Solid Motor Thrust Vectoring

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Gip-Gip

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Hey all! Long time no see, just been busy with life and working on side projects (actually primarily working on v3 of my telemetry computer now that the chip shortage is over, but still)

Was wondering if anyone has attempted to throttle solid rocket motors by flowing a coolant through the propellant at a controlled rate. I'm going to produce a prototype when I have the time/resources (currently making a batch of high purity rocket candy by dissolving the ingredients and dehydrating it) but it would be cool to know if this has been previously attempted before.

Going to 3d print a test jig and etc. I already have the electronics more or less ready, including a pressure sensor and load cell.
 
Interesting concept. I haven't heard of anyone doing this. Why are you wanting to throttle a solid motor?

It sounds like it would require pressurized coolant at a pressure above the motor internal pressure. Might need to match the motor, propellant, nozzle, etc. to the coolant system. Should be easy for a test stand, but might be harder for flight hardware.

Let us know how it works. I haven't heard of anyone doing this so definitely let us know how this works.
 
Handeman said:
Interesting concept. I haven't heard of anyone doing this. Why are you wanting to throttle a solid motor?

It sounds like it would require pressurized coolant at a pressure above the motor internal pressure. Might need to match the motor, propellant, nozzle, etc. to the coolant system. Should be easy for a test stand, but might be harder for flight hardware.

Let us know how it works. I haven't heard of anyone doing this so definitely let us know how this works.
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The primary drawback of solid motors, afaik, is their inability to throttle, shut off, and reignite. I personally think if we are able to throttle solid rocket motors that it will make rockets in-general cheaper, safer, and more accessible to people.

But for hobby scale it would make a cool self-landing rocket more achievable.

Eitherway I plan to use air-pressurized water for the tests which should exceed the chamber pressure of a sugar candy rocket, in theory. In practice, on an actual (most likely medium or high powered) rocket it's most likely to be pressurized by a electric pump of some form, which with modern BLDC motors shouldn't be impossible to do.

Not sure how it will work out in practice but I'm excited to give it a shot :)
 
Currently CAD'ing up the test stand, parts are printing out
 

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sr205347d said:
Your question was about throttling, but the thread title was about TVC. Check the BPS.space web site for more on that.
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Oh my bad I got the words jumbled up in my head lol, they all start with Ts I was halfway close
 
You are not going to get anywhere by throttling a solid rocket motor by pumping coolant in it. First, the inside of a solid rocket motor is under high pressure and you will need to exceed that pressure to pump anything into it. Second, solid propellant burns underwater. Cooling it does nothing to the burn rate. In fact, increasing the pressure by pumping something into it is going to make it burn faster.

The US army has experimented with throttling a solid rocket motor by moving a pintle inside it to change the throat area. This does work, but is not practical because of the added inert mass, and the fact that you get the performance of a solid motor at the cost of a liquid engine.

To control the burn of a solid rocket motor, you do it ahead of time by making your propellant grain a shape whose burning surfaces will burn at a known rate to get you the pressure, and therefore thrust, you seek at any given point during the burn.

I have a degree in aerospace engineering, and have worked on solid rocket motors in a professional capacity and have worked with the people who did those pintle throttling experiments.
 
Antares JS said:
You are not going to get anywhere by throttling a solid rocket motor by pumping coolant in it. First, the inside of a solid rocket motor is under high pressure and you will need to exceed that pressure to pump anything into it. Second, solid propellant burns underwater. Cooling it does nothing to the burn rate. In fact, increasing the pressure by pumping something into it is going to make it burn faster.
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Yes the propellant burns underwater, but it still takes energy away from the reaction nonetheless and usually cooling reactions slows them down, or so is my hypothesis. If I'm wrong please correct me though.
 
Gip-Gip said:
Yes the propellant burns underwater, but it still takes energy away from the reaction nonetheless and usually cooling reactions slows them down, or so is my hypothesis. If I'm wrong please correct me though.
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So you've managed to squirt water inside a 600psi motor, say at 800psi. And then that volume of water flashes to 1500x volume.

