Large Motor Ingintion

[oriansbelt]

Hey guys, this is my first post in years so its good to be back.

 

 

I am starting the design process for a fairly large hybrid rocket engine as a senior project. This is a group project but I think I will be focusing on the fabrication and implementation of the combustion chamber. This job includes ensuring ignition of the hybrid engine.

 

First, here are the facts about the rocket engine:

-hybrid engine design

-Nitrous Oxide(NOX) oxidizer

-Hydroxyl-terminated polybutadiene (HTPB) rubber fuel: we will probably add AL powder to the HTPB to enhance the motor's impulse

-M class rocket engine; >250 lb of thrust

-The combustion chamber is roughly 18-24" long and 1.5" diameter

-designed as a second stage motor so we are going for mid-air ignition

 

The hard part about this design is the in-flight ignition since we need to ensure quick ignition of the HTPB fuel grain to prep it for the NOX. Most hybrids use a small chunk AP rocket fuel to preheat the fuel grain. This would take too long for us and plus if the NOX is injected into the chamber while the AP is still burning it might cause an over pressurization of the chamber and cause a CATO.

 

To solve this problem I want to use copper thermite suspended in a HTPB matrix as a preheating coating on the inside of the combustion chamber. Copper thermite burns very quickly and does not produce any gas so it seems like it should work well. However, I'm not sure that the thermite reaction will still occur evenly and quickly when it is suspended in the HTPB. I would like your thoughts on the matter.

 

I suspect that the reaction will be greatly slowed if not halted. Ideally the entire combustion chamber would be ignited in under 2 seconds but I don't know what to suspect from doing something like this. Does any one have experience or thoughts on the matter?

[Nessalco]

I've flown a couple hundred hybrid motors, both commercial and research, and have (unsuccessfully) attempted a 2 stage flight. It is NOT an easy task.

 

I don't believe cupric thermite would work for the application you're planning. As you know, it's a high energy compound, goes up quickly, and in a motor the size you've described you'd be limited to somewhere around 4 grams (to avoid over pressurization). I don't believe it would have enough duration to preheat the grain, burn through the valve (I assume this is a U/C motor), and initiate the reaction. If you mix the thermite in another compound (such as HTPB) it will indeed slow significantly - enough so you might as well use APCP. Also, when I've compressed thermite or placed it in a matrix, it becomes quite difficult to light.

 

Ignition lag is an inherent problem with hybrid motors. The fastest to light that I've seen have used 50 grain pyrodex pellets as the preheaters (as many as 6 in an M motor). These work well since they will ignite reliably from an ematch, where APCP it is far less certain.

 

I assume you're using some kind of electronics for flight control - another approach would be to light the preheater a bit in advance of first stage burnout. Since the thrust of a hybrid motor tails off so seriously during vapor phase, you'd lose almost nothing in impulse.

 

Good luck!!

 

Kevin

 

PS> You do want the N2O to enter the combustion chamber while the preheater is still burning - otherwise you'll likely quench the motor. The initial pressure spike of any hybrid motor can be problematic - I've blown a couple of cases just by changing the ignition geometry of a motor.

 

PPS> Have you considered catalytic ignition? Would be hard to pull off but would certainly give you 'instant on'

Edited May 22, 2011 by Nessalco

[DeepOvertone]

How about a step priming approach. IE Main propellant on the outside, Couple mm of main propellant mixed with granular magnesium in the inside of that, mm or so of AP fuel as an inner coating on the core. I'm not sure how easy it would be to cast a grain that way. I think the granular magnesium might work because it would react slower than a powdered MG and still give a lot of heat.

[oriansbelt]

DeepOvertone:

With the way I'm currently planning to cast the grain a layered system should be do-able. I think I might just cast a layer of APCP for preheating on the inside of the rubber fuel. I would custom formulate the comp to be plenty hot to ignite the rubber so and additional layer of Mg wouldn't be necessary.

 

 

Nessalco:

I'm glad to have your expertise on this matter.

 

I was hoping to be able to use quite a bit of thermite (~30-60grams) to preheat the grain since it doesn't produce gas. I had forgotten that it would heat the gas in the chamber to high pressure and could cause over pressurization. I will have to run through the calculation to see how much pressure that will actually produce.

 

I can't think of how you would use a U/C valve on a second stage hybrid. We will be using a servo actuated valve to supply the NOX at a precise time after stage separation (we use separation charges to sever the stage connection). This means that we just need to preheat the grain and then allow NOX flow. I don't think we have had quenching problems with our static-test motor so I suspect that we will inject the NOX after the preheating setup is completely extinguished. Another cool thing about the servo valve is that we will close the valve before all the NOX is used to control the apogee of the rocket (our goal is the get to 25,000ft, not over or under)

 

 

Catalytic ignition would be really cool! Do you have any idea on how to do that? I have never heard of that before with NOX.

