Moving forward with OHKLA | Project planning and misc | Forum

Well, voting on the OHKLA propellants has closed, with PE/N2O being the winner by a huge margin.  I will make a blog post about this later tonight.  The poll results were in broad agreement with the sentiment discussed on the forums, but also we saw more votes than there are people regularly active on the forums, so I am going to assume a few lurkers read through everything and saw no obvious problems.  This is a fairly good sign that our high-speed, tightly-focused approach to taking care of the propellant choice issue worked well and didn't devolve into a simple popularity vote – or maybe I'm just being optimistic?

Anyway, this effort has definitely worked to get more momentum behind the OHKLA project and I am keen to see it continue.  So I think that in short order we should do exactly the same thing with another design decision.  It probably makes sense to focus for now on decisions relating to the rocket engine, since that brings us closer to the point of being able to build small scale prototype engines which are similar to the real engine in meaningful ways.  Some "open" design tasks which we could immediately begin profitable work on are:

• Oxidiser tank material
• Combustion chamber material
• Nozzle cooling mechanism

I'm not sure if it would be best to choose one of these and have the entire community focus on it entirely, or if it would be better to delegate individual tasks to individual members and have us cover more ground quickly.  At any rate, I think we should come up with some strategy that ends in us doing a one week long, poll-assisted decision similar to the propellant one sometime in the next week or two tops, to keep our momentum up.  What do people think about this?

On an unrelated note:  I dont' know how many of you saw it, but our contact at PSAS said a while ago said that he felt PSAS would best be able to help us on OHKLA with regards to communications and electronics, and asked how we would feel about "splitting" the project so that CSTART (and possibly others!) works on the rocketry and PSAS works on the avionics.  This didn't seem to get much attention from the CSTART community: perhaps now is time to change that?  If we can continue to make good progress on the rocketry front and have PSAS working on the avionics front simultaneously, we might be able to really move ahead on OHKLA quite quickly.  This would only require a little work on our part to set out the precise requirements we need from the avionics.  I see no reason not to proceed in this manner, what do others think?

Rocket-To-The-Moon said:

I think that the oxidizer tank and the combustion chamber can be grouped into one lump.

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I'm not too sure about that.  I mean, if we were to decide to manufacture both items ourselves, then I can see a strong argument for using the same material for each.  But at the same time, we may be able to buy a pre-built tank (as you mention below), in which case that argument disappears, and even if we are manufacturing them both ourselves, the fact is there are distinct requirements and situations (large, high pressure, moderate temp vs small, moderate pressure, high temp) and so possibly different tradeoffs in the option space.  If we're going to take the engineering process seriously, I think they need to be considered separately.

Rocket-To-The-Moon said:

Are we planning on using gaseous or liquid N2O? Liquid I would assume. Here is a phase diagram. It looks like we could get liquid N2O at around 8Mpa (~1200psi) so it will take a relatively strong tank. I think that the ideal solution would be a pre-made aluminum tank similar to a SCUBA tank.

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The idea is that we use liquid N2O, but at room temperature a small portion of it boils into gaseous, pressurising the tank, which is one of N2O's selling points: we don't need a separate high pressure helium system to force it out of the tank.  Most of the figures I have read have been around 700 psi for a room temp N2O tank – I asked Nick about his university project and he said they saw about 750 psi.  The phase diagram you linked to is a little hard to read because the vertical axis is logarithmic.  Anyway, whether it's 700 or 1200 psi, it does need to be a strong tank.  I like the idea of using a pre-made tank, especially if we can find one that's aluminium and of an appropriate size and length:radius ratio.  We should probably look into this.  Perhaps we should replace the "oxidizer tank material" task on the tree with a "home-made or pre-made oxidizer tank" task?  To make that decision we could research the range of pre-made tanks out there and see if there is anything out there that meets our criteria.  If we decide to go pre-made, then the next node on that subtree would be to choose an actual tank – if we go home-made, then the next node would be material.  What do people think about this?

Rocket-To-The-Moon said:

The combustion chamber could easily be aluminum also, maybe with at thin steel insert to help protect against heat in the event that the fuel burns all the way to the casing.

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I am also currently inclined toward aluminium for the combustion chamber, but definitely want to see us do a proper pros vs cons analysis.  Aluminium is trickier to weld than steel, for instance, and if the difference in mass is small enough that may become an important factor.  And then there's the temperature issue.  If aluminium would require O-rings to keep combustion away from the wall and steel would not, that's another push in the direction of steel.  We should definitely do the research on this, especially asking Nick questions about his project's combustion chamber.  Maybe this decision would be a good one to make the next push on?

Rocket-To-The-Moon said:

My thoughts are that we should hold off on the nozzle cooling because that could be a fairly technical challenge.