Now it's also a steam rocket that has to lift the pump and tubing weight as well as all that water mass.

Maybe vectoring was achieved, maybe not. Miniaturizing the pump and associated power supply is much of the challenge.
 
Gip-Gip said:
Yes the propellant burns underwater, but it still takes energy away from the reaction nonetheless and usually cooling reactions slows them down, or so is my hypothesis. If I'm wrong please correct me though.
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I already did correct you. It's wrong. Water will not slow the reaction and the pressure increase from pumping something into the already-pressurized combustion chamber is going to cause the burn rate to increase, very likely by enough to cause a CATO.
 
Antares JS said:
Cooling it does nothing to the burn rate. In fact, increasing the pressure by pumping something into it is going to make it burn faster.
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Well, that depends on how much you pump through. Successful extinguishment of solids - whilst generally achieved by rapid pressure loss - has been successfully achieved with chamber dousing by either an inert fluid or water (sometimes with a detergent added for better wetting).

If I was to embark on a throttleable solid project, I would definitely be looking at liquid injection as the mechanism, but using the liquid injection to increase burn rate eg. utilise a propellant with a higher pressure exponent and inject something energetic like HTP to increase chamber enthalpy. The higher the exponent, the lesser the injected fluid required to achieve dF.

TP
 
Thrust measurement jig is halfway complete, I'm at least going to run an experiment to see what the results are, because I'm curious. Monkey brain satisfied by rocket testing, whether that is the smart way to do it or not.

Even if it increases thrust, that would be a desired effect imo since it is still adjusting the thrust of the engine. The mix/geometry could be tweaked to burn very slowly then add pressure to increase burn rate.

And I am working with sugar rocket fuel at this scale, so I'm not overly worried about a rapid motor disassembly. Though percautions will be taken and I will most likely have to wait for the next sunny day to test this outside.
 

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If you're injecting liquid into a solid motor, make it nitrous oxide. 😎
 
Correct me if I'm wrong, but couldn't you control the thrust of a hybrid by controlling the flow of the N2O? I would think that might be easier than trying to do "something" with a solid fuel motor... note that "easier" does not necessarily imply "easy".
 
cerving said:
Correct me if I'm wrong, but couldn't you control the thrust of a hybrid by controlling the flow of the N2O? I would think that might be easier than trying to do "something" with a solid fuel motor... note that "easier" does not necessarily imply "easy".
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Correct. If you wanted a fluid as an injectant for *active* solid propellant thrust adjustment, you probably wouldn't choose N2O as your 1st pick. There maybe some density impulse, plumbing, c* and specific impulse advantages choosing to actively throttle a solid over a hybrid, but they wouldn't be significant... or there'd be a lot more of them around... then again, it's not as if the military & commercial aerospace worlds are overcrowded with hybrids either.

Also, there are performance and stability limits to varying O:F ratios for things like hybrids. True, that monoprops like N2O and HTP are pretty generous there, but the fluid injection into a solid should make even less difference to performance given it's an apples-apples monoprop fluid.

TP
 
I wouldn't inject water nor nitrous oxide, because both of those compounds contain oxygen, which is going to increase your thrust. Water will flash into steam, and you end up with a steam-thrust motor. As for Nitrous Oxide, havn't you watched any of those fast 'n furious movies? All those turbo racer boys use NO2 to boost the compression in the engine and create more power (and when they do it wrong, throw pistons right through their engine block).

If you're going to inject *anything*, it has to be non-reactive. Pure nitrogen would be better than NO2. Argon inhibits burning and would be a better choice. But I'm not a chemist, nor an engineer. In fact, I'm a complete dummy. But even I know not to use water.
 
techrat said:
I wouldn't inject water nor nitrous oxide, because both of those compounds contain oxygen, which is going to increase your thrust. Water will flash into steam, and you end up with a steam-thrust motor. As for Nitrous Oxide, havn't you watched any of those fast 'n furious movies? All those turbo racer boys use NO2 to boost the compression in the engine and create more power (and when they do it wrong, throw pistons right through their engine block).

