TCS Daily

Overhyping Hypersonics?

By Rand Simberg - March 31, 2004 12:00 AM

After years of failed attempts, NASA successfully flew their Hyper-X X-43A vehicle on Saturday. It's no doubt a noteworthy milestone in flight, and with its revolutionary scramjet propulsion system, it broke all previous speed records for airbreathing aircraft. Unfortunately, like the old magic trick in which the Looneytunes character quaffs gasoline and ingests dynamite with a match chaser, it could only be done once -- at least with that vehicle. As planned, it plunged unrecoverably into the Pacific at the end of its flight.

The scramjet, of course, was the point of the flight. We have had ramjets (in which combustion is sustained by the pressure and heating that results from decelerating supersonic air flow into a combustion chamber) for decades, but as the article points out, a supersonic combustion ramjet (scramjet), in which the combustion occurs at speeds greater than sound, has been viewed as the holy grail of propulsion by many engineers since the dawn of the jet age.

However, it's possible to overstate the achievement, and both the program personnel and the media are doing that. For instance, one would have the impression from the coverage that a scramjet had never been previously tested in flight, when in fact the Australians did this almost two years ago, with a tiny fraction of the Hyper-X budget. The significance of this flight was not that it was an in-flight test of a scramjet, but that it generated sufficient thrust to actually accelerate the vehicle.

More dismaying, at least to me, are the overinflated claims as to potential applications. The technology is being advertised as revolutionizing commercial flight and spaceflight, when in fact it may end up having negligible effects on either.

For instance, a program spokesman on the Fox News channel, before the test on Saturday, claimed that it would allow a trans-Pacific flights of five hours.

Well, no.

First of all, even if it turns out to be technically and economically viable, an aircraft traveling at seven times the speed of sound would actually cross the Pacific in an hour and a half. Five hours is achievable without scramjets, with conventional supersonic technology, though it remains unaffordable.

And of course, that's the real problem. A scramjet is just a new kind of engine that allows hypersonic flight in the atmosphere. It contributes nothing to solving the fundamental issues of supersonic flight, which are wave drag and sonic boom, resulting in unaffordable ticket prices and restriction of flight routes. In fact, it would simply make them worse.

Moreover, because neither ramjets or scramjets work at low speeds (note that the test vehicle had to be accelerated to scramjet velocities with a commercial rocket), any commercial hypersonic transport would have to have conventional (and supersonic) turbofans in addition to the scramjet motors, resulting in two separate (and heavy) propulsion systems.

The other myth found in much of the coverage is that this is a useful technology for spaceflight.

Proponents claim that by allowing airbreathing up to high Mach numbers, there is no need to take along as much oxygen for the rocket engines, because they can gather it for "free." This argument assumes that space transportation is expensive because propellants are, but those aren't the cost driver. If they were, space would already be affordable, because liquid oxygen is actually about as cheap as milk. Propellant costs are such a tiny fraction of launch costs that they're down in the noise. If we ever get to the point where they become a real issue (as they are for airlines), we'll have solved the problem.

Their argument also fails on the grounds that collecting oxygen isn't really "free." As the old joke goes, there's no free launch.

If your space transport were to be single stage, you'd now need three propulsion systems -- conventional jet, scramjet, and rocket for when you left the atmosphere (which you must do by definition to go into space). It may be possible to have a scramjet lower stage and a rocket upper stage, but the bottom line is that time spent in the atmosphere (necessary to utilize the scramjet) is time spent fighting drag, defeating the purpose. Rockets want to spend as little time as possible in the atmosphere, and carrying two other kinds of engines along and spending enough time in the air to utilize them, just to save on a propellant as cheap as oxygen, just doesn't make design sense.

In addition, a scramjet engine is designed to operate at a specific vehicle speed, and has poor performance in "off design" conditions, rendering it a poor propulsion choice for an accelerating vehicle.

Does this mean that the Hyper-X program is useless, a waste of money?

No. Like supersonic technology, this is a technology that will find ready military applications, in which cost and convenience are less important, but it's unlikely to be used for commercial transportation for a long time, if ever. What it will be used for is, among others, hypersonic cruise missiles, able to be launched from a continent away and take out a target in a couple hours from warning, or from a neighboring country or location in ten minutes (think Indian Ocean and bin Laden in Afghanistan here). Such systems would have the speed of ICBMs, with the capability of being recallable.

Traditionally, X programs like this are jointly funded and managed by NASA and the Air Force when there are perceived civilian applications as well as military. Despite program office hype, NASA recognizes that its proponents' claims for air and space transportation applications are highly speculative and premature at best, which is why they cancelled the follow-on X-43C program. It is viewed (rightly, in my opinion) as having little relevance or application to the president's new space initiative. So while this research should and will continue to be funded, it will be done so in the future by the Air Force Research Laboratory (flight expected in 2008), perhaps with fewer overblown promises for it.

Rand Simberg is a recovering aerospace engineer and a consultant in space commercialization, space tourism and Internet security. He offers occasionally biting commentary about infinity and beyond at his web log, Transterrestrial Musings. He last wrote for TCS about The Concorde's Sonic Boomlet.


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