Ramjet Performance Primer

1. Introduction

Ramjets, at least on paper, have been around since the early part of the 20th century. They have been flying since at least the 1940s with many research and production engines being flown by many nations.
This primer describes, principally, the performance capabilities of conventional (subsonic combustion) ramjets, both demonstrated and theoretical. It is intended to excite entrepreneurs about the possible opportunities and entice engineers to take another look at this stepchild of the jet propulsion age.
This primer does not describe the technology of ramjets, something left for engineers and technicians to research. Nor does it discuss scramjets beyond mere mention because their commercial use is still decades away.
This primer makes reference to it’s sources to distinguish between demonstrated and theorized capabilities but citatations are not included because this is intended to pique interest, not convice.
For those readers who can’t live without the technical details the following web sites are recommended.
- Onera’s page on Ramjets, Scramjets, and PDE
- ARLA Introduction which includes lengthy references and source materials

2. A Brief History of Ramjets

Most of the available literature on ramjets is dated. It appears that there was great interest and research during the 1930’s through the 1950’s but then research tapered off to supporting specific applications. As with most achievements, anything that was demonstrated 50 years ago can surely be done better and easier today. The references here are intended to put the performance measures below into historical perspective.

1913: Rene’ Lorin patented a subsonic ramjet design but had not considered supersonic applications
1928: Albert Fono was awarded a German patent for a supersonic ramjet
1936: Rene’ LeDuc demonstrated positive thrust from a ramjet, though this was probably in a wind tunnel
1941: A 2,000 hp ramjet is mounted and tested on a Mercedes Benz truck
1942? A 20,000 hp ramjet is tested on a Dornier DO 217 aircraft
1946: The US begins ramjet flight testing by fitting two Marquardt engines onto a P-51 Mustang.
1947: Ramjet powered Little Henry helicopter flies
1948: F-80 flies with ramjets on the wingtips
1949: The LeDuc 010, developed by Rene Leduc, is the first ramjet-only powered aircraft, though it was launched from atop a conventional aircraft. Gorgon missile flies at Mach 0.9
1951: X-7 achieves Mach 3.95 at 55,000 ft and Mach 2.54 at 95,000 ft
1952: Ramjet powered Big Henry helicopter flies
1953: The ramjet powered Rigel missile flies at Mach 2
1956: The LeDuc 022 Turbine/ramjet aircraft begins flight testing
1957: The Nord Griffon II flies with a turbo-ramjet, eventually achieving Mach 2.19
1958? The X-7A achieves Mach 4.31. The Talos missile enters service
1959: The BOMARC missile becomes operational
1966: The turbo-ramjet powered SR-71, advertised at Mach 3.2, enters service
1980: The ASALM inadvertantly achieves Mach 5.5 at 40,000 ft

3. Ramjet Performance Measures

Many myths and misconceptions abound relative to ramjet performance measures. Because ramjets have been used almost entirely in “niche” applications most engineers know little about their true capabilities and have come to believe that what they’ve been used for is all they can be used for, which is not accurate.
And to be honest, the vast majority of propulsion applications to date have been better suited to other technolgies. But a new frontier has been opened with (wealthy) tourists buying tickets to space, private suborbital astronauts, and interest into vertical drag racing. These new endeavors need propulsion systems that are cheaper, safer, and more robust than the rockets and turbojets currently available on the market. It is in these new types of markets that ramjets should come into their own.

Velocity:

