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

Post edited 3:20 am – June 16, 2010 by Luke Maurits

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Hi everyone,

In light of:

1. The fact that OHKLA (indeed, CSTART in general) has stalled very badly,
2. Recent discussions elsewhere on this forum making it seem clear that finally making a decision on the OHKLA propellants will allow us to progress to making a lot of other design decisions for OHKLA, putting some life back into the project.
3. The fact that we now have 2 people with good hybrid rocket experience on the boards (nick and sampo)
4. Recent discussions suggesting that there is some support for me, as the most active director, making unilateral decisions in order to spur things forward

I am making a declaration!  We shall decide upon OHKLA propellants before 23/06/10, i.e. one week for today (all of this is in my timezone, sorry – just add one week from today to get your local time).  This deadline will be enforced very simply – if we as a community working together on this forum, the Wiki, IRC, email, whatever, aren't able to come to a rough consensus in one week then I will make the choice unilaterally.   I don't want to do this, but I will if I have to because the plausible threat of it will make sure we get it done properly.

So, here's the situation: we need to choose a fuel and an oxidiser.  That's all.  A list of candidate propellants and things to consider when making the choice can be found in the Wiki here.  That page is kind of the "go to" page for this entire situation.

The current serious contenders are, in alphabetical order:

Fuels:

1. Hydroxyl-terminated polybutadiene (HTPB)
2. Paraffin
3. High Density Polyethyelene (HDPE, or just PE)

Oxidisers:

1. Liquid oxygen (LOX)
2. Nitrous oxide (N2O)

(there are a few others on the Wiki that I haven't listed here because I find it hard to take them seriously due to problems with them – but it's important that we consider all our options).

It's by no means the case that those above and on the Wiki currently are all we should consider – anybody should feel free to suggest extra propellants as well.

What those who want to take part in this need to focus on over the next 7 days is the following:

1. Finding any other propellants we may want to seriously consider.
2. Making sure that the information on the go-to wiki page is as accurate and as complete as we can make it.
3. Deciding what they think is the best fuel and oxidiser choice.

Toward the end of the 7 day period I will set up a wordpress poll at cstart.org with all of the contenders on it and people can vote.  You will need to be signed into your cstart.org account (the same one you use to post on this forum) in order to vote.  This will avoid spamming, etc. and provide some transparency.  There's no need to keep voting anonymous here since this is an objective technical decision we are making.

While the wiki page I've linked to is where we should be putting all of the relevant information to make this decision, this thread is the place we should be talking about it and asking questions, and generally coordinating our efforts.  If we want to talk about it in real time, we can arrange meetings in our IRC channel.  As many meetings can take place between now and the deadline as people want.  If you have an IRC conversation with someone about this, post the logs (or a link to the logs, or whatever) in this thread.

I guess that's everything I need to explain.  I hope that people embrace this aggressive attempt to get a major OHKLA decision made and do their best to contribute and take part.  Nobody be afraid to ask any questions or raise any issues!  Let's do this, do it well, learn some stuff and have some fun while doing it!

A special to note to anybody coming here for the first time from Reddit – please feel totally free to get involved in this!  Register an account for these forums and share your opinions, knowledge and experience with us!

Some data it would be good to have to help make the decision: regression rates for various propellant choices.  My understanding from reading and surfing around is that, with regards to fuels, the ranking of regression rate is:

Paraffin > HTPB > PE

with the difference between HTPB and PE being relatively small, and paraffin being very high above them both (due to the liquid boundary layer effect). It would be nice to see some genuine numbers (m/s) on this, though.  I also don't know too much about how the choice of oxidiser influences regression rate.  Anybody who knows anything about this or is willing and able to do some research should feel very free and welcome to do so.

brmj said:

Some minimal research on HTPB shows that it is sold as a liquid which, with the aid of a curring compound, can be molded into arbitrary shapes. It doesn't ignite very easilly. It and NO2 are a proven combination, having been used in Space Ship One.

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Huh, I didn't realise it was solid as a liquid.  I suppose this makes PE far and away the most convenient fuel to work with, in that it is a solid cylinder right from the get go.  I wonder how ignitability of HTPB and PE compares?

brmj said:

I wonder if it would be practical to prevent parafin from slumping som much by embedding something fiberous into it. If we used thin alluminium wires or something, it might even function as a nice performance adative. My primary concern is that it might burn unevenly or cause crumbling along weak planes where slump would otherwise occur.

