Preliminary OHKLA design | Rocket body discussion | Forum

brmj said:I'm pretty sure the standard is to use liquid nitrous oxide rather than gasious. At 184.2 kelven and .1013 MPa, Wolfram Alpha says it will have a volume of about .08036 m^3. Using a diameter of 200 mm, that gives a tank 2.558 meters long. In practice, it will need to be a little longer because it won't be quite as wide as the outer diameter of the rocket, but the difference won't be huge.

Hmm.  I am not at all certain, but I suspect that this may actually not be the norm.  184.2 Kelvin is quite cold.  At that sort of temperature one would probably need an insulated tank, cryogenic-friendly valves, etc.  All the stuff you would need for LOX, basically.  And if that were so, why would anybody use N2O instead of LOX, when LOX offers better Isp?  Also, I recall that SpaceShipOne's engine (which used N2O) had a composite oxidiser tank (there are photos of it being made on Scaled Composite's website somewhere), which suggests that they weren't doing anything at particularly low temperatures.  After some brief Googling, this page lists as an advantage of N2O over LOX: "Unlike liquid oxygen, not cryogenic. Oxidizer at -60 C or -40 C is much easier to manage. Composite tanks can potentially be used" (as an aside, the same website has another page just on N2O which looks like a good source of knowledge/safety tips, and a page on paraffin and PE wax fuels).  All of this seems to me to suggest that gasous N2O is the norm.

brmj said:

Space ship one used liquid N2O, supporting the idea I had gotten that that was the standard.

Ah, fair enough!  I'm happy to accept that as fairly definitive.  I don't really understand well enough the interactions between pressure, temperature, density and solid vs liquid phase to think about these things clearly, I haven't had to think about it since undergrad physics.  I should probably brush up on it!

So, 4 metres long, at least very approximately.  On the current diagrams the tank looks quite a bit shorter than that.  Still, this isn't too big a problem: we've seen that we could afford to decrease the combustion chamber a little bit, and the currently allotted empty space for parachutes looks to me (admittedly based only on intuition) too large.  We should be able to find room for a roughly 4 m tank if we need it without increasing the height of the rocket much.  6 m should definitely be a workable upper limit.

I'm starting to feel like we are quite close to having the approximate dimensions and masses of most things sorted out, at least to within ballpark estimations.  The details will change, but at least we can be sure nothing is going to drastically change in size now (except maybe the fins).  The height and width of the rocket relative to the person standing next to it in the renders conveys the right impression.

This is motivating me to do more work on the pages to support propellant choice, so we can move on to more detailed considerations!  I need to focus on the SpaceUp presentation I'm supposed to be preparing for CLLARE first, though.

For what it's worth, right now I think quite strongly that N2O/Paraffin is the correct choice for our needs.  N2O because it is self-pressurising, removing the need for a Helium pressurant, and because it can be stored at sensible temperatures, removing the need for cryogen-friendly valves etc. (and overcoming what, based on readings in my hybrid rocket textbook, was a common problem for the AMROC people: LOX valves freezing stuck in half-open positions shortly after launch, leading to crashes).  Paraffin because, besides being very cheap, safe, etc., it has a very high regression rate compared to other solid fuels like HTBP (3-5 times higher!), which means that simple, single-port grain geometries can provide adequate thrust.  The "slumping" problem does worry me, but I'm sure we can come up with a solution.  The other benefits seem to make this small challenge worthwhile.

Just a small bump on this thread, since it has been suggested elsewhere that we really ought to just settle on propellant choices for OHKLA: this would let us (i) begin construction on relevant small static hybrid engines, and (ii) let us finalise the dimensions of the rocket which can facilitate more important work on things like aerodynamic stability, etc.

I think we should try to get this decision made in the next 7 days.  We can have IRC meetings if required.

I still think N2O is the way to go for oxidiser.  As far as I can tell, everybody agrees with this – at the very least, nobody has offered any convincing arguments against it.

With regards to fuel, things are a bit less clear.  Feed back from Copenhagen Suborbitals on paraffin fuel has made it sound like the higher regression rate that paraffin affords comes with some complicating factors.  That's not to say we should rule it out, but I don't think it is any longer an obvious or natural choice.

I just wanted to make a small comment on polyethylene (PE), another option for the fuel.  The Isp of N2O with PE is almost exactly the same as with paraffin (247 s vs 248 s), and the optimal O:F ratio is also the same (8:1).  The density of PE varies but it seems to be in the 0.8-0.9 g/cm^3 range, which is also pretty close to paraffin.  Basically the design we were converging to above could be changed to accomodate PE with very little effort.  The big difference is that PE will have a lower regression rate than paraffin – but at the same time it will be much easier to build multi-port grains (since PE is solid stuff you can just drill through), so this is not too big a problem.

The reason I bring up PE is that it's super easy to come by because it is used in a huge range of non-rocketry applications.  I think this is quite distinct from HTBP, which is mostly used for rocketry and hence harder to come by.  I found a place in Australia that sells rods of solid PE which are 1 m long and 200 mm in diameter – which is exactly the diameter of the rocket in our design above!  This means we could basically buy pre-fabricated rocket grains of exactly the size we need – all we would need to do is drill the ports down the middle.  This could, I think, be done pretty easily with a fairly standard drill press.  This is much easier than paraffin, which we'd have to buy in blocks, then melt it down and pour it into molds, etc, etc.  So it's definitely a strong contender.  Probably the biggest problem I can think of is that burning PE probably produces some moderately nasty fumes – but I doubt HTBP is all that much better in this regard (though I could be wrong).

If we were to have a meeting on making final propellant choices some time in the next week, who would be able to make it?  This invite extends to anyone who is interested, not just directors – antinode, KellyJ, Nick, you guys are all completely welcome to show up.  The more people who do, the better.