Hybrid oxidiser system HELP?!?

[jjrgray]

Hi everyone,

First post on here so don't bite my head off if I say something completely stupid. The background to this thread is that I need some help with designing the oxidiser system in a hybrid rocket that myself and some other student friends of mine are making. I have dabbled in solid motors before so I think I have a few core concepts down. Obviously this is the first time myself and my friends have tried to branch out into hybrids so we are seeking some help. We have also enlisted the help of a few aerospace engineering students at Imperial University in London, however they don't have much if any experience in hybrids so they are just helping with nozzle and aerodynamic equations right now. Sorry for that very long winded explanation however I believe full transparency is best. Hopefully some of you guys can help us out with all your knowledge!

1. Can someone please explain why using liquid nitrous oxide would be better than using gaseous nitrous oxide as the oxidiser (is it because of a higher density)?
2. Is it necessary when using a nitrous oxide system to also include a pressurising vessel holding helium in order to further pressurise your oxidiser?
3. How does one turn gaseous nitrous oxide into liquid nitrous? Is it just down to pressurising the gas to a certain point (critical pressure + temp)?
4. Is mixing glue sticks into a paraffin fuel grain a viable option to stop the wax from melting so easily and expelling un-combusted droplets out of the engine?
5. Has anyone got any experience using aluminium powder in order to increase the regression rates in paraffin fuel grains, and if so is around an 8% aluminium powder 92% paraffin mix to low or high?
6. I have attached an initial sketch of an idea for a nitrous oxide system to this post. Is an idea feasible or would it just not work the way I have laid it out?

I really would appreciate answers/criticisms to any of the above questions and I'm sorry if I sound a bit stupid but we all have to start somewhere.

Many thanks.

 

[prfesser]

Hello jjrgray,

I suggest that you join the Arocket mailing list. The archives have a LOT of information on liquids, hybrids, and solids. Go thru the archives before you ask your questions, as I think most if not all of them are answered in the archives.

The proper place for amateur rocketry on TRF is the (Restricted) Research forum, but as you are (presumably) outside the US it's a non-starter. You must be a US citizen and level 2 or 3 certified in NAR or TRA to request access.

Best -- Terry

 

[Tim51]

Hi everyone,

First post on here so don't bite my head off if I say something completely stupid. The background to this thread is that I need some help with designing the oxidiser system in a hybrid rocket that myself and some other student friends of mine are making. I have dabbled in solid motors before so I think I have a few core concepts down. Obviously this is the first time myself and my friends have tried to branch out into hybrids so we are seeking some help. We have also enlisted the help of a few aerospace engineering students at Imperial University in London, however they don't have much if any experience in hybrids so they are just helping with nozzle and aerodynamic equations right now. Sorry for that very long winded explanation however I believe full transparency is best. Hopefully some of you guys can help us out with all your knowledge!

1. Can someone please explain why using liquid nitrous oxide would be better than using gaseous nitrous oxide as the oxidiser (is it because of a higher density)?
2. Is it necessary when using a nitrous oxide system to also include a pressurising vessel holding helium in order to further pressurise your oxidiser?
3. How does one turn gaseous nitrous oxide into liquid nitrous? Is it just down to pressurising the gas to a certain point (critical pressure + temp)?
4. Is mixing glue sticks into a paraffin fuel grain a viable option to stop the wax from melting so easily and expelling un-combusted droplets out of the engine?
5. Has anyone got any experience using aluminium powder in order to increase the regression rates in paraffin fuel grains, and if so is around an 8% aluminium powder 92% paraffin mix to low or high?
6. I have attached an initial sketch of an idea for a nitrous oxide system to this post. Is an idea feasible or would it just not work the way I have laid it out?

I really would appreciate answers/criticisms to any of the above questions and I'm sorry if I sound a bit stupid but we all have to start somewhere.

Many thanks.