If you're going to inject *anything*, it has to be non-reactive. Pure nitrogen would be better than NO2. Argon inhibits burning and would be a better choice. But I'm not a chemist, nor an engineer. In fact, I'm a complete dummy. But even I know not to use water.
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You would inject N2O or H2O2 to *increase* thrust. That's why they're some of the prime candidates for active throttling of solids. Both liberate energy on decomposition, both are reasonably dense to store, N2O is self pressurizing and both will provide additional chamber pressure upon addition to the chamber and provided the solid propellant has a high(ish) pressure exponent, it shouldn't take much mass flow of the injected fluid to provide a significant increase to thrust (ie. injecting either N2O or H2O2 will increase overall Isp, add injected fluid mass flow and increase solid propellant mass flow via increased regression rate given a positive pressure exponent). Also, this fluid injection can help regulate chamber pressure to suppress overpressurization. Also, both will obviously contribute positively to overall impulse - much more than inert fluids.

And in case it needs spelling out: you obviously design the KN ratio of the solid to allow for the additional injection ie. it'd be lower than an equivalent classical (non active throttlable) solid. I realise that this doesn't relate strictly to the term "throttling", but for all intents, throttling and active thrust control should effectively be synonymous in this case as they're achieving the same purpose and result.

TP
 
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rocket_troy said:
You would inject N2O or H2O2 to *increase* thrust.
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Ah, the OP was originally intending to decrease thrust by injection. But yes, if you want to "throttle" a solid propellant, you could inject to INCREASE thrust, but then, this has already been done. It's a hybrid motor. Hybrid motors are already common in model rocketry.
 
techrat said:
Ah, the OP was originally intending to decrease thrust by injection. But yes, if you want to "throttle" a solid propellant, you could inject to INCREASE thrust, but then, this has already been done. It's a hybrid motor. Hybrid motors are already common in model rocketry.
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I wouldn't call it a hybrid. Technically it could (perhaps) be called that, but it's primarily a solid with <10% of the mass flow coming from the fluid injection.

What's more (as I've already said) hybrids typically suffer from inefficient c* realisation from the complications of finding the right O:F ratios with a changing fuel mass flow rate that's a function of both port geometry and varying regression rates which in turn is a function of oxidizer flux rates. A solid doesn't suffer those particular complications. Also, the oxidizers used in solids tend to offer considerable density advantages for those with volume restrictions.

If you were replying directly to the OP, then fair enough, but the mention of N2O sounded like you were responding to my last post, which itself was specifically responding to the N2O direction.

Nevertheless as I also said, I'm not advocating solids with active adjustable thrust as a serious alternative to the more mainstream forms of chem propulsion, but there maybe niche applications for it.

TP
 
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I sure appreciate your discussion. I have nothing to add, except that it is very interesting, and that kind, thoughtout responses like you are all doing here helps me learn, and makes me want to learn more.

Steve
 
techrat said:
I wouldn't inject water nor nitrous oxide, because both of those compounds contain oxygen, which is going to increase your thrust. Water will flash into steam, and you end up with a steam-thrust motor. As for Nitrous Oxide, havn't you watched any of those fast 'n furious movies? All those turbo racer boys use NO2 to boost the compression in the engine and create more power (and when they do it wrong, throw pistons right through their engine block).

If you're going to inject *anything*, it has to be non-reactive. Pure nitrogen would be better than NO2. Argon inhibits burning and would be a better choice. But I'm not a chemist, nor an engineer. In fact, I'm a complete dummy. But even I know not to use water.
Click to expand...
Nitrous oxide will increase thrust because it's a strong oxidizing agent. Water...no. Just because it contains oxygen does not mean that it will augment thrust the way oxygen does. Water is the "ashes" of hydrogen combustion, so to speak. Ashes do not burn well...

