Pyrovalves / Pyrotechnic valves

[jjrgray]

Hi guys,

Does anybody know anything about pyrovalves being used in hybrid engines. I have done a bit of research and I think Aerotech and Contrail use them in a few of their commercially available hybrid engines? I am designing and building a hybrid engine with a few of my mates (one of who is writing an EPQ on it) and I would very much like to use a pyrovalve instead of a solenoid valve in-between my internal engine tank and my combustion chamber so that I can ignite both the fuel grain and start the flow of oxidiser at the same time in a relatively easy way. I have so far not had any luck finding pyrovalves online for the scale I am working on (indeed for model rocketry at all), with all the hits coming up as commercial rocket pyrovalves which cost literally thousands of pounds/GBP (I live in the UK).

Any help would be much appreciated - I just want to learn.

Many thanks,
Jack

 

[heada]

Search for floating U-C injector. The injector acts as bulkhead between the upper internal NO2 tank and the combustion chamber. The upper tank is filled through the nozzle and injector. A "pyro valve" is placed on the fill tube below the U-C injector so that when the pyro valve is fired, it burns through the fill tube releasing the NO2 into the combustion chamber via the injector. Skyripper used this setup as well as Rattworks and Alpha I believe. I don't know of any monotube hybrid that didn't use a U-C injector with a pyro-valve. I know that the assembly instructions for Skyripper is still rattling around the internet.

 

[jjrgray]

Search for floating U-C injector. The injector acts as bulkhead between the upper internal NO2 tank and the combustion chamber. The upper tank is filled through the nozzle and injector. A "pyro valve" is placed on the fill tube below the U-C injector so that when the pyro valve is fired, it burns through the fill tube releasing the NO2 into the combustion chamber via the injector. Skyripper used this setup as well as Rattworks and Alpha I believe. I don't know of any monotube hybrid that didn't use a U-C injector with a pyro-valve. I know that the assembly instructions for Skyripper is still rattling around the internet.

Thanks for the advice man. Newbie to hybrids here (done some work with solids before though) so could you possibly explain some of the technical terms you used in your answer? Big thanks again.

 

[justforfun]

Found this https://web.archive.org/web/2006062...ocuments/SRS_38mm_Instruction_manual_V1.4.pdf

 

[heada]

Single 6061 Al tube setup into 2 sections; a forward section that acts as the oxidizer tank and holds your liquid Nitrous and an aft section that is your combustion chamber. The forward bulkhead has a small vent in it to allow for air to escape as it is being filled and a stream of liquid Nitrous when the oxidizer tank is full. Between the oxidizer section and the combustion chamber is a bulkhead called a "U-C injector" named after the 2 people that came up with the idea at the same time. That bulkhead/injector seals the oxidizer tank section but also allows for the flow if nitrous into the oxidizer tank section during fill and down to the combustion chamber during fire. It does this by way of a nylon tube that goes through the injector. The nylon tube is cut/burned and allows for the flow of nitrous when a slug of propellant is started. That propellant is your pyro-valve.

Search for the assembly instructions for a Skyripper hybrid motor. Those instructions give step by step assembly for a monotube hybrid motor that should apply to most monotube hybrids that use a floating U-C injector.

 

[jjrgray]

Found this https://web.archive.org/web/2006062...ocuments/SRS_38mm_Instruction_manual_V1.4.pdf

Cheers, I had a look at that, however I think that the fill line/igniter/pyrovalve combo is not going to work with the engine I'm developing. In terms of a pyrovalve, what I need to make my engine work is a valve that is usually closed which is opened (and remains open for the duration of the flight) by the firing of a small pyrotechnic charge. Like a solenoid valve, however rather than being electrically actuated it is pyrotechnically actuated.

 

[jjrgray]

Single 6061 Al tube setup into 2 sections; a forward section that acts as the oxidizer tank and holds your liquid Nitrous and an aft section that is your combustion chamber. The forward bulkhead has a small vent in it to allow for air to escape as it is being filled and a stream of liquid Nitrous when the oxidizer tank is full. Between the oxidizer section and the combustion chamber is a bulkhead called a "U-C injector" named after the 2 people that came up with the idea at the same time. That bulkhead/injector seals the oxidizer tank section but also allows for the flow if nitrous into the oxidizer tank section during fill and down to the combustion chamber during fire. It does this by way of a nylon tube that goes through the injector. The nylon tube is cut/burned and allows for the flow of nitrous when a slug of propellant is started. That propellant is your pyro-valve.

