Observations and comments on Cal/OSHA report (Inspection No: 31081103) on fatal accident at Mojave test site of Scaled Composites at the Mojave Air and Space Port, 26th July 2007.
On 26th July 2007, there was a fatal accident in which three people died. This accident happened during testing of a rocket system intended for use in Virgin Galactic’s passenger carrying space-craft; ‘Spaceship Two’.
The accident has caused much concern in the international rocket-engineering community and resulted in a lot of speculation.
Details of the accident have never been fully released nor published.
The investigation report from California Occupational Safety and Health Administration has recently been obtained under the California Public Information Act.
This is the first time that the rocket-engineering community has had an opportunity to better understand what occurred and the safety issues the accident raised.
Analysis of the information contained in the report raises grave concerns regarding the use of Nitrous Oxide Hybrid Rocket Motors in passenger flight, and explores the complexities and potential dangers of these rocket motors.
In particular the claim (made by Virgin Galactic) that Nitrous Oxide is ‘stable and benign’ is discussed.
The following is a document agreed by an international group of people with many years of combined practical experience in the field of Rocket Motor Systems, and the associated physics, chemistry and engineering, including the use of Nitrous Oxide as an oxidiser for such systems. Much of the supporting information has been gained from background research, including proven and known data, much of which is available on the internet. As the accident report does not contain a large amount of hard factual information, some conjecture and assumption are used. These comments are open to be confirmed or refuted by any further data or expert opinion that may come to light. This document is not intended to assign blame or responsibility to any party and is for the private use of the recipient.
Observations and comments on Cal/OSHA report (Inspection No: 31081103) on fatal accident at Mojave test site of Scaled Composites at the Mojave Air and Space Port, 26th July 2007.
Authors: G Daly, C. Knight, K. Mason. Reviewed and endorsed by: B. Berger, F. Ratliff, M. Stacey.
We have decided to break the matter down into two major sections. The first is the hard data in the report that sheds light on the conditions and methods surrounding the ‘cold flow test’, and the second the matters raised by the report’s general content and omissions.
Summary of reported facts
The test was conducted at an open-air and unshaded facility containing a testing rig and various support and ancillary equipment, surrounded by a chain-link fence. It was conducted at approximately 2:20 pm. The ambient temperature is reported as 105 + degrees F. The records show that the ambient temperature recorded at the nearby Mojave Airport peaked at 115 F on that date.
There was a holding tank on site containing more than 10,000lbs of Nitrous-Oxide which had been filled on a prior day.
Some unknown quantity of the N2O was transferred to the ‘TST’ tank, which composed part of the actual testing apparatus. A tank was filled the night before. It is not clear which tank this refers to, but we suspect this would be a transfer from the holding (Monoxcs) tank to the TST tank. There is no clear indication of how much N2O was in the test apparatus before the testing commenced. The information in the report suggests that the quantity was 10,000lbs?
There were seventeen people present, five of whom retreated to a mobile control unit behind an earthwork some 430′ from the site. The remaining eleven people gathered at a chain-link fence in quite close proximity to the testing rig, to watch the experiment.
We are led to understand that the test involved 10,000 lbs of N2O at 70 degrees F and 360 psig.
This was a ‘cold-flow’ test that was to study oxidiser flows into a ‘balance chamber’ without rocket motor ignition.
Three seconds into the test, there was an explosion which resulted in three fatalities, and several other major injuries. The victims all suffered burns and explosion shock. Todd Ivens, Eric Blackwell and Charles ‘Glenn’ May were killed.
Observations and comments
The reported temperature and pressure (360psi at 70F) of the N2O is not credible. We must assume that the object of the test was to flow self-pressurised liquid N2O through the testing rig. AT 70 degrees F, the vapour pressure of N2O is 770 psig. Either the claimed temperature or the claimed pressure is wrong, but more probably both are incorrect. We wonder where OSHA came by this information?
It can be seen, from the above chart, that a pressure of 360 psig indicates a temperature of 20f (well below the freezing point of water) and a temperature of 70f indicates a pressure of 770psig. It is hard to see how the two figures given are in any way compatible.
If one were to want to conduct a study of oxidiser flow as it would relate to a burning rocket motor, it would be correct to flow the oxidiser through a chamber with an exit restriction that would maintain the chamber-pressure in a range equivalent to that prevailing in a burning rocket motor. In our experience that would be about 400psig. This would require a very small exit orifice to obtain this needed chamber pressure. We are assuming that this is the set-up that was being used. If the input pressure was, indeed 770 psig, the exit orifice would be a little smaller than twice the cross section of the injector. This small orifice would not provide an adequate exit pathway should a pressure-spike occur. We are somewhat bemused as to what the object of this experiment was in relation to the actual performance of an hybrid rocket-motor but that does not mean such ‘cold flow’ tests are not useful. It is our opinion, that once allowed to turn to gas, it is much safer to get the N2O lit and firing a motor than to attempt to work with it as an unlit gas. There is no indication as to the design or dimensions of the balance chamber, or whether or not it contained an H.T.P.B. Fuel grain.