If you're 'starting from somewhere' in the UK (...?) then check out the Aspire Space website: http://www.aspirespace.org.uk/ for the theory behind all things hybrid. Aspire was set up by rocketeers active in the Scottish Aeronatics and Rocketry Association (http://www.sara.rocketry.org.uk). Again, if you're in the UK - there's also a small group of very experienced Level 3 flyers into hybrid in Manchester (the ones who did the Top Gear Robin Reliant spaceshuttle) who regularly fly at Midland Rocketry Club (www.midlandrocketry.org.uk). Bear in mind that using motor hardware that is not commercially manufactured will mean your rocket motor is classed as experimental - the full implications of that (and they are numerous) are set out in the UKRA safety code (see www.ukra.org.uk). Best have a read of that first..

 

[dhbarr]

A word of caution: Nitrous is not nearly so tame as people have a tendency to treat it. Please be sure to use Oxygen-safe practices if you continue down this road.

 

[jjrgray]

A word of caution: Nitrous is not nearly so tame as people have a tendency to treat it. Please be sure to use Oxygen-safe practices if you continue down this road.

Thanks for the advice man. This is why I am asking about liquid vs gaseous nitrous oxide. I've come to understand that liquid nitrous is a lot safer as it doesn't carry the risk of detonation as it does in the gas state? Would that be correct?

 

[jjrgray]

Hello jjrgray,

I suggest that you join the Arocket mailing list. The archives have a LOT of information on liquids, hybrids, and solids. Go thru the archives before you ask your questions, as I think most if not all of them are answered in the archives.

The proper place for amateur rocketry on TRF is the (Restricted) Research forum, but as you are (presumably) outside the US it's a non-starter. You must be a US citizen and level 2 or 3 certified in NAR or TRA to request access.

Best -- Terry

I'll definitely have a look at the Arocket archives. Thanks for the help!

 

[jjrgray]

If you're 'starting from somewhere' in the UK (...?) then check out the Aspire Space website: http://www.aspirespace.org.uk/ for the theory behind all things hybrid. Aspire was set up by rocketeers active in the Scottish Aeronatics and Rocketry Association (http://www.sara.rocketry.org.uk). Again, if you're in the UK - there's also a small group of very experienced Level 3 flyers into hybrid in Manchester (the ones who did the Top Gear Robin Reliant spaceshuttle) who regularly fly at Midland Rocketry Club (www.midlandrocketry.org.uk). Bear in mind that using motor hardware that is not commercially manufactured will mean your rocket motor is classed as experimental - the full implications of that (and they are numerous) are set out in the UKRA safety code (see www.ukra.org.uk). Best have a read of that first..

Yep I do understand the implications of not flying commercially available motors, however we are building the engine because it's part of an EPQ dissertation. I have looked at the UKRA safety code and have lined up a big open space to launch from in the future which should more than satisfy the class of engine we intend to build. I know that as it is experimental it carries extra risk, but we plan to test and launch under a very controlled environment and we are already building the necessary GSE to do both from afar.

 

[dhbarr]

Thanks for the advice man. This is why I am asking about liquid vs gaseous nitrous oxide. I've come to understand that liquid nitrous is a lot safer as it doesn't carry the risk of detonation as it does in the gas state? Would that be correct?

CGA G-4.1, Cleaning of Equipment for Oxygen Service
and
CGA O2-DIR, Directory of Cleaning Agents for Oxygen Service

 

[jjrgray]

CGA G-4.1, Cleaning of Equipment for Oxygen Service
and
CGA O2-DIR, Directory of Cleaning Agents for Oxygen Service

Once again thanks and I'll be sure to read that. Any specifics on liquid nitrous oxide vs gaseous nitrous oxide. I'm just trying to learn and any answers would be much appreciated. I fully understand the need for safety around high pressure gases.

 

[rocket_troy]

I'd be surprised if the details you're asking for wasn't in the links provided by Tim51. Nevertheless, the density difference between liquid and gas N2O obviously depends on the temperature of the fluid - lower the temp, bigger the difference.

