Making a decision on OHKLA propellants - deadline is 23/06/10, one week from today! | Hybrid engine discussion | Forum

Post edited 5:36 pm – June 21, 2010 by Luke Maurits

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Rocket-To-The-Moon said:

 

What are the thoughts on a single port, medium thrust motor? This would dramatically simplify casting and it would extend the burn time.

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Good to see you here, Rocket!

I'm not opposed to this in principle in any way (in fact, if we can show that it will work well enough, I would prefer it), but I would hesitate to throw too much support beore it until we have more numbers.  It depends on how much thrust "medium thrust" is, and how much extra fuel mass we need to include to make up for the gravity loss.  My gut feeling is that one port won't be enough, but that could be very wrong.  I think we should hold off on committing to a port geometry until we've done some test burns.  Let's choose our propellants, build a 1/4 scale test engine, and try burning some grains with different numbers and shapes of ports, record or estimate the thrust these different grains produce and then use some good software, like OpenRocket (or a vastly improved USOFS), to estimate the required fuel mass for full scale versions of these various grains.

The nice thing about HDPE as a propellant is that even if it turns out we genuinely need 7 ports to get enough thrust, it's really no hassle to do that: it's just drilling holes a rigid plastic.  7 ports is no more challenging than 1 port, which probably isn't true when casting fuel grains by pouring liquids over mandrels, etc.

Post edited 7:13 pm – June 21, 2010 by Luke Maurits

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Rocket-To-The-Moon said:

 What are the products of polyethylene and nitrous oxide combustion? Ethylene is C2H4 and nitrous oxide is N20.

 

C2H4 + 8(N20) → ???

 

I am sure that H20 is one of the products, probably CO2 also. The O:F is 8:1 so what I take from that is that it takes 8 oxygens to fully decompose the C2H4 (6 atoms which leaves two spare oxygens…). I'm no chemsit so I emailed my sister to see if she could work it out.

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The N55 rocket page claims:

The fuel for ‘PROTEST’ is polyethylene. Combustion of this material is clean, efficient and predictable.  The gases produced are mainly carbonoxide, carbondioxide and water

However, I think (I read something along these lines somewhere) that in addition to these main products there are a wide range of rather more complex molecules which are produced in much lower quantities.

The O:F ratio is oxidizer:fuel so all I can say with certainty is that it takes 8 N2O molecules to combust one PE molecule.  It may be the case that all the N2O just splits up into N2 and O and so the PE takes all 8 O molecules, but I suppose it might also be the case that some other nitrogen-oxygen molecules are formed as well?  I'm also not a chemist so I don't know how feasible this is.  It is probably well worth our time and effort to find out exactly what all the possible products, are though.  If your sister can help with this it would be fantastic.

Rocket-To-The-Moon said:

A centrifuge could be an interesting way to test for slumping. Depending on the thrust of the motor it would have to be a fairly substantial test rig, but I don't think it would be too complicated. It could easily be self powered by having the thrust vector slightly off center so that it spins in reaction to the thrust. We would then need a simple tachometer to measure the angular velocity. The biggest problem that I see is that the acceleration would vary greatly from the top of the motor to the bottom (perhaps useful to determine the acceleration limit).

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Hmm. If we went with the off-centre self-powered approach, the direction of the artificial gravity induced would not be exactly aligned with the axis of the engine, which would not accurately reflect flight conditions.  We could make the difference in alignment fairly minimal by mounting the engine only slightly off-centre, but the problem then is that only a very small proportion of the thrust vector is used to turn the centrifge, which means we only get a fairly gentle artificial gravity, prehaps too slight compared to flight conditions for the tests to be useful?  I haven't thought this through very carefully, and it may be possible to come up with a self-powered design which doesn't have these problems.

Rocket-To-The-Moon said:

As for construction of the centrifuge, a car or truck axle could be used and mounted vertically. Balancing the centrifuge could be challenging because both the thrust and mass will change with time.

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Gary Schnyder has, as previously mentioned, actually performed centrifuge testing of paraffin rockets, so if we decided that we did want to engage in this sort of research further down the track I am sure he would be able to give us a lot of great advice.

Rocket-To-The-Moon said:

I wonder if there is a way to add another substance (tri-propellent) that would boost the efficiency by reacting with unused substances. If so, I would assume that it has already been investigated. This brings up the subject of some sort of consumable matrix material to help hold the grain together. A matrix that adds to the thrust would be ideal. A reactive metal like magnesium comes to mind.

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I've definitely heard of people adding stuff to grains to increase thrust – and magnesium is a common choice, like you suspected – but this usually comes up in the context of fuels which are cast from liquid form, where it's really easy to mix some stuff in during the casting process.  The only time I've heard of it being done with PE is where small hobby rockets have cast PE grains by buying small pellets of PE, melting the down and then pouring the liquid into a cast.  However, this approach completely eliminates what I think of as one of the biggest advantages of PE: that you can just buy it in solid cylinders off the shelf and there's no troubling casting involved.

I suppose you could insert thin rods of magnesium into drilled holes in a solid PE grain, and that should work.  You can buy magnesium in wire form, right?  That could be an option.  I think this should be fairly simple and cheap to test on our 1/4 scale engines, so we might as well.

I've enjoyed posting back and forth with you in real time, but my netbook battery is starting to get really low so I'm going to have to leave after this post.  There's a lunch break coming up at this conference soon, however, during which I can dash back to my hotel room and get a bit of a recharge, so if you reply to this post I'll probably be able to reply to that sometime in the next two hours.

Rocket-To-The-Moon said:

It must be possible to melt and recast PE. We will probably need experience doing this because the full scale OHKLA motor will be quite large.

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I have no doubt that it is possible, but I consider it highly undesirable.  Doing this basically eliminates the strongest arguments in favour of PE: that you don't need to do any messy or tricky casting.  You just buy it, drill it, and you're done.  If we were to melt and cast it as a matter of routine then I think HTBP becomes a lot more competitive.

I'm also not convinced that it will be necessary.  I have seen HDPE for sale in 300 mm diameter solid rods on the web, with length ranging from 1m to 5m in 1m intervals.  Surely we won't need a grain larger than that?  When we were doing calculations for paraffin, which is roughly as dense as HDPE, we were using a grain diameter of 200 mm, and that was based on a substantial overestimate of the fuel required.  I suspect a 200 mm rod or thereabouts will be entirely adequate.  Even if we require a length of grain which is longer than the longest 200 mm rod we can find (something else I doubt), I suspect we could get away with simply gluing lengths together rather than melting anything.

Thanks a lot for that information, Nick!

Most of those products are relatively familiar as commonly-used combustibles, used in cooking, heating, welding, etc.  This makes me feel a little better about the issue of it producing anything too nasty – although the last three are probably exceptions to this.  Being more complex molecules, I would assume they are produced in considerably lower proportions than things like methane, though.  It seems like PE is more clean burning than one would expect!

Post edited 7:41 am – June 24, 2010 by Luke Maurits

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Hey, great work.  I feel like this list should go in the Wiki somewhere, although I'm not immediately sure where.

EDIT: Having the NFPA diamonds on those pages was a great idea.  I looked at the last three, which sound potentially scary, and they all have a Health danger score of 2: "intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury".  Not too bad.

What do people think of me writing up a brief (<= 5 pages) report on the propellant decision, using the really nice LaTeX template Denis made for us?  Basically putting all the rationale for the decision somewhere a little easier to find and more permanent than the forum thread?