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To be clear, I didn't mean for us to soon tackle the task of actually designing the nozzle: by "nozzle cooling mechanism" I meant us making a choice between the 3 broad types of nozzle cooling we have to choose from: regenerative, ablative and radiative.  Regenerative would be pretty much immediately ruled out by simplicity concerns, I would think, so it's really just a matter of ablative vs radiative.  The decision would largely come down to the cost and accessibility of the materials compatible with each.  Once we've settled on ablative or radiative, further design tasks would be specified for material, shape, etc.

Rocket-To-The-Moon said:

I have no issues with letting PSAS handle the electronics suite.

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Great!  I would want to hear at least one more director or prominent member agree with this before feeling confident going ahead with it, however.

rpulkrabek said:

 

I think the next design task is the O:F ratio. The rest seems to fall into place. We can then continue forward with the CAD model.

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Not entirely sure what you mean by this?  The O:F ratio which yields the best Isp with PE and N2O is 8:1.  That's a fixed fact and not something we get to choose.

We do have to determine the geometry of the fuel grain and the oxidiser mass flow rate in order to achieve an O:F ratio as close to 8:1 as we can, but that's a function of multiple separate design tasks rather than one in and of itself.

rpulkrabek said:

I might be able to handle material choices. I have access to a tool where I can choose material properties I want to maximize/minimize (elastic modulus, density, specific heat, etc.) and then choose the material that most closely matches the result. I just need to know the constraints involved. I think we should use a method like this, as opposed to a simple vote, however, we can narrow down the choices using a certain method, and then have a vote between a handful of choices while providing pros and cons.

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This sounds pretty useful.  I feel like we might as well stick with the voting system, but have the results of your tool's output on the relevant Wiki page, in the "supporting calculations" section.  That way people can read that and take it into account.  If there really is a single obvious choice that trades off best between all the materials I think people will just vote for it.

I guess for both the oxidiser tank and combustion chamber we want to minimise density.  For the oxidiser tank we want to maximise whatever physical properties correlate with "amount of pressure the thing can hold before bursting", and for the combustion chamber we want to minimise whatever properties correlate with "amount of energy the thing can absorb before melting" (a combination of melting ponit and specific heat, I guess?).

Even before we properly set up a deadline, polls, etc, if you wanted to do some work in that direction and post it to the forum, that would be awesome.  We are going to need to choose the materials before we can get precise dimensions, since the heavier the tank and chamber turn out to be, the more propellant we are going to need.

Are you sure that top image is for hybrid rocket fuel grains and not solid rocket fuel grains?

The bottom image is definitely refering to solid fuel grains because the first option, "end burning" makes no sense at all with a hybrid.

Choosing a geometry is a little tricky because you can't do it well purely with numbers – we'd need to actually do some small test burns to see what kind of thrust we get from different numbers of ports.

I also feel like we need to do some more research on exactly how the O:F ratio (which changes along the length of the grain) varies with various parameters before we can design the grain geometry with much confidence.  I'll reborrow that hybrid rocketry textbook soon and have a read in that direction.

For my part I think it makes sense now to focus on tank and chamber materials.  This choice is largely independent of the grain geometry, and once we know it we can start making small test chambers out of the same material as the real one, and do test burns to gather data to inform the grain geometry choice.

 

rpulkrabek said:

What type of polyethylene have we chosen as the fuel grain? UHMW-PE, HMW-PE, HDPE, MDPE, LDPE?

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As far as I have been able to tell, HDPE is by far the most commonly used kind of PE for hybrid rocket fuels.  I think I kind of fell into the trap of using PE and HDPE interchangably throughout the entire process for this reason.  So while at times we may have just spoken about PE, I think in practice we've basically selected HDPE.  Of course, if someone can present some data and/or arguments to suggest that one of the other kinds is a better choice, it's no trouble to change.

rpulkrabek said:

I noticed there was a change to the wiki under OHKLA Resources that had an empty section titled Information on HDPE. I added two links giving material properties for HDPE (extruded and molded). Is it decided to use HDPE? What was the reasoning behind choosing HDPE as opposed to UHMW-PE or LDPE?

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Yeah, I created that page this morning just so I could clear out my Firefox tabs of useful stuff I'd accumulated during the propellant decision process.  I didn't have anything open on HDPE.  As mentioned just above, I think we've defaulted to HDPE because it's the only form I've seen so far actually used in rockets.  This includes in static test rockets built by people at Stanford etc., people who probably know very well what they are doing.

Intuitively, I would say LDPE is a poor choice simply because of its low density.  The lower density the fuel grain, the larger the combustion chamber must be to hold a given mass of fuel, and hence we have a heavier chamber requiring slightly more fuel.  I can't give an immediate answer for UHMW-PE.