Overhyping Hypersonics?

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.

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.

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.

in which combustion is sustained by the pressure and heating that results from decelerating supersonic air flow into a combustion chamber

Years ago, when I worked for NASA; the scramjet did not use combustible fuel. Jet Fuel heated air expands by 15-20%, whereas liquid Nitrogen expands 10,000 times it's volume. The scramjet worked by routing supersonic air, through a channel to create controlled turbulence. This turbulence generated extreme heat. Liquid Nitrogen was added just behind the turbulence area, where it expanded 10,000 times it's mass. The result was tremendous thrust, yet no heat trail, nor pollutants.

BTW, this is NOT Top Secret.... it's in just about every article on Scram Jet technology I've read (except this one).

"Very informative. It debunks some of the more absurd claims I've seen since the test."

Actually, a lot of it is bullshit. The part about the "cost of fuel" being the argument for potential scramjet use in spaceflight is a total strawman. The reason scramjet is attractive for use in spaceflight is because of the REDUCTION IN WEIGHT OF THE VEHICLE due to not having to carry oxidizer in some of the booster stages. ANY time you can get a reduction in weight of the spacecraft, it is a MAJOR advantage.

The reason scramjet is attractive for use in spaceflight is because of the REDUCTION IN WEIGHT OF THE VEHICLE due to not having to carry oxidizer in some of the booster stages. ANY time you can get a reduction in weight of the spacecraft, it is a MAJOR advantage.

Except that the oxidizer+tankage, while heavy, is cheap. And for your scramjet, you are either going to need a first stage (how heavy is that) or separate propulsion system to get to scramjet velocity, also heavy -- and expensive.

Trying to get around having a vehicle that is 95+ percent fuel at liftoff is great, but you have to realize that isn't the biggest problem with space transportation right now.

The article is right on.

while heavy, is cheap.

Total contradiction. In the aerospace industry and ESPECIALLY the space launch industry weight is never cheap. It costs thousands per pound to send stuff to orbit. Every pound less of vehicle you can save saves several more pound needed to get those pounds up to speed. Now I agree SCRAM is no magic bullet to the industry but there is a lot of potential in it.

"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."

Boo Hoo. Does anyone really care? People are used to hearing overinflated claims from the media for years. Anyone who believes everything those idiots say deserves to be taken for a ride.

The article doesn't even mention the downside of using pure H2 as your propellant.

The bulk density of LH2 is one fifth that of LOX/LH2, and about 15 times lower than LOX/Kerosene. Therefore, even if you are using much less propellant (by weight), you can still end up with larger, heavier, more expensive fuel tanks.

The other major unmentioned issue is thrust-to-weight ratio. Common LOX/LH2 rockets can get T/W ratios of about 75:1. Common LOX/Kero engines can exceed 100:1. N2O4/UDMH engines peak at around 170:1. Turbojets perform at about 15:1. Even given the theoretical simplicity of a scramjet, there is little reason to expect a T/W of over 30:1.

Scramjets are interesting, but there is no reason to expect them to replace rocket motors for orbital spacecraft anytime soon.

Every pound less of vehicle you can save saves several more pound needed to get those pounds up to speed.

So I ask again, which is better, a pound of cheap oxygen or a pound of really expensive hardware? Something aerospace engineers apparently don't give much thought to is that ten thousand pounds of fuel might cost a few hundred thousand dollars ( but probably much less), while ten thousand pounds of hardware will costs tens of millions of dollars -- at least. And you get to burn the fuel on the way up.

You can't get off the ground with Scramjets and you can't insert into orbit with them. That means that all the weight you save in oxidizer is going to go right back into those systems.

when I worked for NASA; the scramjet did not use combustible fuel

Then that wasn't a scramjet. 'Scramjet' stands for 'supersonic combustion ramjet'.

You seem to be describing (badly) an air-augmented drag-heated LN2 thruster. It would be much lower performance than a true scramjet, but the lack of pollution and thermal exhaust would make it potentially interesting for both military and civilian applications.

"So I ask again, which is better, a pound of cheap oxygen or a pound of really expensive hardware? Something aerospace engineers apparently don't give much thought to is that ten thousand pounds of fuel might cost a few hundred thousand dollars ( but probably much less), while ten thousand pounds of hardware will costs tens of millions of dollars"

Unfortunately for your logic, the ratio is not 1:1. It is more like 1000:1 (weight of liquid oxygen vs. weight of scramjet engine) or even higher. And yes, payload weight IS more important than system cost, especially if the scramjet stage is recoverable/reusable.

It is more like 1000:1 (weight of liquid oxygen vs. weight of scramjet engine) or even higher.

But you have to add in the hardware to get up to scramjet velocity, the orbital insertion hardware, enhanced TPS, additional fuel to compensate for scramjet inefficiency (yes, LH2 is light), drag losses, etc. It isn't as simple a swapping out LOX+tankage for 1000X smaller scramjet.

I am not saying that somewhere down the road scramjets won't be better. I am not saying scramjets are a bad idea or shouldn't be studied.

They are bleeding edge technology that won't payoff for at least twenty years.

And yes, payload weight IS more important than system cost, especially if the scramjet stage is recoverable/reusable.

What other attitude would you expect from an industry that has priced itself nearly out of existence? Right now, if you could put 20klb into orbit for $2 million ($100/lb) it would be a revolution in space launch. On the other hand, 50klb for $500 million is merely what we have now ($10,000/lb).

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