The ram-air compression (Ramjet), or air-breathing jet engine is not a new concept. The first reference to Ramjet propulsion may be traced back to as early as 1913 during the pioneering era of aviation.
The infamous German V-1 flying bomb marked one of the first operational attempts to launch an air-breathing rocket (using a pulsejet engine). Although the V-1 had a relatively low subsonic speed, which rendered it highly vulnerable to both air and ground fire, it proved to be a useful weapon and paved the way for the development of cruise missiles after the war in both Eastern Europe and the Western World. During the Cold War, both the U.S. and the USSR developed and tested various ramjet designs. One such missile was the U.S. Navy MQM-8G Talos, which entered service in 1959. This long-range surface-to-air missile, which had a top speed of nearly Mach 4 and a range of more than 300 kilometers (around 190 miles), could carry either a conventional or a 5KT nuclear warhead. The Soviets developed their own line of ramjet-powered missiles ranging from the SA-4 and SA-6 surface-to-air missiles to the SS-N-22 and its successor, the SS-NX-26 high supersonic (Mach 3) anti-ship cruise missile, which is still in service. Versions of the latter two missiles have been reportedly sold to China and Iran.
Other countries have also joined the ramjet race. France was especially active in this field during the last several decades and fielded a number of ramjet-powered missiles, including the ASMP long-range nuclear cruise missile, future variants of which are capable of hitting targets at a distance of 600 km (more than 370 miles). The U.K. has developed the Sea Dart missile, a ramjet powered sea-to-air missile, which proved particularly successful in the Falklands War. This missile was also able to destroy an Iraqi Silkworm missile headed for the US battleship Missouri during the 1991 Gulf War, making it the first validated successful engagement of a missile by a missile during combat at sea.
Notwithstanding all these achievements, most ramjet designs have never entered service, and although billions of dollars have been spent on research in this area in Eastern as well as Western countries, ramjets have not yet reached their full potential.
Plaguing most ramjet engines is their requirement for very high initial speeds (usually over Mach 2). To reach these speeds, most ramjet missiles use rocket boosters for initial acceleration. (An exception was the US D-21 reconnaissance drone, piggybacking on an M-21 flying at Mach 3 that was able to use its ramjet engine without the need for a booster). Boosters make missiles larger, more complex, and consequently more expensive, thus eliminating one of the great advantages of ramjet engines – their simplicity.
How does the ramjet work?
Though still not very common, the ramjet may actually be regarded as the simplest form of a jet engine having no moving parts. It is essentially a hollow tube into which fuel, mixed with air, is injected and burned in order to produce thrust. The ramjet is capable of operating only when it reaches a particular speed that permits incoming air to be compressed by being forced into the engine. This phenomenon, called the “ram effect”, occurs when a volume of air is forced into a small space at a sufficiently high speed and is compressed to a higher pressure.
The supersonic combustion ramjet (or scramjet) is a variant of the ramjet engine. Despite the extensive research conducted in this area in the last several decades, it has not yet reached operational stage. All ramjet engines are supersonic, but Scramjet operates at hypersonic speeds, from approximately Mach 5 up to Mach 10-12 (NASA’S X-30 National Aerospace Plane, or NASP, was designed to reach speeds of up to Mach 25 in order to reach orbit, but the program was terminated in 1993 and the concept was never tested). The fastest scramjet ever tested is NASA’s X-43A, which reached Mach 9.6 in a test flight on November 16th, 2004.
A scramjet generally operates in a very similar way to a conventional ramjet, with the exception that the flow of air in the combustion of the fuel-air mixture through the engine happens at supersonic speeds, thus allowing the scramjet to achieve ultra-high speeds. A scramjet engine – just like a ramjet – cannot start operating until it attains a sufficiently high initial speed. To meet this precondition, the idea of constructing a hybrid ramjet-scramjet engine has been raised. Another hybrid concept is the turbo-ramjet engine, which combines a conventional turbojet engine capable of takeoff and reaching speeds surpassing Mach 2 and a ramjet engine for high supersonic flight, like the SR-71, which had two Pratt & Whitney J58 turbo-ramjet engines enabling it to reach speeds above Mach 3.2.