[Nessalco]

A complex project, to say the least.

 

The issue with hybrid motor ignition is not the preheater itself, but the transitional pressures at startup. This will be complicated by the fact the motor you're trying to light will be moving through the air, which will certainly change the pressures in the combustion chamber during the ignition sequence.

 

Before I say any more - do you have flying versions of the motors you well be using? Or proof of concept models that have flown?

 

Also - Is the first stage also hybrid? If so it must be a big honker....

 

Kevin

 

PS> Flow N2O across a heated nickel screen and it will decompose and spontaneously ignite as a monopropellent with an ISP in the mid 140s

[oriansbelt]

Haha, yes. We like to take on project that are probably a little bigger than we can actually chew. If they work it is really awesome and if they don't we learn a whole bunch anyway.

 

Please tell me more about the transitional pressure problem. This is something that I am unfamiliar with.

 

No, we don't have any that have flown. All previous test engines have been just that, static test engines. This is the first one that we intend to fly.

 

Haha, no. The first stage will be a solid, I would love to do a two stage hybrid someday though . Actually this reminds me, I got to see a hybrid fly at the beginning of the month that made it to 60,000ft in one stage. It was about 20ft tall. I believe National Geographic was filming the whole thing so you might want to keep an eye out for that documentary.

 

Hhmmm, I think that experimenting with auto ignition is beyond what we are capable of this year but I would really like to do that with our old static test engine. Maybe I can get some more guys interested and start a side project on that. It would be really cool .

[dagabu]

"Large motor ignition is 90% black magic and 10% dumb luck."

-Jason Newski-

 

I have been reading this thread with both fascination and dread. The two stage hybrid rocket is kind of like Sir Hillary reaching the top of Mount Everest. It is attainable but very difficult. In my time in the late 1990s up to the mid 2000s that I was playing with large model rockets, my group tried three two stage rockets with one of them being a hybrid design. It was always the second stage that disappointed us, the most "famous" of which was the 600mph lawn dart in North Dakota in 2002 that was caused by the igniter not having the heat necessary to preheat and then ignite the second stage on time.

 

It turns out that it is below zero at 8000' ASL even in the summer at times and that it takes a lot more heat to get a J burning at that height then it does at 640' ASL at 92°F. You know what? I really dont miss LDRS much.

 

Good luck and take video!

 

-dag

[Nessalco]

Haha, yes. We like to take on project that are probably a little bigger than we can actually chew. If they work it is really awesome and if they don't we learn a whole bunch anyway.

 

I see you're from California - hopefully you'll be flying way out in the desert. Way, way, way out....

 

Will you be working with RRS or TRA for the actual launch? There are a hatful of permits you need to put anything into the air.

 

 

Please tell me more about the transitional pressure problem. This is something that I am unfamiliar with.

 

There is a fair amount of information out there about hard starts and hybrid motors. It's all about the ratios between injector and nozzle area and initial nitrous pressure. The situation you're considering is complicated by an initial negative pressure in the combustion chamber, low temperature, and complicated ignition dynamics.

 

An excellent series of papers was composed by Rick Newlands of Aspire rocketry. They are available free on line. Also, Bill Colburn's book on hybrid design principles is a must read. It's available from Aerocon.

 

 

No, we don't have any that have flown. All previous test engines have been just that, static test engines. This is the first one that we intend to fly.

 

 

Seriously? And the first flight you're going to attempt is an M class motor in a complex setup??

 

(For the non-rocketeers - to give you an idea of the scale, a mid-range M class motor will lift a 65 pound rocket to 5000'...... Or a 25 pound rocket to 20000' or so.... It could also drive that 25 pound rocket into the ground at Mach+)

 

I often serve as RSO (Range Safety Officer) at regional and local launches. The RSO is the final authority at any launch, and must approve every flight before it can boost. With all due respect, if you were to show up at a launch with a project like this with a *J* hybrid motor in the second stage, and that motor had never been flown - I'd deny the flight. If you came wanting to launch the M hybrid as a first stage motor for its first flight, and had a good paper trail, I'd probably allow the flight, assuming all other airframe and construction guidelines were met. Of course, I'd put you out at the Way Away cell (1500' or more from spectators).

 

Haha, no. The first stage will be a solid, I would love to do a two stage hybrid someday though . Actually this reminds me, I got to see a hybrid fly at the beginning of the month that made it to 60,000ft in one stage. It was about 20ft tall. I believe National Geographic was filming the whole thing so you might want to keep an eye out for that documentary.