Yes, it will flash into steam under the proper conditions...and it doesn't evolve energy in doing so; it absorbs it.
 
prfesser said:
Nitrous oxide will increase thrust because it's a strong oxidizing agent. Water...no. Just because it contains oxygen does not mean that it will augment thrust the way oxygen does. Water is the "ashes" of hydrogen combustion, so to speak. Ashes do not burn well...

Yes, it will flash into steam under the proper conditions...and it doesn't evolve energy in doing so; it absorbs it.
Click to expand...

It doesn't evolve energy, but can evolve gas, hence pressure. The result can be more available energy for pressure/thrust from more energy efficient utilisation of the thermal capacity within the chamber. That's the theory anyway. This has been a concern for those investigating thrust/engine termination utilising water dousing.

"The results of these calculations showed that for reasonable injection rates, water would generate a maximum pressure somewhat less than twice the operating pressure. For Freon ZZ, the pressure increase could be as high as three times the normal operating pressure, and for N2O4, three and one-half times the normal operating pressure."

https://apps.dtic.mil/sti/citations/AD0804269
Of course, theory and experimentation don't always agree and from what I've seen of subscale testing, there's been no noticeable flashing induced pressure spikes from water injection during *termination* testing.

TP
 
One could very well use water. Modify the solid propellant to increase percentage of metals. Metals (Mg for instance, from ALICE propellant) are quite happy stealing the O from the H2O. One could design a solid propellant that will essentially smoulder - like a smoke grain - that requires injection of water to generate thrust. I haven't run the numbers on it, but water at least is reasonably dense, easy to handle, and incidentally is a great solvent for Ammonium Nitrate. So, sort of a hybrid, but not quite. Also lower than rocket oxidizer percentage H2O2 would be useful. 30% is probably still available as a farm supply - useful for cleaning but treat it with care.

I have run some numbers on this sort of thing. It is interesting.

Ditto on N2O injection. Also interesting. Nice ISP boost, and self-pressurizing can be useful.

Lots of ETCs.

HOWEVER, any such techniques are going to require a large enough motor to reap the benefits due to the extra mass requirement of the hardware implementation. In our small motors it is unlikely to be worth it, except for fringe cases. You do get to lose the liner for the portion containing oxidizer so that's a partial mass and volume offset. And if you can come up with a small high flow high pressure pump (please, somebody!) then the oxidizer tank can be rather thin-walled and therefore a lot lighter. Then things might get interesting.

Gerald
 
G_T said:
One could very well use water. Modify the solid propellant to increase percentage of metals. Metals (Mg for instance, from ALICE propellant) are quite happy stealing the O from the H2O. One could design a solid propellant that will essentially smoulder - like a smoke grain - that requires injection of water to generate thrust. I haven't run the numbers on it, but water at least is reasonably dense, easy to handle, and incidentally is a great solvent for Ammonium Nitrate. So, sort of a hybrid, but not quite. Also lower than rocket oxidizer percentage H2O2 would be useful. 30% is probably still available as a farm supply - useful for cleaning but treat it with care.

I have run some numbers on this sort of thing. It is interesting.

Ditto on N2O injection. Also interesting. Nice ISP boost, and self-pressurizing can be useful.

Lots of ETCs.

HOWEVER, any such techniques are going to require a large enough motor to reap the benefits due to the extra mass requirement of the hardware implementation. In our small motors it is unlikely to be worth it, except for fringe cases. You do get to lose the liner for the portion containing oxidizer so that's a partial mass and volume offset. And if you can come up with a small high flow high pressure pump (please, somebody!) then the oxidizer tank can be rather thin-walled and therefore a lot lighter. Then things might get interesting.

Gerald
Click to expand...
ALICE was Al-ice not Mg. The distinction is more than just semantic too. Mg is likely to provide too much condensed phase products, particularly post throat, to offer a practical alternative.
What you are describing there with those examples are very much what I would define as hybrids, yes I'd even shove some of them on the "classical" hybrid shelf.
Agreed on the practical scale limits. Dabbled manufacturing rocket pumps before here but I don't think that's the answer.

TP
 
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