(X-15-3 #672 Accident Report)

June 24, 1960

Following are some of the more significant items of information to come out of the accident investigation to date:

Examination of the wreckage shows that both the H2O2 and NH3 tanks exploded. The bent position of all the brackets mounting the H2O2 tank indicates it moved aft under a strong force before it exploded. Also abrasion marks on the aft bulkhead of the NH3 tank shows that it struck the H2O2 tank before the H2O2 tank exploded. Therefore it appears that the NH3 tank exploded first.

In attempting to determine cause of this explosion detail examination of the various X-15 systems and all available data has been going on at NAA, Los Angeles.

Examination of the engine has revealed no evidence that the engine contributed to the explosion. Burn marks are all external to the engine plumbing and appear to come from some other compartment.

There is no evidence that the electrical system malfunctioned so as to cause the explosion, although 4 fuses were found open after the accident (possibly due to shorting of wires during and after the explosion). One of these fuses provided power for the pilot's NH3 tank pressure gage.

Data shows a gradual rise in pressure of the NH3 tank beginning at approximately the time of engine shutdown. This pressure rise was not observed by the pilot (it has not been determined .whether his pressure gage was operative or not during this time). Following the normal engine shutdown the pilot attempted a restart. (All during this time the pressure in the NH3 tank is building up). The engine restarted, but almost immediately, it automatically shutdown (a vibration shutdown was indicated on the data). The pilot reset for another restart attempt and at about this time the explosion occurred.

The NH3 tank pressure regulator, although severely damaged in the area of the loader, was tested in the lab with a new loader and operated normally with ambient gas. However when tested with chilled He it was made to stick full open in the test lab several times. After heating in an oven (to remove all possible moisture) the valve was retested with chilled He and operated normally.

On the basis that a stuck regulator might have caused excess pressure in the NH3 tank, the question was then asked, "Why didn't the relief valve relieve the pressure?" Examination of the relief valve after the accident showed it to be severely damaged, but the butterfly was still free to rotate. Investigation of the vent system down-stream of the relief valve showed that the pressure drop in the 1" vent line to the aft bulkhead of the ship was 60 psi by actual test. A calculated 65 psi for the GSE hose which attaches to the end of the vent, would make a total of 120 psi pressure drop, in the entire NH3 vent line. Since the relief valve is referenced to the vent line pressure and relieves at 65 psi differential, it follows that the tank could possibly be pressurized to l90 psi. (Far above its burst pressure of about 119 psi). This startling discovery resulted in the directive not to pressurize X-15 #l until a suitable safe vent system was installed. In addition NASA has strongly recommended that the regulator be redesigned to prevent locking up in the open position.

While no definite conclusion has been reached at this time regarding the cause of X-15 #3 explosion, the tank pressurize and vent system is under strong suspicion. The investigation of all aircraft systems will continue for at least 2 more weeks and results will be reviewed again at another melting on July 6.

Action resulting from the investigation to date:

1. It will be determined that the fuel system, LOX system, and H2O2 system vents and relief valves are capable of venting off the maximum flow attainable with full wide open regulators, before X-15 #1 is pressurized. This means that a new vent system is definitely required in the WALC system. It is not thought that any change will be necessary in the LOX or H2O2 vent, although this will be proven before we pressurize.

2. A new redesigned regulator will be provided by NAA and will probably be installed before any further flights. This regulator will have an increased positive closing force and will be able to overcome all but the heaviest icing up.

3. Caution must be exercised to insure that no moisture enters any pneumatic system particularly those using chilled gas. All service personnel should be conscious of this danger. Sensing ports for regulators and relief valves should be dry and clean.