pogapndb.htm JELAC/Pogo - Appendix B


POGO - APPENDIX B


APPENDIX B
POSSIBLE VARIATIONS OF POGOs

POGOs FOR SPACELIFT, AS ILLUSTRATED

The Pogo shown in Figures 1 and 2 in the body of this report was only one possible configuration. It used a central tank with the engines externally mounted around the perimeter (this was used only to highlight the concept of using jet engines). It had simple triangular wings. The interstage mount was a frame. When this study was first begun, this author envisioned the engines within the fuselage (see Figs. 3 and 4 in the body of this report). Mounting the engines this way might be the most efficient for inlets but could create excessive boattail drag. These are but two possible configurations.

SOME OTHER POSSIBLE POGO CONFIGURATIONS

a. Jet engines as strap-on boosters with the fuel in the cowlings. This would allow the engines to perform the same function as the currently used strap-on solid rocket motors. The engines would then be separated at altitude and parachuted to ground for reuse.

b. Expendable shrouds for the upper stages. These shrouds would be consistent in their outer shapes and mounting configurations while allowing support for various upper stages inside. These shrouds would protect the various upper stages from most of the aerodynamic forces during flight through the atmosphere and provide consistent aerodynamics for the Pogo. The upper stage and shroud would separate from the Pogo as a unit and continue on a ballistic trajectory for a predetermined time. The shroud would then separate from the upper stage. This would reduce the amount of testing needed for the Pogo because of the reduced number of aerodynamic configurations. It would increase the operational costs because of the loss of the shrouds.

c. Annular fuselage with a fixed front fairing and the upper stage inside. The upper stage would slide out the back and separate from the Pogo before igniting its engines. This would allow the Pogo to maintain much of its aerodynamics (except for boattail drag) during separation. The upper stage, however, would probably be subjected to strong turbulences in the wake of the Pogo.

d. Annular fuselage, currently existing upper stage inside. This would allow better support for upper stages and improved center of gravity management. The upper stage could be released forward or backward. If forward, then the Pogo would reduce thrust and back away from the upper stage. If backward, then the Pogo would release the upper stage while still at full power, allowing the upper stage to drop back away from the Pogo. The upper stage would ignite its engines when the Pogo was safely out of the way. The aerodynamics of such a radical change in configuration during supersonic flight could be quite challenging.

e. Annular fuselage with extra fuel and oxidizer for the upper stage. This would allow the upper stage to start its engines prior to lift-off but still have full tanks at separation. The additional thrust at lift-off would allow the use of fewer jet engines (or higher payloads) without increasing the size of the upper stage. