Elon Musk, CEO of Space Exploration Technologies – commonly known as SpaceX – gave NASASpaceflight.com an exclusive interview, in which he reflects on the findings of the joint DARPA and SpaceX board which investigated the Falcon 1 maiden launch failure.
In part one of a three part series, Musk talks about the probable cause of the accident, the evidence that led to the conclusion, plus the changes to be incorporated into the next Falcon 1 flight, thrust termination.
NASASpaceflight.com: There was a report this week about the accident investigation. We weren’t slated to talk about that specifically, and I know you guys haven’t put up any official comment on that yet. Is it too early to talk about that right now?
Elon Musk: Ah, no, we can definitely talk about it. In fact I’ve been a little late in posting an update to the web site; it’s been a very very busy period. So I’m actually planning on working on it this weekend. And we also want to coordinate with DARPA to make sure everything we put out there is something they agree with as related to the flight board.
NASASpaceflight.com: According to the AP report, the finding was that it was pretty much a rusty nut that caused the accident.
Elon Musk: Technically, it’s a corroded nut, because aluminum does not rust, but it corrodes.
NASASpaceflight.com: I know in previous statements it had been mentioned that it was a procedural error, and there was an unofficial report of technicians on the pad the day before having done a little bit of avionics work near the engine. Is it a combination of the two, or were the procedural reports incorrect?
Elon Musk: First of all, it is important to appreciate that the DARPA/SpaceX board finding – which was unanimous by the way, there was no dissension – felt that the most PROBABLE cause was stress corrosion cracking of an aluminum nut. That is not to say it was the definitive cause.
The thought, shortly after launch, that it may have been an error on the part of one of the technicians was because we identified very quickly that the leak came from that particular location. Within 24 hours we had established the location where the leak had come from.
We were trying to figure out how this leak could possibly have occurred. One possibility is that the fitting was improperly secured. However, once we recovered all the debris, we were able to find the low pressure fuel line, and the lock wire associated with that nut was still attached. The nut side had been torn off, but you could still see the lock wire.
So it appears that the technicians in fact did secure the nut and lock wire it. It’s not definitive, because you don’t have the nut, but it is improbable that the lock wire would be there if it was not torqued and lock wired in place.
NASASpaceflight.com: So did they find evidence of corrosion in other components, and that is why that was the probable cause?
Elon Musk: Anything which appeared to be corroded – where there was any obvious sign of corrosion on the vehicle – was replaced prior to launch. And of course once we recovered items from the ocean it was kind of hard to tell: did it corrode in the ocean, or did it corrode on the rocket?
We did have a few small components break at one point, with the apparent cause being stress corrosion cracking – there was a small check valve, there was a pressure transducer. We’d never seen an aluminum B-nut crack due to stress corrosion cracking in all the history of the company. And these nuts are anodized for corrosion protection.
So it appears as though a highly improbable event occurred: that in approximately the 18 hours prior to launch, the B-nut failed. It’s not clear exactly when, but upon close examination of the telemetry we are able to determine that the leak started at T-400 seconds.
And we know that there was no leak during the static fire, which was just a few days before the launch. So at the static fire there was no leak and everything was fine, and at launch the leak appeared. In hundreds of engine tests there was actually never a fuel leak of any significance, so it was….
NASASpaceflight.com: Are you still having fun?
Elon Musk: Well, nothing is ever 100 percent fun. I think I’ll feel a lot better once we get something to orbit.
Let me talk about the improvements. There are three types of improvements. There are improvements in the design, for example improving the corrosion resistance of the rocket by replacing the aluminum nuts with stainless steel nuts – which, incidentally, the stainless nuts are cheaper than the aluminum nuts. So it’s sort of ironic that, the thing actually failed because we didn’t use a cheap component, [laugh] we had to use an expensive component.
But let me get back to the three classes of improvements.
One is the design improvements. A lot of corrosion protection improvements. We have fireproof blankets around the engine compartment; there is a kind of carbon fiber honeycomb panel, which is overlaid with aluminum tape down at the bottom of the engine compartment. We’re blocking off the corners; the engine compartment is mostly sealed, and we’re blocking it off with fireproof blankets, and then purging that compartment with nitrogen.
So even if a fire develops, it can’t really attack the particularly vulnerable locations like the pneumatic system or the avionics or the engine bay. We want to be in the situation that even if a fire develops, the rocket just keeps going.
There are improvements to the avionics system; to improve the fidelity of the data, improve the noise essentially. Just a bunch of small things that I think from a system design standpoint would constitute Falcon version 1.1.
None of the changes we’re making have a real impact on cost. In fact, changing to stainless steel B-nuts reduces the cost of the vehicle. [laugh] It makes a tiny reduction in payload, like a half pound reduction in payload to orbit, but there is nothing really that we are doing that effects the cost of the rocket.
Then the second type of improvement is in people and processes. We’re adding aa triple sign-off for all work done on the launch pad, on flight components, and flight critical GSE. You have a technician, a responsible engineer, and then quality assurance will sign the final, record all information, and take photographs of all the work that was done, and then make sure that all information is put into our quality assurance database, which is reviewed prior to launch.
So that does add additional personnel to the launch pad, it adds a couple more people; on the other hand, it’s not as bad as it sounds, because when we just had the double sign-off, which was the technician and the responsible engineer, they also were responsible for doing all the paperwork associated with their tasks…
NASASpaceflight.com: So this frees them up?
The third class of improvements: we are massively increasing the number of aborts. The software health monitoring is going through a huge upgrade. Instead of version one-point-something, I would call this version 2 of the software health monitoring.
The number of checks on the vehicle system has increased by an order of magnitude. It has gone from about two dozen health checks to about 250, and actually if you include all the various sub-categories, in that some things are checked in multiple different ways, it is almost a thousand checks on the vehicle.
This is going to slow us down a little bit for next launch, because we are going to have a few false alarms, where the system thinks something is wrong, but it is actually just a sensor which is malfunctioning.
NASASpaceflight.com: When are you hoping for the next launch?
Elon Musk: That is slated for November.
NASASpaceflight.com: I was interested in the thrust termination system. Are you intending to stick with that? I know there was some concern, or at least talk of concern, at Vandenberg. Whether that is true or not I don’t know.
Elon Musk: Please bear in mind, and you might not be aware of this, but our thrust termination system was first qualified for Vandenberg. So I can’t actually speak to the Cape [Canaveral], but certainly for Vandenberg we worked with Vandenberg to qualify a thrust termination system.
Our vehicle was the first American orbital rocket in the modern era to qualify for thrust termination rather than explosive termination. Russians always use thrust termination.
NASASpaceflight.com: Okay, so that’s definitely staying. You are committed to thrust termination; and also on the Falcon 9 as well?
Elon Musk: Yes, we are committed to thrust termination. I think, frankly, that and thrust termination does not imply any serious increased risk to people on the ground.
Which is worse: is it worse to blow the rocket up, and have essentially a shot-gun effect with shrapnel spread over a wide area, or is it better to have the rocket stay intact and land in a single location, even if there might be some propellant left on-board. I’d say frankly you have a much better containment of debris, a much smaller damage area, if the rocket is in-tact than in fragments.**NASASpaceflight.com Job Opportunities** If you are awaiting a response, please re-send due to e-mail failure.