The test was conducted at 2:30 pm at the hottest part of a very hot day. These was no shade. The test appears to have been conducted on a concrete pad. If the ambient temperature (in the shade) was 110 F, the temperature a few feet above the concrete pad would probably have been in excess of 140 F.
There is no mention of method nor details or schematics of the apparatus. Also no mention of refrigeration equipment or levels of insulation. However the N2O had been in the tank (TST tank?) overnight and all day. It is reasonable to assume that it could have reached a temperature quite close to the actual air-temperature at the facility when the test commenced. Even if it was 40 F cooler than that, it could have attained a temperature of 100 F. That is well in excess of the CRITICAL POINT of N2O. At 96.8 F, N2O becomes a super-critical fluid, regardless of the pressure it is subjected to. Beyond that point, as temperatures increase, pressure increases at a high rate. Super-critical N2O is very susceptible to pressure-shock which will result in a very high velocity detonation during which temperatures can exceed 5,000 F. The presence of H.T.P.B would greatly excaberate the danger of flowing super-critical N2O.
From our own experimental experience we know that N2O in a closed system on a hot day will gain temperature very quickly. N2O is internationally recognised as a very powerful greenhouse gas and it has remarkable insulating properties. It is quite possible for supercritical fluid to form in a volume of N2O which has a mean temperature well below the critical point. In can be hot in one place in a tank and quite cold in another place in the same tank. This characteristic was observed and investigated in the 1980’s but was not widely publicised or fully understood. We do, however, have reason to believe that some of the people present at this test were aware of the research. This has implications for both the ‘Monoxcs’ tank (which appears to have been refrigerated) and the ‘TST’ tank. We know very little about the TST tank.
Unless the tanks contained some type of stirring mechanism, it would be difficult to know the mean temperature of the N2O in either tank.
Even if the N2O in the tank had not gone critical, it is likely that N2O in valves and lines could have. This would elevate the pressure in the system to 1,000’s psig. It is also possible that the temperature inside the balance chamber could have been at or above ambient before the test commenced. This could have resulted in the formation of super-critical fluid inside the chamber as flow was started, creating a possible detonation source.
Opening a valve to allow flow into a highly constricted chamber could, once the valve opens fully, cause a pressure-shock in the balance chamber which would be transmitted to the TST tank resulting in the detonation of all gaseous N2O in the system.
MSDA documents, in their most basic form from N2O suppliers, caution against pressure shock.
Any materials incompatibility in the system would have excaberated the event. Apparently the TST tank was constructed from composite materials and did not have a metal lining. Experiments that have been conducted show that N2O (which is a very powerful solvent) can dissolve the hydrocarbon binders in a composite material. This hydro-carbon, when present in the gas at the top of the tank, will greatly reduce the energy required to initiate a a detonation event. It has also been suggested that friction caused by vibration of a composite dip-tube flange (retained by steel bolts) was the ignition source. This may well be the case. There may also have been other materials incompatibilities in the system, in valves, other ancillary equipment, or in the balance chamber.
There is no indication as to whether the system had any provision for protection against rapid overpressure, either a relief valve or burst disc. Nitrous oxide systems sold to the racing community for use in car engines all have over-pressure protection built in. This has long been considered a basic safety requirement.
It may also be the case that ignition was the result of compression shock of super-critical N2O.
This phenomenon had been described in literature dating back to 1937 and is well known amongst those with expertise in the handling, production, and use of N2O.
With either source of ignition, both materials incompatibility and high temperature would have contributed to the detonation event. The actual source of ignition will be hard to determine, (as can be seen from the aerial photograph of the accident site) as very little remains of the testing apparatus beyond small fragments, all of which will demonstrate exposure to very high temperatures and rupture pressures from the detonation. If the balance chamber was largely intact after the explosion, that would support the theory that the detonation initiated inside the TST tank.
If the balance chamber was also destroyed, then ignition could have initiated at almost any point in the system.
One must also assume that data recordings of temperature and pressure in the system would be recorded in real-time for the purpose of collecting useful data from the test.
One, then, must be concerned at the nonsensical temperature and pressure data provided to the OSHA Inspectors.
