Air Force Mach 17 air-breathing aircraft program

NAS committee (~2005). Air Force wants a Mach 17 non-man-rated engine for cruise-missile-style strikes. Frictional heating to 3000–4000°C against materials limits (W ~3300°C, Ir ~3100°C). Justified by need to strike anywhere in the world in 20 minutes (bin Laden window). Tom: dreamed up by people who believe Star Trek is real.

We're really pushing the limits. But the really interesting thing — I'll take two more minutes. The Air Force explained their goals, and their goal was to have a Mach 17 non-man-rated engine. If it's not man-rated, basically this is for some cruise missile. I said, "So you're going to go Mach 17. We already go Mach 17 in space, what's the big deal?" "No, this is in the air." They wanted to go Mach 17 in air. I said, "Well, have you ever heard of frictional heating?" Okay, I didn't say that. But the problem is, when a spacecraft at Mach 17 is coming into reentry, what happens? They have to have these carbon tiles that absorb the heat. The frictional heating will give temperatures of 3000 degrees centigrade.

2004 National Research Council committee on Air Force propulsion. Air Force requirement: Mach 17 unmanned warhead (motivated by 15-minute window to strike bin Laden) plus 25,000-mile unrefueled range from continental US bases. Tom's analysis: 6,000-degree radiation heat transfer alone exceeds copper melting point, no boundary layer can save it.

You might think it's sort of funny — I thought it was sort of funny until in 2004, I was on another National Research Council committee on aircraft propulsion for the US Air Force. We were supposed to help advise the Air Force on what materials and designs to use for the three hundred million dollars a year the US Air Force had over the next 20 years to develop better motors. They came in and gave us a non-classified presentation — the Air Force wanted a Mach 17 warhead to fly through the air, not through space but through the air, at Mach 17. The reason was — this is before they killed Osama bin Laden, somewhere around 2004-2005 — they had actually located Osama bin Laden and they had 15 minutes to get the ordnance on site. But he was gone from that region within 15 minutes. If they had the right weapon on one of the aircraft carriers in the Persian Gulf, and it went Mach 17, they could have had it on site and blown him up.

Air Force propulsion-research committee Tom served on. Two grandiose goals — 25,000-mile range and Mach 5 / 17,000 mph — justified by a single anecdote about an Osama bin Laden time-to-target requirement. Tom argues this is a cruise missile masquerading as an aircraft, and the material limits make it infeasible.

The great thinkers in the Air Force still have these grandiose plans. About five or six years ago I was on a committee for propulsion — how the Air Force should spend $300 million a year in their propulsion research budget. They had two goals. One was to fly 25,000 miles without stopping. I said, why do you need 25,000? Why isn't 12,000 enough? Half of a great circle will get you anywhere on Earth, and we can actually do 12,000 miles with the latest 747s — the Dreamliner will go half a great circle. They said, we don't think we can ever depend on bases anywhere except in the continental United States.

Air Force propulsion committee, ~2003–2005. Wanted Mach 17 in air to deliver ordnance on Osama bin Laden within 15 minutes from a Gulf aircraft carrier. Skin frictional heating at 6–7,000 Kelvin leaves only three or four candidate materials (carbon, hafnium carbide).

Getting back to the science of material properties — we finished up externalities and we did cost and availability, and now we want to talk about material properties. There are limits to the properties of materials, and some people will design systems that are way beyond whatever we can really do. Did I tell you about the Air Force propulsion program? This is a committee I was on down in Washington about twelve years ago. The Air Force had a couple of requirements. One is, they wanted to have an engine that could go Mach 17. Mach 17 is basically the speed of the satellites going around the earth, 15,000 miles an hour. I said, in air? They said yes. I said, this is man-rated? They said no, it's not. I said, why do you need something that will go Mach 17 in air? Because if you look at the frictional heating of the skin, you get to temperatures of six or seven thousand Kelvin. There's only three or four materials in the world — carbon, hafnium carbide — that don't melt or vaporize at 6,000 Kelvin. You have a very limited choice of materials.