Jet Engine Launch Assist Concept (JELAC)/Pogo
Addendum 1
Mach 5+ Speeds
27 Mar 97
INTRODUCTION
This update is intended to provide information relative to the Pogo concept that has been collected since the writing of the initial Pogo report. It contains information on upper speed potential and additional comparisons with operational aircraft.
UPPER SPEED POTENTIAL
The following two examples of relevant high speed flight have been found.
Mach 5 HYPR Engine. A Japanese-led multi-national consortium (including U.S. participants) has been working since 1989 on the Hypersonic Transport Propulsion System Research Project (HYPR) and should be completed in 1998. The HYPR is developing technology for a co-axial turbo-ramjet Mach 5 transport engine rated at 61,000 lbs thrust. Such engines would power a Mach 5 airplane carrying 300 passengers the distance between Tokyo and New York with a flight time of 3 hours.
Four subscale engines are being built and tested to validate design predictions. One, a one-tenth scale Mach 3 turbojet engine demonstrator is currently undergoing testing. The others, which will test for noise, exhaust emissions, and Mach 5 operation, are in various stages of development and test. The report did not indicate if or when a full scale engine would be built.
Mach 4+ ASALM Flight. The Advanced Strategic Air Launched Missile (ASALM) was developed by the US Air Force in the late 1970s as a possible replacement for the Short Range Attack Missile (SRAM).
The ASALM was an air-launched, ducted integral rocket-ramjet missile. The air inlet was chin mounted with ducting to carry the air into the combustion chamber in the back half of the vehicle. It used the same combustion chamber for both rocket and ramjet mode. After the rocket fuel was expended (approximately Mach 2.5) the inner exhaust nozzle was ejected along with the inlet air duct covers. The ramjet mode was then operated for the rest of the flight.
During the first test flight excessive fuel flow caused the missile to accelerate at up to 2G to well beyond Mach 4 at 40,000 ft. Thrust was terminated when it ran out of fuel. The components were recovered in “good condition with respect to the severe environment imposed by the excessive flight conditions.”
Mach 5+ Potential For Pogo. One of the concerns raised during the writing of the initial Pogo report was that Mach 2.5 could be difficult to achieve. The above examples show that Mach 5 speeds are not only achievable but could well be commonplace in the near future. The HYPR and ASALM programs add strong credibility that Mach 2.5 is not only achievable but probably at the low end of the Pogo’s potential.
Estimated Cost Benefit of a Mach 5 Pogo. Extrapolating from Figure 6 and other data in the initial Pogo report results in the following performance estimates for a Pogo-Pegasus combination:
Comparison of a Mach 5 Pogo Benefit to Pegasus
Vehicle Payload to Percent of Cost/lb Percent to LEO Baseline Payload $ Baseline Cost Baseline Pegasus 794 100 12,594 100 M2.5 Pogo-Pegasus 1535 193 7,362 58 M5 Pogo-Pegasus 2460 310 5,132 41 Note: The baseline and other payloads was for the particular orbit selected and not for all orbits.
AIRCRAFT COMPARISONS
Visual and technical comparisons between the Pogo-Pegasus and the F-4 and F-15 are presented along with some of the details of the F-15 Streak Eagle’s time-to-climb world’s record making flights.
Visual Comparison. The figure below provides visual and specification comparisons of the F-4, Pogo-Pegasus, and the F-15. The Streak Eagle looked much the same as the F-15 pictured but had a much lower weight. This combined visual/specification comparison was not available for the initial Pogo report.
F4 Pogo-Pegasus F-15 Engines J-79 Turbojet F-100 Turbofan F-100 Turbofan Length 58 ft 90 ft 64 ft Width 38 ft 22 ft 43 ft Thrust 35,000 lbs 135,000 lbs 54,000 lbs Weight 53,000 lbs 94,500 lbs 80,000 lbs T/W 0.66 1.43 0.67 Max Speed Mach 2+ Mach 2.5 Mach 2.5
The important characteristics to look for in visually comparing the vehicles are surface area and angularity (both of which contribute directly to aerodynamic drag) and the ratio length/diameter (L/D) (which contributes to reduced aerodynamic drag). Thrust/drag (T/D) is directly related to the maximum velocity, though not linearly. Thrust/weight (T/W) is directly related to the vehicle’s acceleration in flight and its ability to launch vertically.
The F-4 appears to have the lowest surface area and angularity followed by the F-15 and the Pogo and therefore should have the lowest drag due to these factors. The Pogo-Pegasus appears to have the highest L/D followed by the F-15 and the F-4 with an associated reduction in drag.
It must be noted here that the artist’s conception of the Pogo was created expressly to show the use of jet engines, which is what the Pogo concept is really about. An operational Pogo would most likely have the engines inside the fuselage or well blended into the fuselage to reduce the surface area and angularity. This should reduce the total Pogo-Pegasus drag to about that of the F-15.
Specification Comparison. The specifications shown for both airplanes are in Max Gross Weight configurations. Because of the many different models (A, B, …) and minor variations from year to year these are representative of the airplanes but not specific to any one model or year.
….Thrust. Thrust is determined by the type of engine, is directly related to speed, and inversely related to altitude. It is also influenced by the type of engine inlet, though studies for the initial Pogo report indicated that these should not be a major engineering problem (and supported by the more than twice design speed of the ASALM). Turbojets, as in the F-4, have lower thrust/weight at sealevel than turbofans, which are used in the F-15 and the Pogo, but have better relative performance at altitude and high speed.