General: Ramjets have historically been designed for either subsonic or supersonic flight, though rarely for both. For the most part this has been for simplicity of design, especially in the inlets, and for performance optimization. One reference suggested that a subsonic inlet might be sufficient to operate at velocities up to Mach 7. The ASALM, with a fixed inlet which would have been suitable for subsonic flight, achieved Mach 5.5.
Lower Velocity Limit: The literature (including the internet) is littered with various claims for the lower limit on ramjet operation, most of which are tainted by lack of research, unstated definitional constraints, and pure prejudice. In fact, ramjets cannot produce thrust while standing still but can achieve thrust at all velocities above that, though the thrust and efficiency at very low velocities makes them unsuitable for most uses. Practical experience indicates that they have been usefully operated below 200 mph. The literature found says that the LeDuc 010 was launched from atop a conventional airplane at 200 mph in order to operate the engine. The literature on the F-80 tests of ramjets on the wingtips states that they were ignited at 200 mph. And the Little Henry helicopter started it’s ramjets at rotor tip velocities of 150 fps, or about 100 mph.
An amateur ramjet experimenter interviewed stated that he started his rotor-tipped ramjets by stuffing a gasoline-soaked rag in the exhaust, lighting it, giving the rotor a swing with his hand, and letting it accelerate on its own. Interestingly, his test stand was a long rotor on a stand in the middle of a pasture.
One way around this lower velocity limit was achieved by the LeDuc 022 which used a turbojet for takeoff. The 022 first flew in December 1956 and in 1957 flew 141 test flights. Other ways to get ramjet-powered vehicles started include catapults and tow-planes such as those used by gliders.
Upper Velocity Limit: The upper velocity limit on (subsonic combustion) ramjets is debatable but a review of the literature (many sources) and personal interviews with three ramjet experts all agreed that Mach 7 was achievable, though one stated that it may be impossible to keep the airflow from going supersonic inside the engine at Mach 7. About half the references indicated that Mach 9 was achievable and one thought that Mach 11-13 might be achievable.
Practical experience bears some of this out. During the 1950s the X-7 was tested to Mach 4.31. In 1980 the ASALM, which was designed for Mach 2.4 – 4.5, suffered a stuck fuel throttle and accelerated to Mach 5.5 at 40,000 ft over White Sands before running out of fuel. There are enough charts on ramjet performance that go up to Mach 6.5 to justify speculation that someone has actually demonstrated that velocity but that it has not been publicly acknowledged.
Mach Range: One expert stated that a reasonable range of operation for a ramjet is three Mach numbers. Therefore, if a ramjet is to get to Mach 5 then it should be started at Mach 2. However, there was nothing in the literature to indicate that this was due to any physics and so it is probably a practical design tradeoff used for development of military ramjet applications (of which he was an expert on). One NACA report details an analysis that indicated that a normal inlet (which can be used for subsonic flight) could provide positive thrust to Mach 7. The experience of the ASALM seems to support this analysis.

Specific Impulse:

ISP: The theoretical maximum Specific Impulse (Isp), which is a measure of fuel efficiency, for hydrogen fueled ramjets has been calculated at about 4,000 seconds but the vast majority of ramjets that have been built have been fueled by kerosene (or similar fuels) which has a theoretical maximum Isp of about 2,300 seconds. Of the ramjets for which data was available, mostly early designs, the achieved maximum Isp was closer to 1,800 seconds. This is very low compared to turbojets but very high compared to rockets.
Actually, the Isp of ramjets is very much a function of velocity and fuel/air (F/A) ratio. At zero velocity the Isp of a ramjet is also zero. At about Mach 0.5 it is the same as a typical LOx/kerosene fueled rocket, or about 350 seconds. Above that speed the Isp increases to a maximum at about Mach 2.5. From there it declines until it drops below that of LOx/kerosene rockets somewhere around Mach 9. At about Mach 3 the ramjet Isp exceeds that of a conventional afterburning turbojet. One reference estimated that a conventional ramjet may have better Isp than scramjets up to about Mach 8.
While the peak thrust occurs at a stoichiometric F/A mixture the peak Isp does not. The Isp of ramjets increases as the mixture leans out,which is good for keeping the engines cool but may increase the mass of the engine needed to obtain the same thrust. The increased size may also increase the overall drag on the aircraft carrying the engine. On the other hand bigger engines offer higher peak thrusts.
MPG: While Isp decreases above about Mach 2.5 a ramjet will achieve it’s highest range or cruise efficiency (miles/gallon) at about Mach 3.5 because ground speed increases faster than Isp decreases. It also allows the aircraft to fly higher where the air drag is lower.