Search for the assembly instructions for a Skyripper hybrid motor. Those instructions give step by step assembly for a monotube hybrid motor that should apply to most monotube hybrids that use a floating U-C injector.

Ah I get you now. Thanks for all your help however I don't think that the engine I have built is monotube (?). It consists of a metal combustion chamber with end caps on and an oxidiser insertion tube running from the rear endcap (injector) to a nitrous tank. I think when I referred to pyrovalve I meant a valve, like a solenoid valve or ball valve, but rather than being actuated by electricity or hand it is actuated by a solid charge. I intend to place the pyrovalve in-between my combustion chamber and tank so I think that U-C injector solution may not work in this scenario. Many thanks anyways.

 

[jderimig]

Cheers, I had a look at that, however I think that the fill line/igniter/pyrovalve combo is not going to work with the engine I'm developing. In terms of a pyrovalve, what I need to make my engine work is a valve that is usually closed which is opened (and remains open for the duration of the flight) by the firing of a small pyrotechnic charge. Like a solenoid valve, however rather than being electrically actuated it is pyrotechnically actuated.

Respectfully, if you are a newbie to hybrids, then why not design a motor that uses a few proven principles? What is your concept for timing of the valve operation and the ignition of your motor?

 

[heada]

Ah, yes. You're setting it up like a normal liquid motor would be set up. Those don't scale down very well and require much more complex plumbing and flow control (like advanced valves) and so aren't popular in hobby rocketry but are more common in amateur rocketry. You won't find many people here that have worked with a setup like that.

 

[jderimig]

Have you looked at the Hypertek method of initiating oxidizer flow and ignition timing?

 

[jjrgray]

Respectfully, if you are a newbie to hybrids, then why not design a motor that uses a few proven principles? What is your concept for timing of the valve operation and the ignition of your motor?

This is a very valid point indeed. If I could go back and start from the beginning I would absolutely choose a more simple monotube design, however my group is overall more than half a grand into building and developing this engine so it would be a hard bullet to bite to revert back to square 1. To be fair to us this is the first major blunder we have come across at all and with relative ease we have built other parts of the engine. We also do have help from multiple aerospace engineering students from Imperial University London. But I do agree that the monotube design is the way to go. (Slap on the wrist for me)

 

[jjrgray]

Have you looked at the Hypertek method of initiating oxidizer flow and ignition timing?

No thanks for the advice I'll have a little look at it now.

 

[jderimig]

This is a very valid point indeed. If I could go back and start from the beginning I would absolutely choose a more simple monotube design, however my group is overall more than half a grand into building and developing this engine so it would be a hard bullet to bite to revert back to square 1. To be fair to us this is the first major blunder we have come across at all and with relative ease we have built other parts of the engine. We also do have help from multiple aerospace engineering students from Imperial University London. But I do agree that the monotube design is the way to go. (Slap on the wrist for me)

Ok we have to make Lemonade. Explain why you need to actuate with a pyro charge? you know the pyro charge will need to be outside the motor right. You can't have any ignition source on the high pressure side of the motor.

 

[jjrgray]

Ah, yes. You're setting it up like a normal liquid motor would be set up. Those don't scale down very well and require much more complex plumbing and flow control (like advanced valves) and so aren't popular in hobby rocketry but are more common in amateur rocketry. You won't find many people here that have worked with a setup like that.

Yes sorry I should really have prefaced my overall engine design on the first post of this thread, it would have saved everyone a lot of breath. To be fair though as it's a hybrid I am only dealing with one pressurised substance (Obv my fuel grain is solid) so the plumbing so far has been bearable... up until now. I may just have to use a (normally open) solenoid valve and forget about using a pyrovalve, because as you said and as I am painfully finding out now these kinds of valves don't scale down very well.

 

[UhClem]

While Aerotech hasn't manufactured their hybrid reloads for almost two decades now, documentation describing them is still available on their web site.

 

[jjrgray]

Ok we have to make Lemonade. Explain why you need to actuate with a pyro charge? you know the pyro charge will need to be outside the motor right. You can't have any ignition source on the high pressure side of the motor.