The Pogo, as illustrated, has 2.5 times the initial thrust of the F-15 and 3.9 times that of the F-4. Both the F-4 and the F-15 have two-dimensional, adjustable inlets while the Pogo is shown with fixed conical inlets (optimum inlets for an operational Pogo would be determined during vehicle design). The Pogo shown has five F-100 engines but could have more or less. The initial Pogo report described both 5-engine and 10-engine Pogos and later in this addendum a six-engine Pogo is compared to the Streak Eagle.
….Drag. Actual estimates of vehicle drag were beyond this author’s expertise but a rough (and reasonable) comparison is offered based on visual appearances. As illustrated, the Pogo appears to have much more drag than either the F-4 or the F-15. However, as stated earlier, the drag of an operational Pogo should be comparable to the F-15. Based on the available data and this author’s experience with aircraft, the F-15 appears to have about 30% more drag than the F-4.
….Weight. Weight is inversely related to both acceleration and drag due to lift during subsonic winged flight. During supersonic flight the drag due to lift becomes negligible, leaving only acceleration affected. The Pogo-Pegasus weighs about 80% more than the F-4 and 18% more than the F-15. A wingless Pogo would probably weigh about that of the F-15 due to elimination of the wings and related components.
….Thrust/Weight (T/W). The F-4 and the F-15 have almost the same T/W. The Pogo-Pegasus has over twice the initial T/W of each. For at least the early portions of the flight the Pogo-Pegasus would have over twice the acceleration of either airplane.
….Thrust/Drag (T/D). T/D is the most important factor in determining the maximum speed of the vehicle. This is a non-trivial relationship because neither thrust nor drag are constant or simple to calculate. For each engine type thrust varies non-linearly with altitude, speed, engine inlet, and other factors. Calculating true vehicle drag is similarly difficult.
Selecting a Pogo with the same engines as the F-15, and assuming similar drag it can be estimated that the Pogo-Pegasus would have over 2.5 the T/D of the F-15. This would give the Pogo-Pegasus a much higher speed potential than the F-15. Even if the estimated drag is off by a factor of two (unlikely) the Pogo-Pegasus should still have a higher speed potential than the F-15.
Selecting a Pogo with turbo-jets, as in the F-4, and keeping the initial thrust the same as with the turbofans this would give the Pogo-Pegasus a lower T/W ratio (due to the lower T/W of the engines) but a somewhat better T/D at altitude and speed. Assuming a 30% lower drag for the F-4 this would give the Pogo-Pegasus 3 times the T/D of the F-4, again resulting in a much higher speed potential.
Streak Eagle Comparison. The high performing Streak Eagle (an early F-15 in a very light-weight configuration) broke several World’s Time-to-Climb-to-Altitude Records in 1975.
On its second flight, with a T/W of 1.6+, the Streak Eagle climbed nearly vertically (80 degrees) passing through 20,000 ft at Mach 1.05 and through 40,000 ft at Mach 0.9. On its last flight, with a T/W ratio of 1.4+, it climbed to 37,000 ft, accelerated to Mach 2.2 in level flight, then climbed at 60 degrees. It’s speed had dropped to Mach 0.7 by the time it reached 98,425 ft (30 km) with an elapsed time of 3 minutes, 27.8 seconds.
The Streak Eagle’s goal was a minimum time-to-climb rather than a maximum speed at some altitude, which would be the Pogo’s goal. The T/D was not given in the reference but due to the angle of climb (and therefore vertical acceleration component due to gravity) the T/W became an important, if not limiting, factor in the Streak Eagle’s speed at the different altitudes.
A six-engine wingless Pogo-Pegasus would have a T/W of 1.6+. The T/W, T/D, and climb angle of the second Streak Eagle flight, described above, would be almost identical to that of a wingless Pogo-Pegasus. It can therefore be safely assumed that they would have almost identical performances. This would allow the Pogo to give the Pegasus slightly better altitude and speed than the L-1011 currently gives it. It would also give the Pegasus a better launch inclination (which is the principle reason for the wing on the Pegasus) but the benefit, in terms of payload, would probably be minor and not cost effective.
Interestingly, the Boeing concept,[6] with 10 engines, no wing, and a T/W of 1.8, was expected to have achieved Mach 1.7 at 50,000 ft in vertical flight. This is probably due to a recursive relationship between early acceleration and higher thrust. That is, with the higher T/W comes a higher acceleration, resulting in higher speed at each altitude level, resulting in greater airflow through the engines, resulting in higher thrust, resulting in higher acceleration, and etcetera.
CONCLUSIONS
1. The information gathered about the HYPR, the ASALM, the F-4, and the F-15 indicate that Mach 2.5 is probably at the lower end of the Pogo’s capability. With current off-the-shelf turbojet and turbofan engines the Pogo is more likely to achieve a Mach 3.5 or better speed during it’s short dash. With modified or new turbine engines and/or the addition of ramjets the Pogo is likely to achieve at least Mach 5. This would significantly increase the value of the Pogo to spacelift over that previously estimated.
2. A vertically ascending wingless Pogo, with currently available engines, would probably have a maximum speed in the Mach 1 to 1.7 range making its value less certain. A different engine scheme could make high mach number vertical flight easily achievable.
3. An operational Pogo would probably need to have the engines within the fuselage or well blended in to keep drag to an acceptable level.
4. Despite this additional, and favorable, information the calculations necessary to verify the cost and performance of an operational Pogo remain beyond this author’s ability.