Thrust:

General: Ramjet thrust is dependent on many factors such as velocity, air density (altitude), F/A ratio, and efficiency. As a baseline the available literature offers up two empirical measures of thrust for making rough estimates. One is for 90 lbs of thrust for every square inch of inlet and the other is for 20 lbs for every square inch of combustion chamber cross section. Since the combustion chamber diameter is often about twice that of the inlet these two measures are roughly consistent. This says that a three-inch diameter ramjet would produce 140 lbs of thrust and a three-foot diameter ramjet would produce 20,000 lbs of thrust. The literature is not clear but these estimates are probably based on sea-level, Mach 2.5 flight conditions with stoichiometric F/A mixtures.
Velocity: Below Mach 2.5 the thrust drops off to about 1 lb/square-inch of combustion chamber area at 200 mph. Above Mach 2.5 the thrust increases until other factors begin to reduce it, somewhere above Mach 5.
Altitude: At 40,000 ft the air density is about 1/4 that of sea level and therefore the thrust would also be about 1/4, given otherwise similar flight conditions. At 80,000 ft the air density is about 3.6% that of sea level with an equivelant reduction in thrust. However, the reduction in thrust due to altitude can be compensated for by an increase in velocity, within a certain limits.
Fuel/Air Mixture: Maximum thrust occurs at a stoichiometric F/A ratio and drops off as the engine is operated otherwise, particularly when operated lean. Operating fuel rich will decrease the thrust due to the cooling effect of the unburned mass but this will be quite gradual.
Efficiency: Such factors as internal drag, expansion ratio between inlet and combustion chamber, and fuel mixing will affect the efficiency of the engine and the thrust it produces.

Altitude:

Cruise: The cruise altitude limit for ramjets is not clear in the literature. However, the Bomarc CIM-10B had a service ceiling of 100,000 ft at Mach 3. Because both aircraft lift and ramjet thrust increase with velocity a ramjet powered vehicle should be able to cruise at least at 120,000 ft and possibly in the 140,000 ft regime under the right conditions.
Zoom: 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).

Weight:

There are no physical limits to the minimum weight of a ramjet other than design and materials. The 1950’s Marquardt RJ43-MA-7 had a thrust/weight (T/W) ratio of about 40. With today’s engineering and materials that could probably be brought up to 150-200 without too much effort. Such T/W ratios would make ramjet powered vehicles excellent accelerators.

Cost:

Per pound of thrust ramjets are the lowest cost jet engines that can be built (when operated near their optimum). They have no moving parts, no compressors, no turbines, and have extremely simple combustors. They are, simplistically speaking, just light-weight aerodynamic shells.
Because ramjet propulsion systems (including hardware and fuel) are lighter and safer than either solid or liquid rockets, the launch systems will have lower overall costs and higher turnaround rates. Regulatory restrictions should also be lower than for rockets.

Development Time:

Because there are so few engineers with ramjet experience the first ones built for commercial use will carry the cost of learning (textbook and practical experience), research and development, and flight testing. However, much of the early technical data is still available to anyone who wants to read it and any experienced jet propulsion engineer should be able to grasp the specifics and begin design work within weeks. Flight testing could begin within a few months and a rudimentary Mach 4 production design proven within a year.

4. Ramjet Performance Improvement Potential

During the past couple of decades there has been some research into ejector ramjets, combined cycle (rocket, ramjet, scramjet) engines, and pure scramjets. There is some ongoing development of military ramjets but the only associated research is specific to the designs under development. Overall, ramjets seem to be passe’ within the research community even though there are so many questions begging answers. Below are some research activities that could extend the performance of ramjets well beyond that which has already been demonstrated.

High Velocity: If ramjets can, as some suppose, operate in the Mach 7-13 regime then their ability to dramatically reduce the cost of spacelift could revolutionize that industry. They could also make space tourism a near-term reality.
Low Velocity: Very low velocity ramjet operation could almost eliminate the biggest weakness of ramjets, thereby lowering overall systems costs. As a spin-off these could be used for auxiliary engines for gliders, increased takeoff weights for transports, and more.
Velocity Range: Little has been done over the years to identify the true Mach range of ramjet designs. Such research, combined with extending the high and low Mach numbers of ramjets could make them competitive with other forms of propulsion in more applications.
Noise Reduction: Ramjets that could operate quietly at subsonic speeds might be suitable for a wide variety of commercial and private applications.
Efficiency: There’s always room for improvements in efficiency.

5. Summary

Ramjets have demonstrated capabilities that exceed those of jet engines and rockets throughout portions of the atmospheric flight regime. Their limitations, real and perceived, along with their unknowns, have unnecessarily prevented them from wider applications to date.
There are some emerging applications, one in particular being sub-orbital space tourism, which could make ramjets the engines of choice for many entrepreneurs. Ramjets could open up whole new industries and markets for those with imagination and the willingness to work through their limitations. This could well be the century of ramjets.