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With regard to your other concerns about paraffin, I don't know if you were around and reading the forums at the time, but a while ago I emailed the Copenhagen Suborbitals guys about these sorts of issues, and their reply was not super encouraging – it is the main reason why I am not as keen on paraffin as I was when I first read about it.  Here's the relevant part of their reply:

> Could you briefly describe the process by which you cast your
> paraffin fuel grains? We have been told that this can be tricky due
> to the way paraffin contracts and expands during the process.

Its very tricky - especially we learned at big size.

We gave up on paraffin and use a polyuretane elastomer from DOW
chemicals. Its burns and behaves much like HTPB, but has a shorter
potlife. Its cast directly in the motorcase around steel mandrels that
are pulled out after curing. The HEAT 1X must have a very short
burntime of just 25 sec, so we need to have a 13 port grain for that.
It has a wagon wheel crossection.

> * Has CS ever flown paraffin fuelled hybrid rockets, or only performed
> static tests? We have been told that flying paraffin hybrid rockets
> on high-g trajectories can cause "slumping" of the fuel grain as
> paraffin becomes soft at high temperatures, and wonder if you have
> experienced this problem?

No flights. Even in 1 g enviroment we observerd propellant screading
and made the decition to go for polymer fuel. This is much stronger and
much more flexible the standard solid fuels som not problems are
expected due to g loads. Cost is 1.5 times paraffine. The compromice
is that with paraffine we had combustion rates up to 4 - 6 mm sec and
could cast the motors with one central combustion port. This is very
simple. However the combustion of paraffine was incomplete and chunks
was blown out. With polyuretane we have a sublimating fuel giving off
gasses that burns much better than droplets - but the combustion rate
is low - about 1,5 mm sec at 10 g / cm2 / sec oxygen flux. This leads to
complex multi hole fuel grains. We have made a lot of seven hole grains
over the years that work very well - so I have all the hopes for the multi
hole grain to be fired on May 16th and flow on august 30th.

It  certainly seems like paraffin can be tricky.  It's true that the single port geometry it supports due to high regression rates is attractive for its simplicity, but the complexity of additional ports feels to me like it would be a lot less than all the various – untested and potentially unreliable – ideas for "fixing" parafin.  This is especially the case for PE, where adding ports is a simple matter of drilling.

brmj said:

On the oxidizer issue, O2 is dirt cheap, while NO2 is less so. NO2 is less efficiant as well, of course. However, I think its ease of handeling and simplicity advantages are a very good thing.

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I really think N2O is the better choice of oxidiser.  The fact that it i self pressurising makes the oxidiser system considerably simpler and less massive.  Not only is it not being cryogenic better for safety, it will also be good for reliability – I have read that AMROC, who did a lot of the early R&D on hybrid rockets, had a lot of problems with LOX valves freezing in place.  The decreased performance is unfortunate, but in practice (see the calculations on the Wiki page) it comes down to a matter of saving about 10kg on propellant – not an awful lot.

Here's a paper from Stanford and NASA on the regression rates of a few common solid hybrid fuels (including HTPB with N2O oxidiser, a really good resource for us.

It includes a glowing reference for N2O:

Nitrous oxide (N2 0) was chosen as the oxidizer for this hybrid motor because it is non-toxic, storable at
room temperature, self-pressurizing, inexpensive, and easy to handle. This list of attributes far outweighed
the slightly inferior performance of nitrous oxide when compared to liquid oxygen or exotic, more hazardous
alternatives.

It also discusses the static test rig they used to get data in a decent amount of detail, including specifying the particular aluminium alloy they used for the combustion chamber and various measurements, which could be data for us.

Most importantly, though, and the thing that I was actually looking for, it has a lot to say about regression rates.  It turns out HDPE has a relatively low regression rate, just a bit more than half the rate of HTPB.  This is unfortunate and definitely a strike against using PE, but I'm not sure it's enough of a strike to cancel out the fact that PE is cheaper, easier to acquire and easier to work with than HTPB.  Obviously higher thrust is better from the point of view of efficiency (minimising gravity loss), but we don't necessarily need super high thrust and we shouldn't sacrifice simplicity and minimalism too much for the sake of thrust.

Hi Nick,

Thanks a lot for posting, all the data you gave us relating to HTPB is fantastically useful.

