The choice of propellants: a similarity analysis of scramjet second stages.
by J. Pike.
Phil. Trans. Royal Soc. Lond. A. (1999), 357, 2357-2378.

Scramjet powered vehicles with similar shapes and the same second stage trajectory to orbit are used to establish relationships between the vehicle volume, the vehicle launch mass and the payload mass. It is shown that increasing the fuel density or the payload density can increase the payload mass, while simultaneously decreasing the vehicle launch mass. The maximum payload mass without a volume constraint is found to be half the dry mass. Practical volume constraints are shown to reduce this fraction.
To increase the scramjet thrust, addition propellant is often injected into the scramjet flow. It is shown here that for a hydrogen-fuelled vehicle, replacing this additional hydrogen with neon or with a hydrogen-oxygen mixture can significantly increase the payload and decrease the launch mass. It is shown also for less rigorous assumptions, that replacing the scramjet hydrogen fuel with a hydrocarbon fuel can have the same effect, such that a vehicle to place a given payload in orbit can be both smaller and less costly.

When denser fuels than hydrogen are used, the vehicle can become smaller, with a consequential reduction in the drag. Even though the replacement fuel is less efficient than hydrogen when producing thrust, the smaller vehicle size can more than compensate for this, giving an increased payload.

Published in Great Britain by the Royal Society, 6 Carlton House Terrace, London SW1Y 5AG.
ISSN 1364-503X
Full text available from The Royal Society
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Last amended: Nov 2013