Nitrous oxide is not ideal for use in the large rocket needed for the launch of the manned spacecraft.
It must be stored at very high pressure, and has a low density.
The HATV shows the effect of this - with just 70 kg of useful propellant the rockets itself weighs empty 130 kg.
This is all the steal in the big nitrous tank that causes that.
Exchanging nitrous oxide with liquid oxygen is needed to get the delta V required to reach suborbital space.
Liquid oxygen is far more complex to work with than nitrous oxide - but it can be used at any desired pressure because it is chilled down to - 183 C.
Most important it is, unlike nitrous oxide, not an explosive.
A 6 kg propellant test setup for liquid oxygen testing build in early 2009.
This test system was named Baby Heat. A long series of tests with it has been conducted.
The test data form the babys indicated that getting a hybrid combustion process efficient would be more difficult than with nitrous oxide. We chose initially to use paraffin wax as fuel - for its excellent environmental properties.
While the wax worked resoneble in small scale test, once we went full scale - 64 cm or 25 inch diameter the propellant failed mechanically.
Thus in the spring of 2010 wax was replaced with a synthetic rubber of the polyuretane family.
This is an example of giving a environmentally benign propellant a chance before resorting to a less benign solution.
Polyurethane rubbers are manufactured form toxic prepolymers - and needs very special care during mixing and casting.
However - polyuretane forms exelent hybrid propellants. In a test with the HATV it showed nearly perfect combustion.
Photo: Thomas Pedersen
COPENHAGEN SUBORBITALS 2013 - ABSOLUTELY NO RIGHTS RESERVED