“Space is hard, and today was a tough day.”
George T. Whitesides, CEO, Virgin Galactic October 31, 2014
[Note added 2014-11-05.]
This week was a terrible reminder that complex engineering projects that harness immense energies can go terribly wrong. No enterprise illustrates this like rocket propulsion.
- On Tuesday, October 28, an Antares rocket was bound for the International Space Station. A few seconds after lift-off, there was clear trouble, leading a range safety officer to destroy to rocket.
- On Friday, October 31, Virgin Galactic’s SpaceShipTwo was making its first powered flight using a new rocket engine. There was apparently an engine explosion a few seconds after it was dropped from WhiteKnightTwo mothership. One pilot was killed; another was able to deploy a parachute, but was seriously injured.
[11/5 – Note: “apparently” was not a strong enough word to express doubt about an engine explosion. Many news reports assumed this was what happened. However, certain credible witnesses (e.g., Stu Witt) reported that they did not detect such an event. Given the fourth NTSB briefing, the root cause is almost certainly not related to the engine. When I wrote this, I did not want to take lots of time to explain why an “explosion” was in doubt. That was not the point of this article. But at the time, there seemed no simple way to say, “ignore that for now because it might not be correct”; at times like this, you know your writing skills are still lacking.]
And earlier this year,
- On Friday, August 22, a SpaceX Falcon 9R rocket was testing improvements aimed at reliable pinpoint landing and reusability. A sensor failure provided faulty guidance information, tilting the rocket. An automated flight termination system detected that it was about to leave the test area, and cut the engine. Up to that point, the engine itself had performed without incident.
A rocket engine is a sustained chemical explosion which is shaped to lift objects or people to defy the gravity well of the Earth. From the moment of ignition, it provides an instantaneous kick which doesn’t let up until the propellant burns out, or in some cases, the engine is throttled and shut down. (By comparison, a nuclear fission reactor starts up slowly, heating its core, before it is brought on-line to provide electricity.)
As a result, starting a rocket engine is always dangerous. The bigger the engine, the greater the danger. The challenge to the rocket designer is to keep the propellants away from the ignition source until needed, only ignite within the engine chamber, and never let the sustained explosion spread to anywhere else. By the time the exploded propellants are converted into gas and pushed through to an expanding nozzle, they have accelerated from a virtual standstill to a few thousand meters/second in roughly the blink of an eye.
Thus, when failure happens, it is dramatic. Sometimes failure may not start with the rocket engine itself; it could start somewhere else. A weak structure may buckle, skewing stresses placed on the engine, leading to compromise of the containment of the controlled explosion. A control actuator may fail, causing the engine’s thrust to be off-center of the vehicle, and taking it off course; at that point, an automatic thrust termination system may take over, shutting off the engine or detonating the propellant.
Accident investigation boards are then convened to find out what went wrong. When loss of life is involved, it has a chilling effect on the entire space industry. The sobering truth is: failures are to be expected of any rocket test program. The trick is to catch them before they become catastrophic events.
The sooner failures are caught, the better. Given today’s computer aided design tools, design and simulation is intended to catch failure conditions before any manufacturing takes place. From there, component testing prior to assembly may weed out bad parts and build subsystem confidence. Some components require special tests, such as individual rocket engines, which are subjected to live fire static tests. This is followed by full vehicle or system integration and test.
Assuming nothing catastrophic happens, each subsequent test phase is more expensive than the previous one. If catastrophe strikes, not only do you lose the vehicle; you may lose the test facility.
Friday saw the loss of Scaled Composites pilot Michael Alsbury, who was helping pioneer private commercial space flight. In the past, NASA has had its share tragedy:
- Shuttle Columbia crew / STS-107 (February 2003) – Husband, McCool, Anderson, Brown, Chawla, Clark, Ramon
- Shuttle Challenger crew / STS-51-L (Jaunary 1986) – Jarvis, McAuliffe, McNair, Onizuka, Resnik, Smith, Scobee
- X-15 pilot (November 1967) – Adams
- Apollo 1 crew (January 1967) – Grissom, White, Chaffee
Space is hard. Rocket engine failures are hard lessons. Human physiology in space is hard as well, as are logistics for deep space flight.
Given the difficulty, when one endeavors on a program leading to space, it is important to understand why. Otherwise, it can easily not be worth the cost.