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Experiments with nitrous oxide hybrids date all the way back to the mid-thirties. The fact NASA has never used nitrous oxide in any rocket system should have been a clue.

Under the right conditions a detonation wave can be initiated in liquid nitrous oxide that will propagate all the way back to the storage tank unless a check valve or some other form of trap stops it. Jet car builder Wayne Knuth once told me an account where Les Shockley was experimenting with injecting nitrous oxide into the afterburner on a jet dragster when, during a run he felt the wind around the car go FORWARD! After Les got out he discovered the nitrous oxide line was split almost all the way from the afterburner to the cockpit. It stopped at a check valve just before it got to the nitrous oxide tank located in the cockpit.

Below is a link to a 1935 paper that documents the explosion potential of nitrous oxide. Extrapolating from the data in the paper, eliminating the explosion potential of nitrous oxide requires storing it at pressures too low (under 13 atmospheres, 188.5 psi) to make it practical as an oxidizer for large scale rocket systems.

http://www.iop.org/EJ/article/0959-5309/48/1/326/prv48i1p189.pdf?request-id=0PAS-PnJ3BG0c7MJ3Ai7Kg

Using concentrated hydrogen peroxide, thirty years ago lone individuals working on shoestring budgets instead of multi-million dollar funding designed and built fully throttleable fully reuseable rocket motors for dozens of manned rocket vehicles yet managed to never blow up anybody.

http://www.draglist.com/stories/SOD%20Mar%202002/SOD-031402.htm

http://www.arvilporterprojects.com/index.htm

http://www.dragracingonline.com/features/miller_1.html

http://www.the-rocketman.com/RPV/Courage_of_Australia.html

http://www.the-rocketman.com/RPV/x1.html

http://www.the-rocketman.com/RPV/free-sprint.html

In terms of propulsion system, The Blue Flame land speed record car with its throttleable reuseable hydrogen peroxide and liquid natural gas motor remains the most sophisticated private manned rocket vehicle.

http://members.aol.com/KingstonChas/TheBlueFlame.html

The first manned hybrid rocket vehicle of any kind – air or ground – was the Budweiser hydrogen peroxide rocket car.

http://www.artemisimages.com/gallery.aspx?photomode=3&categoryid=4255