’ll probably want to gather up a couple friends and have a well-stocked makerspace to operate out of. All told, [Half Cat] estimates you should be able to build a Mojave Sphinx for less than $2,000 USD, but that assumes everything is done in-house and you don’t contract out any of the machining.
The design is the result of years of research and development that was aimed at distilling a liquid-fueled rocket down to its most basic form. There’s no gas generator, no turbine, no pumps of any kind. Controlling the flow of propellants within the rocket requires only a pair of servo-actuated valves thanks to the ingenious use of dual-acting vapor pressurization. Put simply, the rocket uses one large vertical tank that’s internally divided by a movable piston, with the oxidizer — nitrous oxide — on one side and the fuel — nearly any flammable liquid, such as alcohol or gasoline — on the other. The high-pressure nitrous oxide pushes down on the piston, which in turn pressurizes the fuel.
To get the most out of your investment, the Mojave Sphinx is designed to be entirely reusable. Assuming it makes a soft enough landing, you just need to refill the tank and launch it again. In practice it’s a bit more involved than that, but the team of [Half Cat] say they’ve managed to fly the same rocket multiple times in a single day. The handbook even has a handy maintenance schedule that tells you how often you should check or replace different components of the rocket. For example, it advises replacing the propellant piston o-rings every third flight.
The downside? There’s only so much performance you can wring out a rudimentary propulsion system like this. When compared to more simplistic solid-propellant rockets, the higher mass of the Mojave Sphinx puts the maximum altitude of the 96 inch (2.4 meter) long rocket at around 10,000 feet (3 kilometers). Still, we know plenty of folks who would call that a worthy compromise for being able to say they built their own liquid rocket.
Thanks to [concretedog] for the tip.
That is pretty cool, but I would like to see an animation of how it works.
At any given temperature, the nitrous oxide has a fixed vapor pressure. If the oxidizer chamber gets larger, then more nitrous will boil off to keep the headspace full of gas at that pressure. That applies as long as there’s enough oxidizer left for there to be some liquid. Since a given amount of nitrous oxide at its room-temperature vapor pressure takes up on the order of 20 times as much space in gaseous form than in liquid form, boiling some off to provide pressurizing gas won’t eat into your reserves too much.
The fuel, on the other hand, is incompressible and has a relatively low vapor pressure, so the fuel side of the tank will always be just liquid fuel.
Since the piston can move freely, the pressure on both sides of the tank will be the same.
Assume that the oxidizer outlet valve is at the bottom of the tank, in the liquid part of the oxidizer. If you open both valves, liquid fuel and liquid oxidizer, both under identical pressure, will be forced out of their respective parts of the tank at rates determined by the sizes of the orifices they’re passing through. The tank pressure will stay roughly constant as the oxidizer continues to boil (it’ll presumably actually drop off some because the tank will cool down, but it’s basically constant at any given temperature). So the flow rates of both fuel and oxidizer will both also stay roughly constant. And since whatever the pressure is, it applies to both the fuel and the oxidizer equally, the proportions of the two should stay even more constant.
The animation would just show fuel and oxidizer flowing out of the tank in liquid form, the oxidizer boiling, and the piston moving toward the fuel side.
That’s a very good explanation! Thank you!
Another thing I am not getting, is how does the oxidizer get to the nozzle if it is above the fuel?
Okay, so the oxidizer is below the fuel, and the gaseous oxidizer pushes the piston up forcing the fuel out. So there is a pipe or channel that delivers the fuel to the nozzle?
Excellent explanation that gave me an idea. The thrust is determined by the mass and velocity of the exhaust gas. How to do this? One possible way would be to install a second piston, rotated 180 degrees to the first, thus creating a 3rd chamber. Now pressurize the middle chamber with an inert gas such as CO2, Argon, Nitrogen, Helium. A side benefit is there is now an inert gas between the fuel and oxidizer in the event of an O-ring leak. (Possible safety feature to the original design.) If the pressure is equal to or greater than the NO2 gas pressure, you will now may be injecting liquid NO2 into the combustion chamber so the flow needs to be metered. A possible mitigation to this issue is to wrap the NO2 feed line tightly (weld/silver solder?) around the exhaust cone which will increase the generation of NO2 gas from NO2 liquid and would have the additional benefit of cooling the exhaust nozzle. Fuel line wraps would heat the fuel increasing thrust and also provide nozzle cooling. The biggest issue with all of this is the vapor pressure of NO2 is approximately 300 PSI@5 degrees F to 1000 PSI@100 degrees F. Perhaps the simplest solution to more thrust is to heat the loaded tank to some temperature that is 1/2 or less the proof pressure of the tank. That means one would have to build and hydrotest a number of tank bodies in order to find out the burst pressure. It’s an interesting problem.
Wonderful. I am glad to see technology really coming to model rocket propulsion. It used to have such a stultifying focus on premade solid motors with no active stabilization, basically getting rid of most of the interesting parts of rocketry.
One big piece of that was safety and at the time of the introduction of model rockets, to try and keep them form being classified as fireworks, which had a pretty bad reputation. Sadly even today fireworks have a pretty bad reputation, when the vast majority of “accidents” I read about are caused more by human stupidity than the pyrotechnic device.
Various rocket motor topologies have appeared and the gap between model and amateur rocketry has closed a bit. It is not unusual at a launch to see some rockets flying with nitrous and some kind of a fuel. There are also a lot more electronically enhanced rockets, doing everything from firing the chute charge to sending full video back and recording lots of telemetry of the flight.
There are also people playing with active stabilization to the point where one group seems to be able to get their rockets to pretty reliably land standing up.