The idea for the pyro charge is so that both the ignition of the fuel grain and the flow of nitrous oxide into the fuel grain are done pyrotechnically therefore decreasing the amount of solenoid valves I need to use. I say this because I cannot think of a good way to get a solenoid valve to do the job of the pyrovalve and open the combustion chamber up to the flow of oxidiser as an always closed solenoid valve would require coding or an external signal to remain open during the flight after ignition, both of which are one more thing to go wrong on top of this engine and rocket being experimental, and an always open solenoid valve which is energised and closed before filling seems pretty damn sketchy.

 

[jderimig]

But you can't have your pyro valve in the high pressure stream before it is expanded in the combustion chamber, otherwise your motor will explode spectacularly. You can have your pyrovalve at the injector but not too much before. What is your injector design?

 

[jjrgray]

But you can't have your pyro valve in the high pressure stream before it is expanded in the combustion chamber, otherwise your motor will explode spectacularly. You can have your pyrovalve at the injector but not too much before. What is your injector design?

My injector design is basically a cylindrical end cap which tapers down from 2 inches to 1/2 inch which I then connect to a short length of 1/2 inch pipe (in order to stop flames from the combustion chamber backing up into my hose and causing a flashback into the tank) and from there to a hose which connects to my oxidiser tank. It aint pretty but it works.

 

[jjrgray]

But you can't have your pyro valve in the high pressure stream before it is expanded in the combustion chamber, otherwise your motor will explode spectacularly. You can have your pyrovalve at the injector but not too much before. What is your injector design?

That thing about explosions, and spectacularly at that, may be an issue I didn't yet see.

 

[jderimig]

So can you get a rod up through the nozzle that had a conical end that will fit into the injector and seal it off if the rocket is tied down to the pad with some tension?

 

[jjrgray]

So can you get a rod up through the nozzle that had a conical end that will fit into the injector and seal it off if the rocket is tied down to the pad with some tension?

Yes I think so.

 

[jderimig]

Then you tie rocket down, fill, get a good fire going in the combustion chamber, release your tie downs with a pyro charge and you're off to the races.

 

[jjrgray]

Then you tie rocket down, fill, get a good fire going in the combustion chamber, release your tie downs with a pyro charge and you're off to the races.

Sounds good boss . Thanks for everything.

 

[G_T]

Perhaps, perhaps not. I'd want to see more details of what you have in mind. For instance compute out the cooling effect of the expanding nitrous in your combustion chamber, and whether your "good fire" gets simply blown out the nozzle by the very cold gas stream.

How long is your combustion chamber, what is your fuel, and how are you getting it lit? A pyrovalve ignites the start of the N2O stream. Once the N2O is decomposing it releases enough heat to keep it going. Simply doing what amounts to a quick open of a valve allowing large gas flow into the chamber which has fire in it is more likely to quench the fire IMHO. But, it all comes down to the details of the design.

Gerald

 

[jderimig]

Then how about leaking a wee bit of N2O through the seal rod into the combustion chamber during the fill? Timing is probably almost everything. This method of ignition is about half the Hypertek system. If you have gaseous oxygen then you can put steel wool in there with some O2 and get a REAL good fire going.

 

[jjrgray]

Perhaps, perhaps not. I'd want to see more details of what you have in mind. For instance compute out the cooling effect of the expanding nitrous in your combustion chamber, and whether your "good fire" gets simply blown out the nozzle by the very cold gas stream.

How long is your combustion chamber, what is your fuel, and how are you getting it lit? A pyrovalve ignites the start of the N2O stream. Once the N2O is decomposing it releases enough heat to keep it going. Simply doing what amounts to a quick open of a valve allowing large gas flow into the chamber which has fire in it is more likely to quench the fire IMHO. But, it all comes down to the details of the design.

Gerald

So do you think it would be better to use my other idea of having a solenoid valve before the injector to initiate the flow of oxidiser and then having an ignitor/solid propellant charge in the combustion chamber to ignite the mix? If I did it like this I could stage the opening of the solenoid valve so that the the oxidiser starts flowing before the solid propellant igniting, which would then mean the cold stream of gas would be less likely to 'blow the fire out' because the fuel grain is being ignited in an already oxidiser rich environment?