In their press-release subsequent to the accident, Scaled Composites states;
“The body of knowledge about nitrous oxide (N2O) used as a rocket motor oxidizer did not indicate to us even the possibility of such an event.”
This would seem to indicate either a lack of due-diligence in researching the hazards surrounding N2O (negligence) or a wilful disregard of the truth.
In the light of the above, one would be cavalier to advertise Nitrous Oxide, and its use in rocketry as ‘safe’ or ‘benign’.
Consider then, the current advertising claim on the Virgin Galactic Website:
“Hybrid motors offer both simplicity and safety. This is the type of motor that SpaceShipTwo will employ and that was used by SpaceShipOne. It means that the pilots will be able to shut down the SpaceShipTwo rocket motor at any time during its operation and glide safely back to the runway. The oxidizer is Nitrous Oxide and the fuel a rubber compound; both benign, stable as well as containing none of the toxins found in solid rocket motors.”
Cal/OSHA subsequently required Scaled Composites to introduce new safety protocols, and accepted that these new protocols ‘abated’ the hazard. Having studied the protocols published by Scaled Composites, we find no mention of avoidance of the critical point of nitrous oxide. While adiabatic compression shock is mentioned, it’s relationship to temperature is overlooked.
Spacedev (now absorbed into Sierra Nevada and the present subcontractor providing rocket motors for SpaceShip Two), in its document relating to safe handling of N2O does recommend storage of N2O at O deg F but then goes on to discuss the heating of N2O as desirable for rocket motor use.
It must be said that Spacedev does display a good awareness of hazards, including compression shock, but does not seem to understand the significance of the critical point and its consequences.
As neither of the above mentioned documents fully discuss the dangers of high temperatures, or the insulating properties of N2O, nor do they discuss the significance of the critical point, one cannot agree with Cal/OSHA’s conclusion that the hazard has been ‘abated.’
While it is most advisable to apply the established safety protocols relating to liquid oxygen, such protocols, in themselves are not sufficient to ensure the safe handling of Nitrous Oxide. The unique physical properties of N2O require further protocols above and beyond those used for liquid oxygen.
Safety protocols for N2O, in a rocket motor system, should include (in addition to the protocols used for Liquid Oxygen)
- The detailed study of materials compatibility of all components in the system
- Avoidance of high temperatures at all points in the system
- Stirring of large tanks
- Avoidance of the gaseous phase both during apparatus filling and in use
- Purging of lines and valves immediately prior to ignition
- Not using any component that may have previously absorbed N2O – especially fuel grains
We are not confident that, even with these additional precautions, that we yet know enough about N2O to consider it a safe oxidiser for use in passenger flight. In the light of what we do know, safety must remain a major concern.
The investigation report
At the outset it is proper to observe that Cal/OSHA inspectors cannot be expected to have all the needed specialist knowledge in such an esoteric field as the use of N2O in rocket-motor systems. It must have been very difficult for them to evaluate the quality of the little information that was provided to them, and they cannot have known what data to ask for.
The report concentrates on health and safety violations, and no effort is made to draw lessons which would aid future safety for others operating in the same field. Future safety and lessons learnt are a part of Cal/OSHA’s mission, but, probably due to lack of specific expertise and under-staffing and lack of funds, that part of the job has not yet been done.
It should also be noted that the document is an interim report and, by no means, final. It is the required practice to produce a final report for filing with Federal OSHA. This has yet to happen.
What is striking is the lack of witness co-operation. No-one wanted to talk. In particular the impression was gained of ‘guarded’ responses. Please note the business cards of legal people and a company (Engineering Testing Corp.) specialising in the mitigation of accident consequences.
Of the seventeen people present, the vast majority were not interviewed.
The investigators did collect a variety of business cards. It is not clear exactly when they were collected. The cards include George Wittinghill, owner of a small aerospace company with a long career in hybrid rockets. Mr Wittinghill was probably the most expert person present and was in the control vehicle at the time of the explosion. He is not interviewed. It is not clear whether he was subcontracting his services to Scaled Composites or present in some other capacity.
Also present was Al Cebrian a local specialist in fluid process engineering and fabrication. He is also listed as ‘in the scum truck’ (control vehicle). He listed as being interviewed, but no interview is shown in the report. We have received information that Mr.Cebrian was not in the control vehicle at the time of the explosion, but at the chain-link fence along with other spectators and that he was amongst the first to render aid to the injured people.
Also in the control vehicle was Luke Colby, noted as ‘running the test’. He is not interviewed.
It would appear that not one of the people in positions of responsibility were interviewed.
It may well be that these and other people were formally interviewed at a later date. The content of any such interviews is vital to a complete understanding of events.
