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Data from decades-old missile tests suggests ramjets could have better performance than expected, which has implications for their use in launch systems.

The ramjet mystery

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In an earlier article of mine published here, I discussed the relevance of an old movie, Destination Moon, to the current maneuverings over the question of going to Mars (see “Echoes from the past: the Mars dilemma”, The Space Review, June 6, 2016). I pointed out that the Destination Moon storyline had private business take up the challenge of the first landing on the Moon in order to spur the government to do something about it, and that it rather looked as if this was going to happen with Mars.

The ramjet has faded into memory as a propulsion system that only works at supersonic speeds and is generally inefficient when it does.

That still seems possible (but not quite as probable as it did a few years back), but my point is that the quality of many of the comments about the article led me to suspect that some readers of The Space Review were space-mad 14-year-olds exulting in being able to sound like a real grown-up while remaining safely anonymous. In this I did you a disservice; other, later articles that had less appeal to aspiring space cadets brought numerous very sensible comments (over 70 in the case of one of them.)

Knowing this, I would like to try to turn this wealth of experience to my advantage by asking if anybody can suggest the reasons for a mysterious, but well-recorded phenomenon concerning ramjets on the edge of space. Which is, why do they still work up there?

I should explain that ramjets are a key intermediate step in my spaceplane proposal, the Swala vehicle. This uses a linear motor, such as the US Navy now employs for its catapults, to enable it to be launched from a carriage at something over 400 kilometers per hour at which speed its ramjets will have enough thrust to enable them to lift the vehicle up and away. These then take it up to about 30 kilometers (100,000 feet) and Mach 5, at about which point they will—should—flame-out and are parachuted back for re-use. Thereafter a big solid fuel motor carries the vehicle and its half-ton payload to low Earth orbit. Once rid of the payload, the vehicle, by now a mere shell, re-enters at its leisure, ultimately to be recaptured back on the launch carriage, so doing away with the need for it to lug an undercarriage around.

The most detailed documentation I have seen on ramjet operation comes from the French government’s aerospace laboratory, Office National d’Etudes et de Recherches Aérospatiales (ONERA). In 1965, it issued a report on high-speed, high-altitude experiments with ramjet missiles. This operation received the code name of “Stataltex” and the document recorded that four out of ten missiles reached a flight speed of nearly 4,600 feet per second, i.e., about Mach 5, between the altitudes of 12,200 to 35,000 meters (40,000 to 115,000 feet).

But thinking on ramjets at that time had been conditioned by their near-exclusive use as the second propulsion stage of surface-to-air missiles, the first being a solid fuel motor to bring the vehicle up to Mach 1 (and usually nearer Mach 3) to enable the ramjet to function. Several missiles used this system, such as the Air Force’s Bomarc, the Navy’s Talos, the British Bloodhound, and the ONERA Scorpion. But the arrival of the ICBM rendered bombers obsolete, along with their countervailing missiles, and the ramjet has faded into memory as a propulsion system that only works at supersonic speeds and is generally inefficient when it does.

The first of these statements is not true and the second doesn’t matter. Ramjets have flown successfully at about 320 kilometers per hour, and when you are going to reach the upper limit of their operating envelope in under a couple of minutes, you can afford to be wasteful in the matter of fuel. By contrast, those earlier missiles were intended to fly up to several hundred kilometers. (It is probably necessary to add here that, in the case of the Swala vehicle, the ramjet’s supersonic shock-front problem is overcome by a flattened inlet below a “chin” under the wing, as the artist’s impression above indicates.)

But just how high and how fast can ramjets go? Here is an interesting quote from “A Ramjet Primer” written by the late Glen Olson:

In 1951 NACA launched a ramjet powered missile which reached an apogee of 159,000 ft. This missile was launched at a 75 degree angle and ran out of fuel at 67,200 ft and Mach 2.92. This missile could have reached astronaut-wings altitude with almost any combination of a) steeper launch angle (it was still doing over 1,000 fps at apogee), b) more fuel (it started with only 25 lbs or 11% of its mass), and/or c) bigger engines (it had two 6.6 inch diameter engines).

This zoom performance was indeed remarkable. Its ballistic trajectory took it up to about 2.3 times the altitude achieved with powered flight, and the missile was still travelling at Mach 1 at apogee. But when trying to get to orbit, height is good but speed is vital. What speed might the missile have achieved if it had more fuel? I had hopes that the Stataltex tests might tell.

This raises the remarkable possibility that orbital altitude, if not velocity, might be attainable without rockets.

The sixth and ninth of these tests reached Mach 5 at just under 30 kilometers. A key point about them and all the other tests at the time was that the trajectory was intentionally a flat one at that height: these were anti-aircraft missiles, not space vehicles and 30 kilometers was their goal. But what was startling was that, until their fuel ran out, they were still accelerating at this altitude, the sixth test at 0.8g and the ninth test at 1.2g. How could that be?

The reason for sounding querulous is that there seems simply not to be enough air at that altitude to generate thrust. At sea level, 400 kilometers per hour is a practical minimum for a subsonic ramjet, when its specific impulse is down to about 200 seconds compared to over 800 seconds at Mach 1. Here are the comparative numbers (using the 25-centimeter inlet diameter of the Stataltex ramjet):

Altitude Sea Level 30 km
Air Density kg/m3 1.225 0.01841
Missile Speed, km/h 400 5,500
Ramjet inlet diam. M 0.25 0.25
Ramjet inlet area m2 0.049 0.049
Ramjet air mass flow, kg/hr 24,056 4,971

So up there it seems that only a fifth of the amount of air is needed for the ramjet to perform at least as vigorously as if it were at sea level. This is not because of the reduction in drag; that is more than compensated for by the increase in velocity. Here, again, are the numbers, this time for the Swala vehicle. They assume a drag coefficient of 0.3 and use the simplified drag equation - a x 0.5c x ρ x v2/1000:

Altitude Sea Level 30 km
Missile Speed, km/h 400 5,500
Missile Speed, m/s (v) 111 1528
Vehicle frontal area, m2 (a) 4.5 4.5
Assumed drag coefficient (c) 0.3 0.3
Air density, kg/m3 (ρ) 1.225 0.018
Calculated drag force, kN 10.2 29.0

So what is going on? One remote possibility is that the amount of monoatomic gas at 30 kilometers is far greater than expected, and this re-association of oxygen and nitrogen into the diatomic forms in the ramjet is providing the heat energy needed to drive it forward in that tenuous atmosphere. But this assumption would put such gases about 70 kilometers below the level where they normally exist, and anyway their concentrations appear to be well under those needed for a significant contribution to thrust.

I realize that I may be overlooking something important and am prepared to get egg on my face as a consequence. But the stakes are very high. Consider this: if ramjets can continue to accelerate in an environment where the air density is under 20 grams per cubic meter, then perhaps they can sustain powered flight to 40 kilometers, where the density is only 4 grams per cubic meter. If thereafter they do flame out, their ballistic trajectory, based on the 1951 NACA experience, could reach an apogee of about 90 kilometers. This raises the remarkable possibility that orbital altitude, if not velocity, might be attainable without rockets. Then perhaps there really is a chance of monatomic gas propulsion. And just possibly then, an altitude could be attained so great that it that would allow a ballistic acceleration to orbital speed…

Well, dream on, but your (adult) comments will be very welcome.

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