 

[rocket_troy]

Simply doing what amounts to a quick open of a valve allowing large gas flow into the chamber which has fire in it is more likely to quench the fire IMHO. But, it all comes down to the details of the design

It does indeed depend. But that's essentially the way Kline and Pooley did it with their valve. Lots depends on the initial flux rate and chamber length, but providing all those boxes are ticked, the *products* of the (pre-nitrous flow) combustion also act to catalyse the N2O decomposition assisting the chamber's heat capacity.

TP

 

[OverTheTop]

Make sure you have plenty of distance between whatever you build and test and people or items you want to keep safe. I have been at a nearby pad (not too near thankfully) when a hybrid detonated during launch. Big bang. Butterflied the tank open, trashed the airframe and completely bent, badly, the launch rail.

 

[jjrgray]

Make sure you have plenty of distance between whatever you build and test and people or items you want to keep safe. I have been at a nearby pad (not too near thankfully) when a hybrid detonated during launch. Big bang. Butterflied the tank open, trashed the airframe and completely bent, badly, the launch rail.

Understood. I already have a big field where we static test and I'm working on making more and more of the GSE remote so there can be a big exclusion zone in case of explosions and shrapnel. All about that safety!

 

[G_T]

Pre-releasing N2O has its own issues. It's a bit of an organic solvent. It's a bit of a sensitizer for some organics. Actually there is quite a list of things it isn't really compatible with. You need to use grease and O-rings that are suitable for an oxidizer rich environment. Pre-releasing N2O around a fuel grain made of some materials makes the grain potentially mildly explosive rather than simply combustible. Depends on all the details. So anyway I'm not at all in favor of pre-release of N2O through the combustion chamber, where it would essentially soak things you are about to light. To me it has BAD PLAN written all over it!

For that matter, get a little bit of organic contaminant in your flight oxidizer tank and you greatly lower the required temperature and/or pressure for detonation.

N2O is a monopropellant and can propagate a blast wave through the liquid. It - usually - doesn't go back through injectors when the size is small like anything we would use. Rule of thumb as I understand it is 1" pipe is about where normal liquid N2O starts to be possible for a blast wave to propagate back to the tank. In other words N2O doesn't scale well to large motors.

A typcal hobby (and commercial and university research from what I've read) hybrid rocket motor has a low particulate percentage in the combustion chamber and has a lot of acoustic energy present. That is, resonances. Think of it like the inside of a steam whistle or fog horn. It can make a lot of sound. Sound is pressure waves. The chamber pressure of your average underdesigned hybrid is oscillating all over the place. Liquid goes in through the injectors. It decomposes releasing heat and then burns with the fuel releasing more heat. Chamber pressure goes up. Less liquid oxidizer goes in now because the pressure drop across the injectors has decreased. So now chamber pressure decreases because there is less monopropellant oxidizer flow to feed the combustion. Now that the pressure drop across the injectors has increased, more flows in... Rinse and repeat.

If you somehow manage to get backflow through the injectors during these oscillations, boom. Not CATO. Boom. Big Boom.

The difference is your typical CATO is simply an overpressurization event when you get down to how it failed. The hardware couldn't contain it so it ripped open. It's quite possible the rest of your fuel didn't even keep burning. In any event it wasn't instantly consumed to feed an explosion.

Boom = detonation. The pressure is much higher. The timescale is much shorter. The fragments are much smaller, and moving faster.

Funny but inclusion of perhaps just 10% O2 into the N2O makes it a better oxidizer, while eliminating that particular mode of boom (as long as there are no contaminants). Unfortunately it introduces other issues.

N2O is a monopropellant and energetic enough in its own right to be able to detonate. Most oxidizers used in commercial rocket motors are not going to be monopropellants. So they won't have that mode of detonation failure.

They still have a second mode of detonation failure. If the fuel and the oxidizer tanks get ruptured such that the resultant gasses or gas and vapor mix, the ratio could end up within the range needed for a fuel-air explosion. And of course there is plenty of ignition source sitting right there. If you can't find your test stand with a liquid motor, that's probably what happened.

Solid propellant motors that are decently designed can have very smooth combustion.

Liquid propellant motors are notoriously difficult to get to burn smoothly.

Hybrids are in between these two, closer to the liquid side of things.

There are things you can do about it. My EX hybrid burns as smoothly as a very good solid propellant motor. Perhaps I got lucky; perhaps I made some good design decisions. But that would be getting out of allowed subject area for this open discussion forum. Sorry!

Gerald