Forum | Rough agenda for an upcomming "work burst"

There has been some discussion lately of targetted "work bursts", where everybody gets together on IRC or the like one weekend at a pre-arranged time and we spend as long as we can working together in real time to knock out one or two key issues, specified in advance.

There seems to be good support for this idea, with the biggest problem at the moment being finding dates and times that work well for all of us.

Supposing that we do actually go ahead with something like this in the near future, I would like to propose that our first "burst" be dedicated to the problem of finally selecting an engine type for our modular boosters.  Basically, we want to answer the question "Solid fuel, liquid fuel, or hybrid engine?".  Until we answer this we're not going to be able to do much in the way of designing booster module insides, estimating masses, costs, thrusts, etc.  It's really important that we finally get this out of the way.  In this post I hope to outline a rough agenda for this burst so that when it goes ahead people are prepared.

First of all, some prerequisites:  In order to make useful contributions to the burst, people will need to have read, at a bare minimum, the following three Wikipedia articles: Solid fuel rocket, liquid fuel rocket, hybrid rocket.  Reading more than just these would be strongly encouraged, the more reading the better, but those three should provide an adequate base to at least follow the conversation and make observations.

Things we will need to consider are:

Simplicity: All other things being equal, we should choose the simplest design possible.  Moving parts are evil.  Precision machined parts are evil.  A tenacious preference for simplicity in all design decisions is probably our best bet at keeping things cheap and reliable.

Fuel issues: For each of the different kinds of engine, what sort of fuel do we need?  Can we buy that fuel commercially or will we need to make it ourselves?  If we can buy it, what does it cost?  If we need to make it ourselves: What sort of facility and equipment would we need?  How safe is the production process?  Do we need a government license to make the fuel in most countries?  What are the consequences of imperfections in making the fuel (depending on the kind of engine these can range from reduced efficiency to rocket-destroying explosions)?  Regardless of whether we are buying are making, can we store the fuel for long durations?  How high is the risk of accidental ignition during transport/storage?

Fuel pressurisation: If we go with a design that involves liquid fuel or oxidisers, we need to decide on a method of generating pressure to feed those liquids into the combustion chamber.  The standard options for this are using turbopumps or using a compressed inert gas of some sort.  I believe pumps lead to better performance (but I'm not sure why), but the gas solution is of course much simpler and cheaper.  A third option which seems less common but which we should consider is vapor pressurisation (aka "Vapak").  This involves gently heating the liquid fuel/oxidiser until some of it turns to vapour.  If the tank is small enough, the pressure of that vapour is sufficient to force the liquid into the combustion chamber.  As the fuel level decreases, the space filled with vapour increases in volume, decreasing the vapour pressure which lowers the boiling point of the liquid, allowing more of it to vapourise, which continues to push the liquid through.

Cooling: The nozzles of rocket engines, understandably, get incredibly hot.  In order to avoid melting, the nozzle either needs to be made of a super heat-resistant material (probably quite expensive) or it needs to be actively cooled somehow (if we are using cryogenic liquid fuels, these can be circulated past the nozzle on their way to the combustion chamber to help with cooling).

Control: Our booster doesn't just have to send its payload straight up, it also needs to give it a high velocity in a direction parallel to the ground.  This means we need some way to gently roll the rocket onto its side during flight.  This is possible in diferent ways for different engines – some engines can be throttled easily, which lets us use differential thrusting for control, others can't which introduces the need for gimballing.  The simplicity of the control mechanism for various kinds of engine should be something we consider.

Failure modes: What are all the different ways that the various kinds of engine can fail?  How severe are the consequences for each kind of failure and how easy is it to avoid them?  Ideally we would want to choose the solution for which huge explosions are as hard to cause as possible.

If anybody can think of any other important issues that we should consider in coming up with a solution to this problem, feel free to post them here.