Experimental rocket societies have a more than 60-
year history of providing some of the few hands-on hardware development training grounds for the nation’s space industry scientists and engineers. Most famous among these organizations is the Pacific Rocket Society (PRS), which was founded in 1946
and has been an incubator for many notable personalities now operating companies in the new commercial space movement.
PRS members have built, tested
and flown every type of rocket engine imaginable, including liquids, hybrids
and solids. At an amateur rocket launch in the Black Rock Desert,
in 2005, several PRS members witnessed
the explosion of their large HTPB/Nitrous-Oxide hybrid rocket.
In terms of design, the nitrous-oxide tank was loaded using a tube that passes
through the combustion chamber and plugs
into the injector face. At launch, the tube detaches from the injector, allowing the nitrous oxide to flow into the combustion chamber. A burning ignition grain is present to start combustion.
During nitrous loading the tube seal at the injector face was leaking, spraying the nitrous oxide onto the fuel grain. At ignition the fuel grain blew up. Later, it was determined that the detonation was due to the leak that sprayed nitrous oxide onto the HTPB fuel grain, thereby saturating it, and turning it into a volatile, detonable solid. This engine design was later modified to use a polyethylene tube to fill the tank through the combustion chamber; the ignition grain burns through the tube to start the nitrous flow
and to start ignition.
Knowing that nitrous oxide sprayed on an HTPB fuel grain can saturate that grain and turn it into a substance as volatile as TNT,
have several concerns with the current SpaceShipTwo design, and have some recommendations to improve its safety
based on how the nitrous-oxide hybrid engine disasters that I personally have witnessed could have been avoided.
Venting nitrous oxide through the engine should be avoided at all costs. One possible scenario of high concern is the potential event of an aborted launch requiring the venting of the onboard nitrous-oxide propellant to reduce the vehicle weight for landing. In this type of situation, either the White Knight Two would have to land with a fully loaded SpaceShipTwo, or SpaceShipTwo would have to land with a full nitrous-oxide load thus damaging the landing gear. In both of these cases, the venting of nitrous oxide through the engine to lighten the vehicle weight should be avoided
. Venting should be performed through a separate external vent, not through the grain.
In the event of a misfire of the SpaceShipTwo motor, and if nitrous oxide has been vented through the motor without successful ignition, a second attempt at ignition should not be tried. Redundant ignition methods should be used
to make sure that ignition occurs when nitrous first flows; otherwise, out of concern for public and passenger safety,
the launch should be aborted.
In questioning the ill-fated July 26 ground test,
one could ask: Where did ignition occur? When venting a dry gas
such as nitrous oxide
over a surface, a static charge can build up. If this static discharges, it can ignite the explosive fuel grain, or in the case of nitrous oxide, can ignite the oxidizer itself.
In the Mojave Desert that day, conditions were extremely dry and hot (temperatures hovered around 100 degrees
); static build-up was likely to occur.
Within a narrow range of temperature and pressure combinations, nitrous oxide in and of itself
can undergo rapid, explosive decomposition and can auto-ignite. If, as is claimed, there was no fuel present that day, the nitrous oxide alone could have provided the destructive power that devastated the personnel and equipment at the Scaled Composites test site.
Was this a case of mischaracterizing the nature of the propellant? Were workers led to believe that nitrous oxide should be considered “safe”? After all, didn’t the X Prize Cup organization include nitrous oxide on their list of safe propellants? All people with hands-on rocket experience know that there is no such thing as a “safe” rocket propellant. Unless these substances are treated with respect and scientific precision, they will harm, and in rare and tragic events like the Scaled episode, kill.
Also, as a matter of good practice, it is not safe to flow an oxidizer through a propulsion system with unprotected personnel standing nearby. If the oxidizer is exposed to any organic contamination it can become explosive and detonate during the test. All future flow testing using an oxidizer should be done with the personnel located in a well-fortified blockhouse.
Burt Rutan had
commented that they had vented nitrous many times in a similar way, and no explosion had occurred. This type of statement is commonly made in conjunction with accidents. The Apollo 1
is a case in point:
NASA had performed many high-pressure pure-oxygen cabin leak checks on all the Mercury and Gemini capsules with no resulting fire. But the conditions were right that day for an Apollo 1
fire. Unfortunately for those who died or were injured in the Mojave mishap, the conditions were right again, if just for that one horrible moment.
Mark Holthaus is a safety engineer for The Boeing Co. and so works as a pyrotechnics operator – rockets 1st class with the California State Fire Marshal’s Office.
He has performed
safety analysis on the space shuttle, space station and X37, and has extensive experience with large hybrid rockets through his field work with the Pacific Rocket Society.