TP

 

[jjrgray]

I'd be surprised if the details you're asking for wasn't in the links provided by Tim51. Nevertheless, the density difference between liquid and gas N2O obviously depends on the temperature of the fluid - lower the temp, bigger the difference.

TP

Thanks for the graphs mate it makes a little more sense now. So it looks like if I keep the vapour pressurised and cooled then I should be able to pump liquid nitrous from my ground tank into the vehicle's internal tank? Would this then therefore mean having a helium pressurisation tank is necessary due to the oxidiser already being under pressure?

 

[dhbarr]

Thanks for the graphs mate it makes a little more sense now. So it looks like if I keep the vapour pressurised and cooled then I should be able to pump liquid nitrous from my ground tank into the vehicle's internal tank? Would this then therefore mean having a helium pressurisation tank is necessary due to the oxidiser already being under pressure?

Not trying to be a debbie downer, but please read this before continuing:
https://www.csb.gov/file.aspx?DocumentId=6022

 

[jjrgray]

Not trying to be a debbie downer, but please read this before continuing:
https://www.csb.gov/file.aspx?DocumentId=6022

Thats quite a long report indeed and I shall take the time to read it. I understand that there are risks to using nitrous oxide, and I know that a considerable amount of danger is carried with those risks. However as I stated before myself and the others working on our project are working to ensure a safe launch and test environment which will include remote firing/filling/venting/dumping of the rocket and an exclusion zone in the middle of a quite large rural area. I completely get that you aren't being a Debbie downer and I appreciate that you are just trying to ensure safety. I do not question at all that your issue you are raising is a valid one, however I think the whole reason I came onto this forum, like I have others, is to try and find solutions to these safety issues. So as you can see from my questions on the first post of this thread, I am merely trying to ask others for their experienced opinions and advice so that I have the specific technical knowledge in order to conduct safe construction and tests of the engine and vehicle.

 

[rocket_troy]

You only need external pressurants if you're really chasing density ie. are chilling your N2O below practical feed pressures for engine operation. In most instances, no you don't need them.
You have a fill cylinder that sits on the ground and an engine tank within the rocket that gets filled and a hose that connects the 2 for the transfer for the fluid. You typically have a vent orifice (tiny hole) at the top of your tank to let gaseous N2O to vent. This controls the temperature (and in-turn pressure) of your N2O within the engine tank. The larger the orifice, the cooler your tank's N2O will get. This temperature difference between your fill cylinder and engine tank will create the pressure difference to allow for the N2O to be pressure fed from fill cylinder to engine tank. You obviously need either a siphon in your fill cylinder or to invert the fill cylinder to ensure you're feeding the *liquid* into your engine tank.

TP

 

[jjrgray]

You only need external pressurants if you're really chasing density ie. are chilling your N2O below practical feed pressures for engine operation. In most instances, no you don't need them.
You have a fill cylinder that sits on the ground and an engine tank within the rocket that gets filled and a hose that connects the 2 for the transfer for the fluid. You typically have a vent orifice (tiny hole) at the top of your tank to let gaseous N2O to vent. This controls the temperature (and in-turn pressure) of your N2O within the engine tank. The larger the orifice, the cooler your tank's N2O will get. This temperature difference between your fill cylinder and engine tank will create the pressure difference to allow for the N2O to be pressure fed from fill cylinder to engine tank. You obviously need either a siphon in your fill cylinder or to invert the fill cylinder to ensure you're feeding the *liquid* into your engine tank.

TP

Thanks for all the help, you definitely cleared a lot of questions up for me. In terms of the vent orifice, is that kept open all the way through filling your flight/engine tank, or does it act like a pressure relief valve only opening at a certain pressure and closing when the pressure decreases? And I understand about how the temperature differential leads to a pressure differential therefore helping you fill, but how is that pressure differential maintained if there is essentially an open hole/orifice in your flight tank? Also if your GSE fill tank is pressurised then surely you could just pump nitrous straight into your flight/engine tank?

cheers

 

[rocket_troy]

In terms of the vent orifice, is that kept open all the way through filling your flight/engine tank, or does it act like a pressure relief valve only opening at a certain pressure and closing when the pressure decreases?