Cal/OSHA did make the right steps in terms of preserving evidence. The ‘Orders to Preserve’ require the preservation of all tools and equipment used at the test-site. This should include the control vehicle and its contents. One hopes that this means that the hard data regarding temperatures, pressures and real time sequence of events is still available. As Cal /OSHA has not finalised its report, we would suggest that these data should be retrieved and examined by a person/people with specific expertise. This is where the vital lessons for the future safety of workers and others will be learned.
We have read document P&P C-170 & 170A (DIVISION OF OCCUPATIONAL SAFETY AND HEALTH POLICY AND PROCEDURES MANUAL /Investigations). Under the standards laid down in this document, the accident report is severely deficient. However, we will not go into the details of the deficiencies as the manual is publicly available for all to see, so any reader can draw his or her own conclusions. Our opinion is that the deficiencies are many and manifest and should require explanation by Cal/OSHA. The copy of the Manual that we have obtained shows updates subsequent to the date of this accident. Therefore, we do not know exactly what was the prevailing investigation standard at the time. For instance, the up-to-date manual requires reporting of a catastrophic accident to the California Bureau of Investigation. This bureau did not exist at the time of the accident. It would be helpful to see a copy of the standards that prevailed at the time of the investigation.
Cal/OSHA is expected to follow federal guidelines for its investigative procedures. These procedures call for diagrams and photographs to be generated by the investigators. We have been informed that photographs were provided by Scaled Composites, and remain their property. We are also informed that Scaled Composites will not release the photographs. This renders the report deficient in material facts. We have been informed that there was a video recording of the event.
Also present amongst the business cards are two attorneys from the firm Waller Landen Dortch and Davis, and Gregory M. Walton from Engineering Design and Testing Corp. The latter company specialises in the mitigation of consequences of industrial accidents, including litigation.
The report indicates that the Kern County Coroner was on scene. We can find no record of a coroner’s inquest relating to this accident.
A recent telephone conversation with the Kern County Sheriff’s office has revealed that the investigation is still open. As is the usual practice with open investigations, they are not permitted to discuss the case.
We have looked at the various aerial photographs of the aftermath of the explosion. There appear to be no large remnants of the balance chamber or the TST tank. This would indicate that the entire rig detonated. An examination of the images shows that the trailer holding the Monoxcs tank was twisted and the rear wheels blown away from the epicentre of the explosion, while the Monoxcs tank was tipped towards the epicentre. This would indicate that it was struck by a powerful shockwave at ground level, similar to that produced by High Explosives. The photographs we have available are of very low resolution and taken from the air, so it is impossible to make out any fine detail. Higher resolution photographs would be very useful. We have also viewed Google images of the Scaled Composites test site as of 2012 (co-ordinates 35.065129,-118.136308). The site has been cleaned up, enlarged, and an earthwork has been added. In the light of the ‘Orders to Preserve’, one wonders where this physical evidence is now, and in whose possession it is, as this is a vital part of the chain of evidence in a Fatal Accident Investigation which is still open.
One final observation
We understand why so many rockets are tested at Mojave; because you can! It is a place where lesser safety standards are tolerated in order to allow experimental stuff to make it to reality. An hot desert environment is probably the worst possible place for nitrous oxide rocket-motor testing. This is one field where being cold is good and being hot is not. Alaska leaps to mind.
- Nitrous-Oxide Explosive Hazards. Claude Merrill 19/05/2008
- Cal/OSHA Policy and Procedures Manual (Accident Investigation) P&P C-170 & 170A 2/29/12
- Detonation at SCR (Autodiverse)
- Nitrous-Oxide Trailer Rupture July 2nd 2001. Konrad Munke. Linde Gas AG
- Cal/OSHA Accident investigation. Inspection No: 310821103, July 26, 2007
- Various helicopter photographs of accident site available on ‘Google Images’
- N2O Further Safety Issues. Autodiverse
- Summary of Nitrous-Oxide Investigations. Airforce Weapons Laboratory, AWFL-TR-75-231
- Archive of N2O safety related documents, Bruno Berger, Swiss Propulsion Labs.
- N2O Safety Guidelines Scaled Composites 17/06/2009
- Nitrous-Oxide Explosion, Pratt and Whitney FR-5904
- Thermochemical Properties of Nitrous-Oxide, H.R. Ambler and T. Carlton-Sutton, July 7 1935
- Scaled Composites Press-release, August 1 2008
- Handling Considerations of Nitrous Oxide in Hybrid Rocket Motor Testing. AIAA 2008-4830. 21/07/2008
- EIGA Code of Practice, 116 07 E (1)
To view the full 60 page Cal/OSHA report on the incident in the Mojave Desert, Click Here