Cost of paraffin wax fuel grains | Hybrid engine discussion | Forum

Just some very rough back-of-envelope calculations.

Supposing we cast a relatively large fuel grain – say 30cm in diameter and 2.5 meters long (length to diameter ratio of just over 8).  This would occupy about 0.49 cubic meters of space.  I have not accounted for the empty space of the port because I have no idea how large to make it.  Thus, this will be an overestimate.

Wikipedia gives the density of paraffin wax at about 0.9 grams per cubic centimeter, or 900 kg per cubic meter.  This makes the total mass of such a fuel grain about 442 kg.

This site sells blocks of paraffin wax in bulk.  Assuming we want "hurricane outer shell, long burning votive – hard wax", then we can buy this in lots of 27kg boxes.  We'd want 17 of these boxes.  If we buy 20 of them, the price is $3.48/kg (we could get a cheaper deal buy buying a pallet of 34 boxes).

This give an overestimated total material cost for one of these very large fuel grains as $1538.

According to Wikipedia, the solid fuel rocket in CSXT's GoFast rocket cost $20,000!  I haven't looked into the price of nitrous oxide yet, but surely there is no way it could cost so much that our total cost would be that much.  We may even be able to remain an order of magnitude below it.

Which is pretty awesome, but also a little bit curious.  If hybrids really are that much cheaper, why did CSXT use solid fuel?  The website of "Rocketman", their most well known member, suggests that he has built other devices powered by hybrid engines, so obviously it isn't the case that nobody there was aware of their existence.  Was the choice purely for simplicity?

I agree this is probably not the best deal we can get, but it doesn't seem to be a bad deal, and it establishes an upper bound on cost.

I also agree we should eventually look into sponsorship – but maybe after we've done 1 or 2 small (<10 km) flights to establish that we are serious and capable.  I don't think it should be too hard to find places willing to give us a deal on paraffin and nitrous oxide.  I noticed that Copenhagen Suborbitals are sponsored by Air Liquide: I assume that they sell them LOX at a reduced rate in exchange for advertising.  I am sure we could find a large gas supply company who were willing to offer us a similar deal on N2O if we showed them the scale of our plans.  We're probably going to be buying a lot of the stuff.