Personally, I generally keep it open - it's a fixed orifice for most of my smaller flights. You can bang-bang it or modulate the flow or use a PRV to regulate the temp+pressure or just close it for flight - it's entirely up to you.

but how is that pressure differential maintained if there is essentially an open hole/orifice in your flight tank?

The orifice essentially cools the N2O in the engine tank which lowers its vapour pressure. A set orifice throughput will maintain this equilibrium once reached. The fill cylinder is generally much larger with more thermal inertia and greater heat soak through (greater area) cylinder walls and whatnot to maintain a greater vapour pressure than the actively cooled engine tank.

Also if your GSE fill tank is pressurised then surely you could just pump nitrous straight into your flight/engine tank?

You could... you could even gravity feed it in. But be very careful with pumping N2O: the only truly compatible seals for N2O are Kalrez and if you have an impressive property portfolio to mortgage away to pay for the seals, then sure, pump away. See, you can get away with using EPDM o-rings (even Buna/nitrile) for N2O sealing if they're in a static environment with good heat soaking grooves around, but all bets are off for pumping applications. N2O is one of the best solvents going around - it will dissolve (permeate) into just about any elastomeric sealing material going around. What's more, you still need to vent the engine tank, because if you don't, your *fluid* pump will become a *heat* pump ie. pumping heat from your fill cylinder into your engine tank with nowhere to go other than through wall conductivity/radiance - a cycle best to avoid.

TP

 

[jjrgray]

Personally, I generally keep it open - it's a fixed orifice for most of my smaller flights. You can bang-bang it or modulate the flow or use a PRV to regulate the temp+pressure or just close it for flight - it's entirely up to you.



The orifice essentially cools the N2O in the engine tank which lowers its vapour pressure. A set orifice throughput will maintain this equilibrium once reached. The fill cylinder is generally much larger with more thermal inertia and greater heat soak through (greater area) cylinder walls and whatnot to maintain a greater vapour pressure than the actively cooled engine tank.



You could... you could even gravity feed it in. But be very careful with pumping N2O: the only truly compatible seals for N2O are Kalrez and if you have an impressive property portfolio to mortgage away to pay for the seals, then sure, pump away. See, you can get away with using EPDM o-rings (even Buna/nitrile) for N2O sealing if they're in a static environment with good heat soaking grooves around, but all bets are off for pumping applications. N2O is one of the best solvents going around - it will dissolve (permeate) into just about any elastomeric sealing material going around. What's more, you still need to vent the engine tank, because if you don't, your *fluid* pump will become a *heat* pump ie. pumping heat from your fill cylinder into your engine tank with nowhere to go other than through wall conductivity/radiance - a cycle best to avoid.

TP

What kind of pressure do you usually hold your larger fill tank at?

 

[rocket_troy]

What kind of pressure do you usually hold your larger fill tank at?

Not sure what you're exactly referring to with the word "hold" there? There is absolutely no active temperature control of the fill cylinder and generally no passive measures either unless the ambient temperature is outside my comfortable zone. On hot days or very cool days I will utilise a thin reflective insulation jacket over it and I might preheat (warm) the cylinder inside my car in an attempt to achieve something close to 15-20 deg C starting temperature for flights that might be marginal T:W.
As for what the operating pressure ranges of the fill cylinder turn out being throughout the fill - I personally don't measure them, so I can only estimate.
As for T:W though - the N2O is actually working for you because on hot days the vapour pressure of your tank is likely to operate higher which assists countering the lower density and vice versa on cold days - the higher density helps counter the lower dP.

TP

 

[cwbullet]

Be very careful on this topic. Avoid discussing formulas or fuels. This topic has stayed pretty much on the topic of a commercial hybrid-like topic.