Sadly, it is not a poor mans hobby, but it is fun to go to one of the big launches to see what folks have been up to.
Where is the line between “Actively stabilized model rocket” and “Guided missile”?
When you strap a warhead on it.
No, but that will certainly piss off the ATF even more than putting a guidance system on a liquid-fueled rocket. A kinetic missile is still a missile
Possibly but not effectively. Consider the Atlas, Delta, Space Shuttle and Titan rockets to name a few. The center stages are all liquid fueled with attached solid boosters. The solid fueled motors typically provide more specific impulse per pound of weight. However, the Starship’s engines run on LOX and methane but require turbo pumps. Liquid methane is lighter than kerosene so there is a weight saving gain. Perhaps a heater element in the NO2 would increase the vapor pressure and thereby increasing the thrust. Or consider Dr. Goddard’s rocket where the engine was at the top and the engine’s exhaust went onto the propellant and oxidizer containers. This has a drawback as the exhaust is diminished because the exhaust is pushing on the suspended body.
This comment was supposed to reply to Hirudinea. My apology.
I’m speaking about what the famously capricious ATF would think, since their opinion matters a lot more in effect if not in nuance. All those rockets are run by big organizations with legal teams.
I suppose it becomes a missile when you program it to seek something that isn’t the sky
Thank you Gene.
Goddard was using the “motor in front” because he was trying to keep his stabilization on his rocket that way, the motor was to drive and the mass of the tank and fuel was to keep it pointed in the right way. The German VfR club also used this method to stabilize. They also used a lot of shielding to keep the fuel cool. The Peenemunde rockets resolved the stability problem by gyro operated graphite vanes dipping into the exhaust to vector the thrust. This also reduced thrust a bit but it was a simpler solution.
The first one is only designed to go straight up, the other one sends you to prison if you get caught doing it.
There isn’t one. That’s why I’ve advocated to the rocketry community “Do NOT work on active stabilization!” It will lead to undesired regulation. (Remember: governments say ‘no’ and regulate because…they can.)
“When you add a subroutine that makes it want to go near any various objects which are solid” might be the definition that fits
Could I use these as strap on boosters for a solid rocket rocket?
See the comment I accidently posted above.
“The high-pressure nitrous oxide pushes down on the piston, which in turn pressurizes the fuel.”
The caption on the accompanying picture describes it as an “upwards moving piston”. If I’m understanding that correctly the fuel is above the nitrous oxide and the piston is pushed up not down. That way the liquid nitrous can drain out of the fixed bottom of the tank while the fuel is pushed out of the top.
Am I wrong or those looks like the Y Wing engines?
It does! I’m guessing that the people who designed the Y-Wing may have been looking at some old 70s guided rockets, or some other vectored thrust thingie.
Also: make a piece of what?
Wow. I was expecting a completely different kind of liquid-fueled rocket.
https://www.youtube.com/watch?v=GCaiK3Zqs4M
Okay, A couple of comments here :
1) GNC, Guidance Navigation, and Control.
you are prohibited from guidance. Navigation (common in HPR) can
be as ‘Simple’ as GPS. Control is like fins to keep it from swapping ends. Active control is the same thing, Ejection charges and what not to terminate a flight is a form of control. Just look at the words here.
2) I have built and fired a few N2O oxidizer rockets. The idea shown above is commonly called ‘blow-down’ and it is not so great. The N2O going into the chamber is near or at it’s boiling point. That means that as it passes through assorted orifices (injectors, flow limiters, valves, pipe changes, etc) it can boil. Bubbles have much less mass, so the mass flow for the N2O is not easily predictable. The other problem with N2O is that it thermally stratifies, so that throws another problem in the flows. Now it depends on how, and how long ago, you fill the tanks, because that generally changes the temperature and density. Better the have a regulator and a pressurant.
3) N2O with a little bit of leaked hydrocarbon in it is very dangerous. It blows up unexpectedly and vigorously. It killed several people working on rockets. All operations should be by remote control. There should never be a leak path between the two propellants. The sliding piston terrifies me. I have run mixes that worked the first time and detonated every time after that. Flattened out 5000 psi test plumbing.
4) I meet at least one (B)ATF agent every two years since 1998. They have all been very nice. They have all been respectful of my home. They have all been very helpful, They have all been trying to do their job and not get in trouble with their bosses. They are not lawyers and just want you to follow the current procedure. They don’t particularly like it when the procedures change, but they do like it when they can say : “not a problem anymore, do what ever you like with your” – rocket motor, electric matches, lawnmower or whatever ).
I know people who hate the ATF, but I have never met an agent who hated them back. ymmv
-G
Doesn’t matter if a boot is “just doing it’s job” while it’s standing on your neck. Unconstitutional agency is unconstitutional.
And are you sure those rules aren’t just for IATR? Afik there is no legal limitation on guided rocketry within the US.
ITAR?
International Arms Trafficking Regulations. Basically rules in the US about what technology you can legally export. Some courts have decided the publishing designs on the internet counts as “exporting”.
…the maximum altitude of the 96 inch (2.4 meter) long rocket at around 10,000 feet (3 kilometers)…
You don’t need a rocket to reach 3km but, if launched from a plane at 10km, maybe the propulsion system could be useful.
The VfR one-stick and two stick “Repulsors” and IIRC the v-2 and the Redstone system used evaporation of LOX for the initial pressurization of the fuel tanks, though the V-2 and the Redstones both used a rotary pump to feed the motors. The VfR made great strides for rocketry coming from a baseline of pretty much nothing, and I think it has to do with the “open” model it used, of a bunch of motivated volunteers doing it for fun.
Crowd based innovation, which either turns out wildly successful, or wildly odd.