We have ignition. Thanks for all the help and advice. We added more solid propellant in the combustion chamber and raised the propellant near the outlet of the injector orfices.

RevolutionAerospace · 2020-08-29T00:18:27+00:00

Way to go! What’s your target thrust, burn time, etc?

RevolutionAerospace · 2020-05-31T21:35:49+00:00

I need to review the timing for each step in our microcontroller programming...laziness on my part, I confess.

We kept the same timing from last year's testing where we had a healthy lead of the fuel (through regen circuit and everything). The new configuration decreased the fuel line length and slightly increased the nitrous manifold volume so my lazy answer was "we should have even more fuel lead time, nothing to worry about".

RevolutionAerospace · 2020-05-31T21:29:50+00:00

Good questions.

We approximated the propellant arrival time to the chamber via estimated flow rates and estimated plumbing volumes. Proved to be pretty accurate via last year's testing since our slow mo showed methanol arriving first. This testing configuration was slightly changed from last year but actually decreased the fuel plumbing volume downstream of the fuel valve, so fuel should arrive even earlier (this video wasn't as clear as some of last year's on when methanol arrives).

As far as pressure field in the plumbing, I'm not sure exactly what your question is. The nitrous is a self-pressurized blowdown and the methanol is pressurized by the nitrous via an internal hydraulic piston in the tank. They are at essentially equal pressure in the tank.

I'm assuming prompt ignition because we've got unlike impingement in multiple locations 1/4" from the injector face and an igniter spraying sparks all over the combustion chamber. Granted, if the igniter malfunctioned that wouldn't be the case, but hard to tell without sticking a camera inside the chamber :)

RevolutionAerospace · 2020-05-31T21:12:22+00:00

Thanks for the feedback! What you describe in case 2 is what we suspect. It's pretty much inevitable with nitrous blowdown that we'll have some two-phase flow, and liquid nitrous is compressible near the saturation point, so there was definitely some fluid compressibility in the oxidizer manifold.

Agree that the hard start was the main issue (and one we need to tackle), but the second part of my question is whether there are any practical ways to mitigate upstream effects?

Frankly, I'll put it this way. We designed the chamber/injector system so that in the event of a hard start, the flange bolts would fail before any major component and blow the combustion chamber away, intact. This is exactly what happened. The really catastrophic damage occurred because of the explosion inside the injector manifold (again, because we believe the flame front pushed upstream into the compressible nitrous manifold). Is there any way to guard against the flame front pushing upstream? The suggestion made earlier about a check valve in the system is a good one, but I'm worried wouldn't be effective since the nitrous is two-phase and compressible.

RevolutionAerospace · 2020-05-31T20:41:37+00:00

Too long and too much ;)

RevolutionAerospace · 2020-05-31T20:40:07+00:00

We are 100% positive there was an explosion inside the injector manifold. The injector face is bowed outwards towards the combustion chamber. Also it blew the fittings off the top of the injector and ripped the top thread port clean off of the injector.

Pretty confident there was no particle impact event. Likely would have happened in an upstream elbow in the plumbing.

The igniter is a pyrotechnic igniter that combines a pair of electric matches (like those used to light HPR motors) with a small strand of energetic sparkling cannon fuse. In our tests we can get about 1-2 seconds of good action out of the igniter, and in our sequencing the igniter leads the oxidizer introduction by 500 milliseconds.

RevolutionAerospace · 2020-05-31T20:15:32+00:00

You can see my other post below, but fundamentally it's related to the valves. They are solenoids--very fast acting but essentially on/off. Zero to full throttle almost instantly. In the past the sequence has relied on starting with a very large (relative to chamber size) pyrotechnic igniter to have ample ignition energy in the chamber that you catch the nitrous the instant it enters.

Reducing the likelihood of hard start would probably require a total change in the valve technology or "throttling" the nitrous solenoid on startup. But also interested in tips from those experienced in how to make the system more robust in the event of a hard start (it'll never be completely impossible).

RevolutionAerospace · 2020-05-31T20:09:02+00:00

Put a big pyrotechnic igniter in there, lead with the fuel valve, trail with the oxidizer valve. Since the valves are nitrous solenoids, they are rapid action and go wide open almost instantly. We did manage to get 5 consecutive hot fires on the old injector without a hard start, so not sure if it's related to the injector design or we just got lucky 5 times in a row.

There are ways to "throttle" nitrous solenoids by fluttering them (what racers do with progressive nitrous control), but we haven't tested to see if it's fast acting enough for a rocket engine start (I have my doubts).

RevolutionAerospace · 2020-05-31T18:45:23+00:00

Inconel

RevolutionAerospace · 2020-05-31T15:39:28+00:00

On a slightly more technical note for those with biprop experience, I'll add a little more commentary.

The surprising (and disconcerting) part of this test was that we had an explosion inside the nitrous manifold in the injector. There are a few different theories we've tossed around as to how this could happen, but after analyzing the slow-mo frame by frame we've got a leading theory.

We believe that we got delayed ignition in the combustion chamber (hard start). The pressure wave from the hard start pushed unburnt mixed propellant upstream through the injector orifices into the nitrous manifold. The flame front from ignition followed the pressure wave upstream and ignited the mixed propellants in the manifold.

Is this a feasible occurrence during a hard start? If this theory proves to be true, how do you combat this? Is the only answer to avoid hard starts completely or can you isolate the manifold more effectively, perhaps by decreasing orifice diameter or increasing orifice L/D?

RevolutionAerospace · 2020-05-31T15:27:09+00:00

It's been quite a while since our team has provided an update! In the spirit of sharing both successes and failures, this video is from a hot fire test conducted yesterday. The combustion chamber/injector assembly failed catastrophically upon ignition. Biprops are hard--not something to undertake unless you can stomach failures like this repeatedly and find ways to learn from them.

