pogapndc.htm
Several of the engineers contacted felt that the jet engine inlets could be a significant engineering problem. Specifically, they felt that they would be difficult to design and expensive to develop. Some felt that a high enough efficiency could not be achieved to allow the Pogo to reach the altitude and velocity predicted by the comparative study.
Because of such strong concern, a side study, also comparative, was conducted into existing supersonic inlets. This author determined that there were two parts to the inlet problem - efficiency and disturbed airflow.
Supersonic inlets (other than those for supersonic combustion ramjets (scramjets)) are designed to shock the air down to subsonic speeds while increasing air pressure. These are typically most efficient at only one set of flight conditions. Loss of efficiency results in higher drag with reduced overall performance and, in some cases, engine stalls. The more complex and efficient conical inlet cones move in and out to maintain efficiency over a range of conditions. Aircraft such as the F-4 and F-15 have two-dimensional inlets with movable surfaces to maintain efficiency over a range of conditions. These are complicated and require expensive design and testing.
The majority of the supersonic inlets identified, however, were fixed. For example, the F-104, which is an interceptor requiring less maneuvering than the F-4 or F-15, has a fixed, semi-conical, two-step inlet cone and fixed inlet lip. The F-104 operates from zero velocity up to Mach 2.2 and is probably optimized for a narrow flight profile, being an interceptor. In order to engage an enemy aircraft the F-104 is still required to perform a range of maneuvers at various altitudes, velocities, and angles of attack.
There are many existing supersonic missiles with fixed inlets. While most of these missiles are powered by ramjets rather than turbojets this is not expected to significantly affect the performance of the inlets. For example, the Bomarc was a Mach 2.5 vehicle capable of achieving 100,000 ft altitude. The X-7A, which used almost the same engine, achieved Mach 4.3 during testing with a fixed inlet. There was one report of a missile inadvertently exceeding twice its design speed with a fixed inlet. Many other examples are described in Janes' All the World's Aircraft[1], as well as other publications.
This difference between aircraft requiring movable inlets and vehicles with fixed inlets was considered carefully as a possible explanation for the difference of opinions on the performance capability of a Pogo. Fighter aircraft, such as the F-15, push the limits in speed, maneuverability, and other performance factors. This means that the inlets must provide good performance over a wide range of velocities, altitudes, angles of attack, and throttle settings. The F-15 is designed to be the "Air Superiority Fighter." The SR-71 had a much narrower flight envelope than the F-15 but was designed to cruise in excess of Mach 3 and 100,000 ft. The SR-71 inlet was probably designed to be highly efficient in order to achieve the needed range without increasing the size of the vehicle. The SR-71 had the additional problems of low engine (by today's standards) technology and an inability to withstand losing thrust on one engine during flight.
By comparison the Pogo would require little in the way of maneuvering or range. The Pogo is a brute force method of achieving velocity and altitude. Its take-off thrust/weight (T/W) is higher than the best capability of the F-15 or SR-71. Nor would a first generation Pogo need to achieve the velocities of missiles such as the X-7. Developing higher performing Pogos would be a matter of need and economics.
Disturbed, or turbulent, airflow can have strong adverse effects on inlet performance and the performance of the engine. Besides reduced overall engine thrust such disturbed airflow can cause engine stalls and even structural damage.
Review of various aircraft and missile inlets identified a wide range of inlet sizes, locations, and shapes. The F-15 has two-dimensional inlets on either side of the forward fuselage (very common for two-engine fighters). The F-16 has one fixed inlet below the forward fuselage. The SR-71 has conical inlets on the engines several feet out on the wings. The ASALM missile had a chin mounted fixed inlet with the ducting running half the length of the vehicle. The Bomarc had conical fixed inlets on the engines which were mounted below the aft fuselage. The XB-70 had two large two-dimensional inlets underneath the fuselage just aft of the wing leading edge. The Messerschmitt-Bolkow-Blohm Anti-Surface Ship Missile has four half-conical fixed inlets mounted radially around the back end of the missile. A comprehensive list would be extensive and is not warranted for this discussion. On many of the missiles reviewed the vehicle shape and function were similar, while the inlets varied widely as to location and shape. In several cases the inlets were far removed via ducting from the engines to which they provided air. This wide range of inlet designs indicates that there is considerable leeway in the design of supersonic inlets.
Further, the problem of disturbed airflow from the upper stage may be mitigated by placement of the engines and the wing such that the turbulence would not impinge on the engines. Another possibility is to embed the engines within the Pogo fuselage using an annular inlet and plenum such that only laminar airflow reaches the engines. It might also be that such turbulence can be avoided altogether. With a sufficient T/W ratio it might be possible to gain enough velocity in the vertical direction that the transition to lifting flight could be done with a very shallow angle of attack thereby keeping the upper-stage turbulence small and close to the fuselage.
This author's conclusions are listed below. They are based on a comparative study of existing supersonic inlets only. Resolution of this author's conclusions and the opinions of experienced aeronautical engineers will probably require engineering modeling and possibly some wind tunnel testing.
1. Highly complex and efficient jet engine inlets will not be required for the Pogo to achieve velocities in excess of Mach 2.5 and altitudes in excess of 80,000 ft. Simple inlets, such as those on the Bomarc missile or the F-104, are expected to be more than sufficient for a first generation Pogo.
2. Engine inlet placement will require careful consideration. Depending on the Pogo flight profile some upper-stage configurations may be precluded without modifications. This is not expected to be a major impact to the general use of Pogos.
1. Janes' All the World's Aircraft, various editions.
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This Page Last Updated 5 Nov 96