Virgin Galactic’s Spaceship Two exploded over the desert, killing one pilot injuring the other.
At this stage, it *seems* that there was an explosion right after the spaceplane ignited its hybrid rocket and that the vehicle tore apart, with one pilot coming down under a chute.
Coming so soon after OSC’s Antares blew itself to bits right off the pad, it’s expected that some eyebrows will be raised about whether something fishy might be going on. While that’s always possible, a simpler explanation is that both suffered from the deficiencies of their propulsion systems. NOTE: I have no special insight here, this is all speculation on my part.
The first stage of the Antares uses two Aerojet AJ26 rocket engines… which are refurbished Russian NK-33’s. More accurately: these are *Soviet* NK-33’s, built about 40 years ago, transferred to Aerojet, stripped and rebuilt. *Any* mechanism that old will need some careful looking-over. And while having them refurbished is good, they were refurbished by a different company that built them, with little to no tribal knowledge. All kinds of problems can be introduced here.
The SS2 uses a hybrid rocket motor that has been troublesome for *years.* Virgin Galactic has recently switched from the original nitrous oxide/rubber (hydroxyl-terminated polybutadiene, commonly used as a binder in large rocket motors, and perhaps better known as tire rubber) propellant combo to one using “plastic” (polyamide, like nylon). This was due to the engine not providing the performance it was supposed to. Rumors I’ve heard held that the original engine fell *way* short, and shook really, really badly, to the point of worries about injuries or structural damage due to the harsh vibrations. This, sadly, is a not-uncommon problem with hybrids. And this new engine has not flown much (if at all) prior to this flight. It would not surprise me if there was a hard start (basically a small detonation on startup) and the plastic-based fuel cracked or shattered. This would eb all kinds of bad, especially if a chunk got caught in the motor throat. This would cause the chamber pressure to spike until it burst. Additionally, nitrous oxide is occasionally rather twitchy. While often considered fairly sedate by liquid oxidizer standards, there’s a problem: the triple point is just under 100 degrees F. This means a tank of compressed liquid N2O is happy forever with a pressure of 800 psi or so, but if the temperature rises to about 100 degrees, the liquid N2O will flash to gaseous N2O and the pressure will climb to 10,000 psi or so. This *probably* wasn’t what happened here, as the vehicle had been dragged to 50,000 feet or so by the White Knight carrier plane. But the air temp at 50,000 is really, *really* cold, so it may be that the plastic fuel was also very cold… and thus, perhaps, very brittle.
If Virgin Galactic pulls through this, they will have to do some serious redesign. Hundreds of rich folk have given them large sums of money for seats; I can see a whole lot of legal hijinks as various celebs or their legal counsels try to back out or start making demands. It seems to me that VG would be well advised to simply bail on the hybrid rocket. The system has been troublesome for a decade… and its not the first time it has killed people.
VG might do well to consider changing to a liquid bipropellant rocket system. Ironically, perhaps their best choice for such a system would be to contract for such a system from XCOR Aerospace. XCORs engines appear to be the most reliable around… perhaps not the most bleeding edge in terms of weight and performance, but if you don’t need the absolute maximum in performance (and a suborbital vehicle is far more tolerant of performance shortfalls than an orbital vehicle… how many passengers would even notice if the craft only attained 98% of predicted apogee?) and you *do* need the absolute maximum in terms of not-blowing-up, then XCOR seems the way to go.
The irony, of course, is that XCOR was just about the only competition VG had in the suborbital tourism market in the form of their Lynx rocketplane, which is under construction now.
Today is a bad day, have no doubt. A pilot was lost (and another seriously injured). A vehicle was lost. A whole program might well be lost. And perhaps worst of all, long term, is that something horrible has been gained: an excuse by regulators and bureaucrats to add yet more layers of laws on top of this, perhaps heading towards simply banning civilian manned rocket flight in the US. And with the wonders of ITAR regulations, it may well be that American rocket companies, banned from flying in the US, will be banned from transferring launches outside the US. And thus space tourism will belong to other nations, less risk averse.
An aside: I’ve got CNN running right now. Like a silly, naive person, I was hoping that they might have some useful information. But… no. Just the usual blather you get from talking heads who have to fill air time but who have minimal data to impart. Sadly – and inevitably – they’re subtly smack-talking private industry, questioning whether there was insufficient regulation and government oversight