 

[jjrgray]

Not sure what you're exactly referring to with the word "hold" there? There is absolutely no active temperature control of the fill cylinder and generally no passive measures either unless the ambient temperature is outside my comfortable zone. On hot days or very cool days I will utilise a thin reflective insulation jacket over it and I might preheat (warm) the cylinder inside my car in an attempt to achieve something close to 15-20 deg C starting temperature for flights that might be marginal T:W.
As for what the operating pressure ranges of the fill cylinder turn out being throughout the fill - I personally don't measure them, so I can only estimate.
As for T:W though - the N2O is actually working for you because on hot days the vapour pressure of your tank is likely to operate higher which assists countering the lower density and vice versa on cold days - the higher density helps counter the lower dP.

TP

Sorry for the confusion all I meant was what's the pressure of the fill cylinder before you begin filling the engine tank? Is the N2O fed into the engine tank only using the differential temp and therefore pressure (i.e. your N2O fill cylinder is pressurised only to ambient pressure), or is the N2O feeding assisted by the temp/pressure differential but also carried out under the high pressure of the fill cylinder? Sorry for all the questions I just want to be safe when I do it.

 

[jjrgray]

Be very careful on this topic. Avoid discussing formulas or fuels. This topic has stayed pretty much on the topic of a commercial hybrid-like topic.

Understood, I will refrain from asking anymore questions about fuel mixes. Is asking about N2O systems and GSE allowed (these are my main problems)?

 

[cwbullet]

Understood, I will refrain from asking anymore questions about fuel mixes. Is asking about N2O systems and GSE allowed (these are my main problems)?

That is not a problem. Fuel mixes and formulas skate across legal concerns that we need to avoid.

 

[jjrgray]

That is not a problem. Fuel mixes and formulas skate across legal concerns that we need to avoid.

All understood chief .

 

[Voyager1]

I typically maintain the fill tank at between 650-850 psi which generally achieved by keeping the tank temperature between 15-25 C. As Troy suggested above, to keep the temperature down in Summer, I use a reflective blanket (e.g., car windscreen sun cover). To raise the temperature in Winter I will leave it in the sun for a while to get the pressure up into the above range.

 

[jjrgray]

I typically maintain the fill tank at between 650-850 psi which generally achieved by keeping the tank temperature between 15-25 C. As Troy suggested above, to keep the temperature down in Summer, I use a reflective blanket (e.g., car windscreen sun cover). To raise the temperature in Winter I will leave it in the sun for a while to get the pressure up into the above range.

Cheers for the help man. So therefore then you would be filling with gaseous N2O from the GSE tank and then in your engine tank having in turn into liquid N2O by using the vent in order to lower the temperature?

 

[Voyager1]

No, the fill tank has an extractor tube that feeds liquid nitrous from the bottom of the fill tank into the flight tank.

 

[jjrgray]

No, the fill tank has an extractor tube that feeds liquid nitrous from the bottom of the fill tank into the flight tank.

Okay, so the nitrous should be in liquid state already before filling the engine tank?

 

[rocket_troy]

Yes, providing the N2O is below its critical point, there should be a liquid layer (bottom) and above that a (lighter) gas layer within your cylinder. You want to draw from the liquid - hence the siphon tube, or invert the entire cylinder if there's no siphon tube eg. a medical grade N2O cylinder.

TP

 

[jjrgray]

As I have only ever handled medical grade like cylinder during my GOX/paraffin static fires, could you clarify what you mean by siphon tube?

All this help is much appreciated guys.

 

[rocket_troy]

The cylinders with the siphon tubes can be pretty much identical to the medical grade cylinders, only they're used for automotive engine boosting applications (eg. drag racing).
The siphon tube is simply a small diameter tube that extends down from the main valve manifold to the bottom of the cylinder so it draws whatever fluid is on the bottom instead of the top. Much like CO2 fire extinguishers.

TP