The Technion Ramtech rocket takes off
On the morning of May 9th, 2006, following five years of intensive preparations, the student-built Ramtech rocket was ready for its test flight. Some 20 students from the Technion Institute of Technology in Haifa, Israel designed and built the 3.7 meter, 92 kg (200 pound), two-stage solid-fuel rocket. The TFOT team had been invited to observe the test at the Palmachim Test Range that had previously been used for tests of the Arrow anti-missile missile and the Shavit satellite launchers. The test range is situated south of Tel Aviv on one of Israel’s most beautiful beaches, though it was not chosen for its clean sands and calm waters. Israel is a narrow and heavily populated strip of land about as the size of New Jersey, lacking sufficiently large and detached open spaces inland suitable for performing missile tests. Thus, launches are usually performed in the direction of the Mediterranean Sea, where a large strip must be cleared of ships and aircraft prior to launch.
Arriving at the base at around 10:00 a.m., the TFOT team joined a large group of aeronautical engineers from the Technion, accompanied by their families. This was our first meeting with Technion Aerospace Engineering Professor Alon Gany who supervised the development of the Ramtech.
Our first stop was the launch pad where we were able to gaze at the Ramtech and were briefed on the rocket and upcoming test. Since photographing inside the military compound was prohibited for security reasons, pictures were taken by one of the base officers.
Our next stop was the missile control center, a few minutes’ drive from the launch pad. Once inside, we were escorted to the second floor just above the control room, where the launch would be broadcast on several large screens. The launch countdown had begun and we found ourselves under lockdown in the control center as the critical phase of the mission approached.
Every missile test is a precisely orchestrated event, planned months in advance. In most military missile tests, the missile is equipped with a unique transponder enabling the control center to track its trajectory. Due to budget considerations, no such transponder was installed in the Technion-built rocket, so the control center would have to track the rocket directly, via either optical sensors or radar. The problem was that a two-stage rocket undergoes a separation that might cause the radar to loose track of the second stage of the rocket.
As the countdown reached T-minus 20 minutes, a small fishing boat was detected in the test zone. As navy ships tried to approach the vessel and demanded that it clear the area, we were informed that if the launch would not take place by 15:00, it would have to be rescheduled and or even canceled due to the test range’s tight timetable. At 14:30 the O.K. was given and the countdown continued, only to be stopped at T-minus 2 minutes, dashing our excitement once more, as the fishing boat still hadn’t cleared the area, while the time allocated for the launch had almost run out. Past cancellations of Ramtech launches were mainly due to inadequate weather conditions, so that it would have been ironic if the launch would not have taken place on that clear afternoon. Finally, after what seemed an eternity, we heard the familiar 5..4..3..2..1…
Everyone gathered in the control room to glean further information on the test launch and learned that at least 3 of the 5 objectives of the test had been accomplished but final conclusions would require further analysis.
We left Palmachim with a bitter-sweet taste. The successful launch could no doubt be considered a big achievement for a group of students who, with a fraction of the budget of any military program, were able to build and test a unique solid-fuel ramjet rocket. However, the accomplishment was incomplete. But this is probably just what rocket science is all about: learning from experience and past mistakes, then moving on.
Interview with Professor Alon Gany from the Technion
week after the Ramtech launch we interviewed Professor Gany about the rocket, the test, and his future endeavors.
Q: How do you explain that for decades billions of dollars have been invested in research on ramjet engines that produced relatively few operational ramjet engines and no operational scramjet engines?
A: The rocket industry is conservative and tends to take minimum risks and rely as much as possible on mature and known technology. Implementation of a new technology usually occurs when the conventional, common technology cannot provide the required solutions. Ramjet vehicles are more complicated and more sensitive to operating conditions. In addition, they have to be accelerated first by a rocket motor to reach a speed where the ramjet can operate (over Mach 2). Then it would be able to cruise at a relatively constant speed with little acceleration capability. Thus, as long as rocket motors have done the job, they have usually been selected to propel the vehicle. Nevertheless, the superiority of ramjet to rocket engines regarding fuel consumption will have to be taken into account when much longer flight ranges will be required without a substantial increase of the missile.
Regarding scramjets (supersonic combustion ramjets), this technology may be applicable only to extremely high flight speeds (over Mach 5-6) within the atmosphere. This limits its use to an exclusive type of applications that is probably not of first priority. In addition, a scramjet vehicle (typically fuelled by liquid hydrogen) should be extremely expensive compared to rockets, its performance very sensitive to its actual cycle efficiency, and its acceleration capability limited. On top of all that, the scramjet technology is still premature.
