I’d be a little reluctant to consider statistics for missiles, since “mission success” for a missile not only allows for but requires that the payload explode.
Of course, one can also point to fueling and leakage accidents in liquid systems, both caustic storable liquids and cryogenic propellants. We had several accidents with Titan II ICBMs over the years, including an in-silo explosion that ejected the warhead a quarter mile away. The Soviets had even more problems with their R-36M (NATO reporting name SS-18 ‘Satan’). The US Navy quickly went toward solid propellant systems specifically because they posed less of a hazard than liquid propellant rockets.
While it is true that all modern solid propellant grains are essentially high explosive with certain ignition sensitivity to mechanical shock, high temperature flame, electrostatic discharge, et cetera, the sensitivity is not that delicate under normal circumstances. I’ve seen large solid motors that have been shot with a .30 caliber rifle during transportation without deleterious consequences. The biggest problem in terms of hazard is aging sensitivity, where the propellent reaches a sensitivity threshold due to migration of explosive compounds or embrittlement of the plasticizer. ESD isn’t generally a problem with cast grains unless the humidity is remarkably low. Trimming flash of propellant is commonly done by hand on large motors and I’m not aware of an accident; ditto for automated machining of the propellant grain on the Castor 120 or Star motors.
Yes, but the boost vehicle has to deliver the payload successfully to a delivery point, whether it is a satellite or a warhead, and with roughly the same degree of precision. In fact, all but the more recent generation of space launch vehicles are either adapted directly from or derived from strategic ballistic missile (Atlas I-II-III from the SM-65 Atlas missile, the Titan family from the LGM-25C Titan II, the Soyuz/Proton from the R-7 missile, et cetera). Because such systems have to provide adequate surety for strategic deterrence they are tested far more extensively than any commercial rocket system, both prior to and during deployment, which gives a much higher degree of statistical confidence in such system. Of liquids, only the Thor-Delta (and derivatives), Titan II SLV, and R-7 family of boosters have anywhere near enough flights to compare to the “glory trip” data from solid propellant ballistic missiles like MInuteman, Peacekeeper, and the US Navy Fleet Ballistic Missiles. The comparison is directly applicable.
Stranger
I was thinking of delayed failure modes-- If a rocket explodes after some amount of time, and that time is longer than the time it takes to get to target, that’s acceptable for a weapon but not for a vehicle. I’ll grant that I don’t know how common such failure modes are, though.
And strictly speaking, aren’t solid rocket propellants low explosives? As I understand it, a high explosive is one where the burn front travels through the material at supersonic speed, but rocket propellant is designed to have a slow-moving burn front.
Yes, they deflagrate instead of detonating. But the high pressure built up in the rocket housing creates a pressure explosive out of the whole thing.
I haven’t noticed any mention of the political pressure to launch the Challenger before Reagan’s SotU speech. There are plenty of denials that anybody pushed for this launch specifically for that purpose, but it certainly could have shaded the thought processes of those making critical decisions. After all, NASA is a government agency, and every decision has some political aspect to it. That doesn’t mean any engineer or manager chose an unsafe option simply for political purposes, but politics does affect the entire operating environment and can limit the available choices.
There is always pressure to launch because every launch has a political, military or financial objective. Weighing the technical risks against the nontechnical benefits can never be an objective process, because the risks and benefits don’t fall evenly on those involved. But it’s not like in the movies. There’s no evil manager threatening to fire a technician if he doesn’t shred the damning test data. The decision to launch is always a group judgment based on openly shared information. Everyone knows which people never dissent and which ones tend to be the “Chicken Littles” and their opinions are weighted accordingly. The red flags are when someone who’s usually a pushover is expressing concern.
There was a TV movie about the Challenger decision that actually did a decent job of showing what happened.
Things are fuzzier for high-value, unmanned launches. It has been argued that failure of certain unmanned launches could be beneficial to everyone except the taxpayer. With a failure, someone gets paid to figure out what went wrong, and someone else gets paid to build a replacement payload and launch vehicle. It’s not as if people lose their jobs for making bad judgments. Hell, the mission manager on the Columbia accident has continued to rise up the NASA chain with only a temporary demotion/reassignment.
For a ballistic missile, never; all stages of the booster burn out prior to the RV(s) being released on a ballistic trajectory. For MIRVs, the booster actually releases a liquid propulsion post-boost vehicle that has to orient and release the RVs. Premature failure or underperformance of a ballistic missile is by definition a failure as the target will fall way short of the target.
Although the bulk of the material in a solid motor grain are “solids” (typically powdered aluminum and amonium perchlorate as the fuel and oxidizer respectively, plus some burn rate modifiers) along with a polymer binder like hydroxyl-terminated polybutadiene (HTPB) or polybutadiene acrylonitrile (PBAN), some modern solid propellant grains are composite-modified double base or high energy composite, containing high explosive compounds like nitroglycerine, PBX, or RDX/HMX in order to eek out a few extra seconds of specific impulse. Such compositions are generally classified as being Hazard Class 1.1 (mass explosive) while those without are generally classed as being 1.3 (mass fire, minor blast, or fragment). However, these categories are misleading as they refer to their explosive properties without regard to geometry or confinement. Class 1.3 substances can certainly detonate under some situations, one of those being inside of a pressure vessel (which a motor case is) when experiencing a large overpressure pulse, called a detonation-to-deflagration transition, or DDT. From Cooper’s Explosives Engineering:*If an explosive is ignited, it starts to deflagrate, and if it is confined such that the reaction product gases cannot escape, then the gas pressure in the deflagrating region builds up. Burning reaction rates are a function of pressure as well as temperature; therefore the reaction rate increases as pressure increases. The high-pressure forces the hot gases into the surrounding material and the entire process accelerates. Pressure waves generated in the deflagrating region now can compact and compress the explosive material in the path of the waves. This causes greater confinement and hence even greater pressure buildup. The compressional waves will shock-up and, given sufficient time and distance, form the shock conditions to cause detonation.
We call this process the deflagration-to-detonation transition, or DDT. This process can occur accidentally in large explosive charges where the bulk of explosive itself provides the necessary confinement. DDT is utilized intentionally in the design of certain detonators where primary explosives cannot be used…The conditions necessary to achieve DDT depend upon such factors as confinement, particle size, particle surface area, packing density, charge diameter and length, hat transfer, and thermochemical characteristics of the particular explosive.*
Detonation of Class 1.3 substances can also occur by using an energetic penetrator (stinger) from a conical shaped charge or mass static buildup. (Many Class 1.3 propellants are actually more sensitive to ESD than Class 1.1 propellants.)
Stranger
That ma be true at four-letter agencies and Circle-A Ranch, but that definitely isn’t true in industry or the services, where a failed launch that can be traced back to someone making a poor decision or dropping the ball can be a literal career-ender.
The value of failure (beyond milking the taxpayers out of their hard-earned cash) is that you almost always learn something new about what your system can or cannot tolerate, plus you are forced to look at all of the potential failure modes as part of a disciplined failure review process. It is unfortunate that the failure that led to the destruction of Columbia took so long to occur (presuming that it had to occur at all) because it brought to light literally hundreds of problems with the basic design of the Shuttle that were known from the time of the Challenger Accident Investigation Board and even from the early days of the first four operational flight tests. Had we been forced to learn that information before, it may have driving design changes in the STS system that would have moved toward a more functional and reliable system. Every time you succeed, all you learn is that you were some combination of good and lucky, and you never really know the ratio of one to the other. When you fail, you generally have a good idea of just how unlucky you were that day, and how to improve your odds tomorrow.
Stranger