Post edited 4:02 am – December 18, 2009 by Luke Maurits
Shortly after we made our unexpected first contact with Paul and Gary from the OLF, I sent both of them an email asking for their first impressions of CSTART based on our website – is it easy to figure out what we're about and what our plans are, and do we come across as a respectable and realistic project or a bunch of crazies.
Only Gary (OLF's Chief Engineer) has got back to me so far and our email conversation pretty quickly veered away to the topic of rocket design, in particular the various pros and cons associated with hybrid rockets, particularly N2O/paraffin rockets, and also some thoughts on other fuels and liquid rockes. He's kindly agreed to let me repost some of these comments in the forum so that our team as a whole can see them and discuss them. Before I go on, though, for those who want to know, the upshot of his response to the initial question was basically that things look pretty good now, but we could perhaps do a better job of making sure that the reasons we give in the forums for choosing certain approaches/designs are briefly summarised in the Wiki, and that we'll look more legitimate as soon as we have some physical hardware to show off.
Here are some of Gary's thoughts. Gray wants it to be made clear that he is "not trying to 'rain on anyone's parade' nor insult any pet ideas"/p>
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.
Another option is H2O2. which may have some storage benefits or
liabilities. It is what I am playing with right now for a small storable
engine.
Nitric acid and N2O4 are pretty nasty chemicals, so I would skip them.
LOX really isn't that bad for many things. The valves take about 10x as
much force to move, and you need to wear gloves and such, but it isn't
that bad. It doesn't store well, so probably not for space propulsion, but
good for Launch Vehicles
Gaseous propellants are not going to happen when you figure the weight of
the tanks, It just makes the vehicle impossible.
liquids rocket engines are really nice from a variety of perspectives.
1) They are completely testable. Run them for full duration, on several
occasions, and if they don't break, you have a pretty good idea that they
will run again.
2) They scale in impulse very easily. Just add bigger tanks !
3) Small engines are useful, you can cluster them around one tank, or use
them for other aspects of you mission (lander, RCS, Space propulsion etc)
you can use the smaller engine you developed while building bigger engines..
4) Textbooks! NASA SP125-I think
5) Higher efficiency. ISP isn't everything, but it is very important.
6) Lower weight than a hybrid. A hybrid needs to carry all it's fuel in
the combustin chamber, which is built for high temperatures and any
combustion pressure spikes. You also need a post combustion volume
approximately equal to the volume of a normal liquid engine, and of course
the grain ports are wasted space. The injector side also generally has
dead space. To a very good approximation, the mass of a pressure vessel is
directly proportional to it's volume and pressure rating.
7) Control: throttling is not that great of a control because of losses in
propellant efficiencies, and varaible control authorities. gimbals are
very difficult in a hybrid but easier in a liquid. Injection systems have
some losses but may be workable for TVC
8) Regenerative cooling, never seen one for a hybrid, but very common for
long burn time liquids. ablatives can work, but are better for space
propulsion due to the lower chamber presssures when in vacuum.
9) The liquid engine has a well defined O/F ratio. The hybrid is
continually changing, the injector end is Oxidiser rich, the throat end is
fuel rich, or you slide everything around.
[from a second email]
On the liquid propellants front. I always like N2O.C2H5OJ as a propellant.
Talk about friendly, It is a total party blend ! Laughing gas and booze !
As for flammability, from the experiences I've had with alcohol stoves,
I'd much rather be in a alcohol fire than a paraffin one. similarly,
candles are pretty easy to light, diesel can be tough !
Speaking of ignition, Lighting a rocket engine can sometimes be a
challenge. The hybrids we built used a small APCP fuel grain to get the
system going. A friend used Estes D motors to ignite a 10,000 lb thrust
kero/lox engine at Stennis. What works, works. but neither was
restartable.
It is nice to have a small set of valves that will put very small amounts
of propellent into the chamber with a spark generator to warm and ignite
the engine. This was a multi-month learning experience for me in 1998.
Think about a 700 psi cold N2O stream blowing out whatever you have for an
ignition system.
Another point that some think is important: go with lower pressure. 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.
We're quite fortunate to get such a lot of comment on our ideas from somebody with a considrable amount of rocket experience. Obviously Gary prefers liquid rockets over hybrids, but I think the best response we could have to this input as a group is not "Holy crap, hybrid rockets suck and we made the wrong choice, let's switch over to liquids!". Rather we should realise that, while we are all obviously quite intelligent people, none of us has any real experience with rocketry and we need to avoid jumping to decisions too quickly. I think the justifications we gave for using hybrids were good and valid – using hybrid rockets will make our lives easier than any other option with regards to fuel procurement and handling. However, we have to realise that whenever we as a small team of amateurs weigh up the pros and cons of different approaches, we are probably going to overlook some pros and some cons because we only know what little we've read in the space of about a month. So we should make our choices tentative at first and slowly increase our confidence in them as we learn more.
I don't think we should declare "hybrids abandoned" just yet, but some things to consider going forward might be:
- Paraffin may not be the best fuel choice afterall, I definitely overlooked some things (slumping under high acceleration, expansion during casting) in thinking it would be. Which other alternatives might we want to consider?
- How would we feel about using LOX in our hybrid rocket? Cryogenicity is probably the least problematic item on the list of nasty things about rocket fuels. If it is indeed cheaper than N2O (I've had a hard time finding bulk N2O prices online), then the fact that it performs better and can use lower pressure valves which are cheaper and easier to find might make the increased handling difficulty worth while.
- The multiple combustion chambers in our current plans could add a lot of unecessary mass compared to the liquid fuel option. How light can we get these chambers? SpaceShipOne's hybrid engine used a composite material for the chamber, which was probably quite light but also quite expensive. Can anybody find out more details about this option?
- We haven't given any real thought to ignition yet, I'm not sure how the options for liquid and hybrid compare, but we should look into it.
- Suppose we were for some reason forced to use liquid rockets. In this hypothetical situation, which fuel and oxidiser would we prefer (weighing up safety, availability, etc) – how does the cost and performance compare to some existing hybrid engines? Then we can consider whether or not it makes sense to use the least nasty liquid rocket possible instead of a hybrid rocket.
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