Q: Solid-fuel ramjet engines were tested in the past. What makes the Ramtech rocket unique in comparison to other ramjet projects?
A: The Ramtech is unique in that it is the first and only solid-fuel ramjet rocket that has been flight-tested in Israel.
Q: Would you describe the origins of the Ramtech project at the Technion and the different stages of this project?
A: It was started as a student design project, similar to other challenging projects that students undertake during their final year as undergraduates in the Faculty of Aerospace Engineering at the Technion. We do our best to stimulate students with new ideas and cutting edge technologies and concepts. Over the last three years, there were three teams of students (altogether about 20 students) who performed the different phases of the project. The first team (some 12 students) did the initial design, including the overall form, weight, stages and operating techniques. They have also made a full set of drawings, designed wind-tunnel models, and performed wind-tunnel tests to obtain the aerodynamic parameters. The second team performed igniter tests as well as structural design experiments and pyrotechnic separation test. The last team did all the final design and construction, performed additional ignition tests, and conducted full-scale static firing tests of the ramjet engine in a free jet. In addition, this team of four students performed all the safety requirements and wrote the safety and operation reports. At the final stage they made all the preparations required for flight tests, including the necessary operations during the launch time itself, and the analysis of the results after the test.
Q: Would you describe the Ramtech rocket: its operation, its engine, and the choice of fuel?
A: Ramtech is a two-stage vehicle, total weight 92 kg, length 3.70 meters. Its first stage consists of a conventional solid rocket motor. Its second stage is propelled by a solid-fuel ramjet engine. The solid rocket motor serves as a booster. It initially develops an acceleration of over 20 g. Its operation lasts 8 seconds, where it brings the rocket to a very high speed of about Mach 2.5 at an altitude of 3 km. Within less than a second, a pyrotechnic device is actuated, causing separation between the first and second stages. At the same instant, the ramjet engine receives the ignition signal. The ramjet is then ignited and is planned to operate for a number of seconds, keeping approximately the same speed.
The Ramtech fuel is based on the commonly used polymeric fuel polybutadiene, with some additive which increases its burning rate and flame-holding capability.
Q: In retrospect, how do you regard the Ramtech test? What went right and what went wrong, and what did you learn from it?
A: I see the Ramtech flight test as a great success. All the operating phases exhibited almost perfect operation: rocket motor ignition, very smooth launch, then about 8 seconds of the rocket booster burning, separation, and ramjet motor ignition and combustion. The ramjet engine itself operated for a shorter time than planned: less than 3 seconds instead of 8 – 10 seconds. Nevertheless, it is not the ramjet’s blame. It was designed to operate at a flight speed of Mach 2.7, with a capability to still operate stably at a flight speed as low as Mach 2.5. Below this speed, the air inlet is expected to be unstable. Processing of the tracking camera data reveals that the rocket’s maximum speed was about Mach 2.3, too low for a stable inlet operation. This caused extinction of the ramjet engine earlier than planned. However, as a first test in a research and development program, it would be considered a remarkable success, as the results can lead to a 100% success in a second test.
Q: Will there be a Ramtech II or any sort of follow-up program based on the experience gained from this test?
A: We think that there must be a follow-up program. We believe that ramjet technology will sooner or later play an important role in propulsion of high speed, long-range flight vehicle. Israel must have the technical knowledge. We believe that we have stimulated and created interest, and ways will have to be found to meet this challenge.
Q: What sort of applications do you see for the ramjet technology you developed and can you estimate how long will it be until these applications will become operational?
A: Ramjet engines can replace rocket engines in missions characterized by relatively long operating times, long-range, and high (supersonic) flight speed within the atmosphere. Naturally, different missiles can fit these characteristics (surface-to-air, air-to-air, cruise missiles, etc.). Projects using ramjet propulsion can start within a few years. In the distant future, one may expect also operational scramjet vehicles, but in my opinion it will take at least a generation.
Q: Finally, what else are you currently working on?
A: I have a number of on-going research programs (I should emphasize research and not projects or system design). These include combustion of metal and metal-hydride particles for high energy and propulsion application, solid fuel ramjet combustors, supersonic combustion of solid fuel (an original scramjet-related research), combustion and combustion instability of solid propellants, hybrid rocket combustors, and synthesis of nano-size powders.
TFOT is planning a follow-up article on a novel technology currently under development in Professor Gany’s lab.