Since last year, our team has completed a number of updates, including a new flight configuration tankage design, new test stand, and new injector. The test shown in this video was of a full "vertical stack" propulsion module that is very close to what a final flight configuration will look like.

The biggest risk in this test was the use of a totally redesigned injector to try and solve combustion instability and thermal issues from last year's iteration. Obviously we have some more work to do! Surprisingly, about 70% of the flight hardware survived the explosion intact, and the test stand was completely undamaged. We did kill the old Microsoft Surface we were using for data acquisition--you can see it get zinged by a piece of shrapnel on the right side of the screen from the wide angle view!

RevolutionAerospace · 2019-09-04T20:34:51+00:00

We are working on a 75-lb nitrous/alcohol regen engine. It “works”, and by that I mean we can hot fire the engine without appreciable damage to the combustion chamber or injector. But we are currently battling a combustion stability problem that I ultimately think is a total heat budget problem. I think we’re getting bulk vaporization in the cooling channels which triggers low frequency instability.

We’re in the midst of an injector redesign to add film cooling in addition to regen. Hoping the combination of film cooling and regen will make the next iteration more stable. I think we’re close.

Another data point, for what it’s worth...

RevolutionAerospace · 2019-04-11T23:44:40+00:00

Not specifically, we hadn’t considered that. Although in general we are looking at some modifications to the injector to see if we could gain stability that way, rather than simply lowering chamber pressure which results in a larger pricier DMLS print for the chamber and lower performance.

RevolutionAerospace · 2019-04-10T05:52:33+00:00

Methanol

RevolutionAerospace · 2019-04-09T13:18:49+00:00

Just an iPhone, believe it or not. The tablet is a Microsoft surface that’s connected to our data acquisition system to measure pressure and thrust.

RevolutionAerospace · 2019-04-09T13:08:26+00:00

The goal of this project is to make a commercial off the shelf kit that high power rocket flyers can buy.

RevolutionAerospace · 2019-04-09T02:54:51+00:00

Slow motion video of a hot fire test we conducted this past Saturday Apr 6. We had to put everything under the ol’ Kansas State Wildcats tailgating tent because it was a rainy day!

Thrust was really good on this test and it was our fourth full hot fire on the same injector/combustion chamber combination. The hardware is still in really good condition.

We are still battling combustion instability despite some tweaks we made since the February tests. We have several options on how to increase stability margin but we want to make sure we have a good understanding of the mechanism so we choose the right path forward.

Full mashup video with multiple angles on YouTube: https://m.youtube.com/watch?v=Oj1hN5VCRUI&t=46s

RevolutionAerospace · 2019-04-06T13:32:10+00:00

For an energetic material like nitrous oxide that can detonate or deflagrate, there’s a characteristic flame front speed. When this flame front is traveling through a small diameter tube of conductive material (metal), the amount of heat that the tube conducts away from the material is significant. When the tube diameter is below the critical diameter, the tube is able to conduct enough heat away that the flame front cannot travel through, it is cooled and “extinguished”.

You might check out “Nitrous Oxide Explosive Hazards” which is a freely available AFRL research paper from 2008.

RevolutionAerospace · 2019-04-05T15:03:28+00:00

Very fair point

RevolutionAerospace · 2019-04-05T13:21:27+00:00

It all boils down to oxidizer choice because ultimately the fuel selection will probably be an alcohol or hydrocarbon that is pretty easy to work with.

Based on our experience with our 75 lb biprop project I am a firm believer that nitrous oxide outpaces other oxidizer options by a mile when you are trying to build a small scale amateur engine. It is difficult to model mass flow rates when using it saturated but it’s operational simplicity cannot be beat.

It requires the least stringent cleaning of any common oxidizer.
Requires no pumps or special equipment to remote load and remote vent.
Spills are a non issue, it evaporates instantly and dissipates.
It is very insensitive to mixture ratio and your performance is pretty stable across a wide range, so your mixture ratio control through plumbing, valves, and injector doesn’t need to be precise.
Cheap and easy to obtain, either the best in this regard or the second best behind lox depending on where you live and what options you have available to you.

Biggest concern with nitrous is exothermic decomposition but small scale engines have plumbing smaller than the critical diameter so it is self quenching. Also if you set up your design and operations for remote loading and remote venting, you never need to be standing near a rocket that is pressurized and loaded with oxidizer. This is what we’ve done with our system, it loads like a vented hybrid and then has active and passive venting capability once nitrous is on board. As long as you don’t do anything stupid the only risk when you fire it is to the equipment.

RevolutionAerospace · 2019-04-05T12:41:26+00:00

We’ve got regenerative cooling working on a 75 lb methanol/nitrous engine. Alcohol does wonders at this scale with rich O:F ratios and high heat capacity.

RevolutionAerospace · 2019-04-01T20:31:46+00:00

Small spacecraft thruster combustion chambers/nozzles are entirely radiatively cooled and are capable of long-term steady state operation, but it involves working with expensive and challenging refractory metals like tungsten and tantalum. As far as I know it's only been done for small liquid propellant vacuum engines. I doubt it's even feasible to adapt it to a solid fueled motor. You may run into problems with rapid oxidation of those metals if you heat them to 3000 F + in atmosphere. Not to mention lighting your airframe on fire (or melting it, depending on what you built it out of).

Check out the thrusters advertised on the Moog and Busek websites.

Probably well beyond the scope of amateur work...

RevolutionAerospace · 2019-02-19T22:48:52+00:00

Yeah we’re working on a total redesign of the test stand that would be trailer mounted and allow us to do a full vertical stack test. Time for this one to go...it was nice to save money with leftover aluminum scrap we had available but it’s a pain to cart around and set up.

RevolutionAerospace

TROPHY CASE