 

 

Sounds like one of Jeff Jakob's projects. He flies in Southern California, and is probably the leading American flyer of research hybrid motors. He sells stuff on-line if you need pieces-parts.

 

Hhmmm, I think that experimenting with auto ignition is beyond what we are capable of this year but I would really like to do that with our old static test engine. Maybe I can get some more guys interested and start a side project on that. It would be really cool .

 

Look, I don't mean to come across as a pedantic ass, but PLEASE be careful in your efforts. The energies potentially available in the event of a catastrophic failure are far more than you might think - just ask the folks on the SpaceShip One project. Consider failure modes carefully - if you don't design the motor to fail correctly, what you've created is a fragmentation bomb.

 

Using servo valves is rarely done in this sort of application - they are not reliable under the pressure and temperature changes that occur, and are very heavy. A servo valve with the flow capacity to run an M motor would weigh ~6 pounds - plus batteries. Also, the tiniest traces of moisture in the system can freeze and block the valve's proper function. Pyrovalves are much more frequently used, and could serve exactly the same purpose you've described - with a much lower risk of failure. An appropriate pyrovalve for initiation (normally closed) can be designed into the injector, which essentially automates the nitrous release and would carry very little weight penalty. The main flow valve (normally open) could come in at around 6 ounces.

 

Another alternative is a pneumatic system and high-pressure ball valves, though you'd have to take precautions against moisture in the lines freezing up the actuators. The advantage here is the system could serve the dual function of being part of the recovery chain. There are partial systems available on-line.

 

Avoid the temptation to make the whole system overly complex. For example, to get back to your initial question about motor ignition - if I were attempting this, I'd use a cluster of Pyrodex pellets well secured in the fore bulkhead of the combustion chamber as the preheater, initiated with (redundant) ematches. I'd then use a pyrovalve in the injector to initiate the flow and light the motor (google Alpha hybrids for an example) and a second pyrovalve to terminate the flow when desired.

 

I'd bet such a system would come up to full operation in less than four seconds, which is comparable to many large APCP motors. If ignition timing is critical, use your on-board electronics to start the ignition sequence a little earlier in the timeline. In short, rather than trying to overcome an inherent charectaristic of hybrid motors (ignition lag) - accept it, work with it, and go from there.

 

Finally, unless you're very experienced at casting HTPB I'd discourage you from using it, especially doped with any metals. The regression rate of the fuel is too high, the possibility of splintering quite high, and metal doped fuels are prone to runaway reactions and chamber overpressure. If you have a leak at the injector face HTPB fuels can also become saturated with nitrous and form explosive compounds. PVC or ABS are better choices for both safety and performance reasons, and allow design of more compact combustion chambers.

 

Like Dag said - take video. It will either be cool or REALLY cool.

 

Kevin

[Nessalco]

Here's an analysis of why my attempt at a 2-stage hybrid was "really cool". The 1000 people watching thought so, especially since the rocket flew *away* from the crowd.

 

Kevin

 

PS> Sorry if this is getting too far off topic.

Fail Final.pdf

[dagabu]

Its on You-Tube as well under Two Stage Hybrid Rocket. At least the second stage lit.

 

-dag

[oriansbelt]

I see you're from California - hopefully you'll be flying way out in the desert. Way, way, way out....

 

Yes, we would be waaaayy out in the Mojave desert. We have access to a special launch site, it is in military air space so the normal regiment of permits are not required. This site is the best launch site I've every seen, it has bunkers that everyone gets in for the more dangerous launches as well as a bunch of other support infrastructure. It looks like a commercial rocket test site to me. The last time we were there the military launched a cruise missile that was supposed to fly right over us .

 

 

Seriously? And the first flight you're going to attempt is an M class motor in a complex setup??

 

Haha, I know. It is still up in the air if we are going to actually fly this engine. We are designing it so that it could be flown but it will go through probably 6 months of bench tests before we could decide if it was flight worthy. As I mentioned, this is a senior project so it is going to have quite a bit of paperwork backing it up as well as safety guidelines.

 

 

I think your design would probably be a good design to stick with. We are just in the first brainstorming parts of the design sequence so what I've mentioned are just the preliminary thoughts on how things will work. We are going to look in the different options for valving the engine. I think that pyro valves are the way to go but we are going to explore other options form a large hybrid design company(I believe they make the hybrid engine for SpaceShipTwo).

 

 

Thanks for all the input, I'm really glad that I started this thread. I gained a lot more incite as to where we should go with this design than I expected. We will definitely have a lot of decisions to make over the next 6 months. I will try and keep you updated on our testing regiment, I'll post some of the bench test firings on here. I'll definitely have a video of the flight if it ever gets that far .

 

 

That was a pretty awesome failure BTW. It actually could probably be called a limited success considering the second stage when up .