There is so much disinformation and lack of information going on regarding the SS2 crash and their choice of engines, that I am going to take a major risk and jump into the discussion. I have not been a part of their team but I know many many of the players and worked with some of the people who were there in the beginning. Hopefully we can raise the signal to noise level a bit regarding what happened. This is a slight modification of what I wrote in a reddit.com thread today.
This is also specifically in response to a Reddit thread on Joel Brenner’s appearance on CNN yesterday.
As someone who worked with many rocket scientists, and who has designed the power systems and avionics for, and worked with those who have built and fired engines, let me say this.
Burt Rutan with the original Spaceship 1 brought in the hybrid engine design SPECIFICALLY because it was sold to him as being the safest type of engine. A hybrid engine with a solid fuel and a liquid oxidizer has the ability to be shut down like a fully liquid engine, without some of the problems that come from a fully liquid or solid design.
That was the theory. The truth of the matter is that there is no such thing (at our current level of technical maturity) as a perfectly safe rocket engine. ALL rocket engines are an exercise in design compromises between cost, operability, and complexity as integrated into the larger system.
I worked with the German rocket scientists in Huntsville Alabama and they (specifically Konrad Dannenberg) told me that one of the reasons we were successful in our engine selection and reliability of the Saturn V launch vehicle was that when they were at Peenemunde they literally built and tested engines with every single combination of chemicals. The Germans did not use solid propellants as for launch vehicles solids are low performance with poor total impulse and hybrids were never seriously considered due to the nature of the chemical industry in Germany at the time.
Hybrid engines do not have the pedigree of either all solid engines or all liquid engines in terms of number of hours of operation. Many observers, myself included have stated that an all liquid design might be better for their application. HOWEVER, we are not on the ground with them, nor do we have full insight into the compromises that have be be made (ANY ENGINEERING DESIGN INVOLVES COMPROMISES) for the application of suborbital tourism with that type of airframe.
Joel Brenner is a friend of mine as well but she is not an engineer and thus listening to many armchair (or professional) rocket scientists discussing the SS2 system design, she does not have the experience to judge the nuances involved when you are in the room with the engineers working the design at the system level.
I know for a 100% certain fact that for Burt Rutan (and I was present at 2 out of the 3 test flights of Space Ship 1) safety was and is always paramount to any design that he does or is involved with. His safety record with composite aircraft is stellar and it is a testimony to his talent that composites are now taking over the entire aircraft industry (the Boeing 787 is the latest implementation).
In terms of an all liquid design, which is what many armchair rocket scientists (as well as professional) say that VG should use, the answer is not at all as clear cut as some would declare. They are not a walk in the park. Just take a look back at our friends at SpaceX. They lost their first three vehicles from various issues related to propulsion systems. Just this past week a mature all liquid design flown by Orbital Science failed 16 seconds after launch. In the early days of rocketry we lost far more vehicles than made it to orbit. As in no other business developing vehicles that travel from the surface of the Earth, from zero velocity to space, has never been easy.
When we were (at Space America in the late 1990’s), developing a pressure fed LOX/RP-1 engine, which is basically the simplest possible liquid engine it was not a cake walk. Not only did we blow them up (even with the technical backing of several incredibly experienced Huntsville rocket scientists involved) at the test site on the stands, we blew the engines clean off the vehicle on the pad in the west Texas desert. We did this when there was only five moving parts in the entire vehicle. Here is a picture of us there in the desert. Note that this is the same site that Jeff Bezos later purchased for Blue Origin.
An all solid design is basically a non starter due to the safety problems of not being able to turn it off after it is lit.
If you look at the many companies that have come into being and then failed trying to build launch vehicles for unmanned payloads much less humans, the only conclusion to draw is that it is an incredibly complex engineering problem where not only do you have to get the small things right, you have to get all of the small things right and the big ones as well.
A choice at the power point level or even preliminary design can doom you to failure years later because of an unwarranted assumption. This is what happened to NASA in the Ares 1 design that was part of the constellation program. Some would claim that the choice of a hybrid design fell prey to the unwarranted assumption that a hybrid was inherently safe. This may be true. However, even if true it does not imply either incompetence or malice as sometimes the only way to find out these things is to build and fly them. Many lives were lost trying to break the sound barrier simply because we did not understand that a taller tail would provide more stability at the sound barrier pressure interface. An X-15 crashed just a few miles away from where SS2 crashed as we continued to learn about space.
Thus the point to be made is that there is no such thing as an inherently safe rocket engine (as the Antares disaster earlier in the week testifies to), and that there is nothing inherently unsafe about the design of a hybrid engine. My friend Joel Brenner allowed the emotion of the moment to get the best of her in her CNN interview and it is probably not good to ask questions of people who are deeply involved in the heat of the moment.