I know there has certainly been some some "real" (i.e. non-garage) work done with parafin.  NASA and (yet again!) Stanford have done a lot of research on it.  I will try to hunt down some papers on it for you later on today and post them here.  In summary, though, I think the regression rates seen with parafin are generally around 5 times greater than seen for HTPB (according to both theoretical models and experimental data, which are in fairly good agreement for parafin).  Parafin fuel grains typically have just a single, circular combustion port for this reason.

I am curious: what are your thoughts on us overcoming all the issues related to slumping etc. with parafin?  You seem completely unconcerned about this and if there's a good reason for it I'd love to hear it!  I have basically gravitated away from parafin and toward PE exclusively because of my concern about slumping.  There are a lot of ideas out there for how to counteract this, but I don't think any of them have really been well tested, so to my mind there's kind of a "risk" factor about them.  We could always try parafin and switch to PE if we can't get anything to work, but that could delay the project and increase the cost considerably.  Although, perhaps not: parafin and PE are very close in terms of Isp, optimal O:F and density, it may be the case that a combustion chamber could be made which is suitable for both fuels?

With regards to oxidiser, it is starting to look like N2O is pretty much a given.  I am certainly in favour of it, Nick has explicitly voted for it, and while nobody else has explicitly voted for it there also hasn't been anybody who is explicitly opposed to the choice.  Brmj, it looks like you are a little on the fence about the issue?  Can other people who are currently happy to make a definite endorsement one way or the other please comment and let us know what you are thinking?  I know the deadline is still some time away, but if we can make it clear early on that there is a definite, strong preference toward N2O then we can just declare that part "done" and focus the remaining time purely on fuel considerations.

For the sake of having all relevant stuff in one place, I am going to repost here a few comments that we got from OpenLuna's chief engineer Dr Gary Schnyder on the subjects of paraffin and N2O in hybrids.  The original thread (which also includes some discussion on liquid rockets) is here.

I had a 17 point list of arguments against N2O/Paraffin hybrids, but I
Lost it with a misplaced keystroke so I'll just give a few.

Paraffin fuel. Paraffin was supposed to be useful as a hybrid fuel because
of the much more rapid regression rate (how fast the surface burns down)
Fuel grains don't scale up very well. A small motor could use paper or
some rubber style grains, but a larger motor needs a faster burning fuel.
The old Amroc vehicle had one oxidiser tank and a ring of fuel motors
because of this scaling problems with building a big motor.

The paraffin is mechanically weak. This causes it to slump in the chamber
under acceleration. I tested this with a 20Gee centrifuge and actually got
results that seemed to match some experimental test flight failures. Our
idea was to add some structural material (carbon foam). It sort of worked,
I think it would be useful for special, low regression rate areas but not
everywhere.

The paraffin isn't really cheaper than gasoline, diesel, E85, alcohol,
methanol, etc. and the paraffin shrinks when it solidifies which
complicates your casting process.

The N2O oxidiser is much more expensive than LOX. and it is a poorer
propellant in many ways. it requires at least a 700 psi tank and burns
pretty hot for its performance level (lots of Nitrogen). I don't know for
sure, but I *FEEL* that the Nitrous compounds have some explosive
characteristics.

Common valving has a max working pressure (solenoid drive or pneumatic) of about
300 PSI. Nitrous at 700 psi requires heavier, more expensive, and harder
to find equipment. There is a lot of stuff off the shelf for 300psi or less.

Obviously these are the opinions of someone who has a strong liquid > hybrid bias (I don't mean to imply it's an irrational bias, I have no doubt Gary knows what he is talking about), but nevertheless they are issues we have to consider.

Nick, you have worked with N2O before: how much trouble did you have finding 700 psi-friendly valves etc. and was it much more expensive than 300 psi stuff?

With regard to Gary's suspicion that "the Nitrous compounds have some explosive characteristics", this is true.  N2O can decompose into N2 and O2 in a reaction which releases a lot of energy, but this reaction is only possible at very high pressures and very high temperatures.  It can't happen in a properly running hybrid engine but if something goes very wrong it can happen.  I suspect this is what caused the N2O explosion that killed a few Scaled Composites employees a few years ago.  This is a risk, but everything I have read on the subject has been very quick to downplay the risk, stating that this reaction is statistically very rare and the vast majority of N2O rockets have never had any issue with it.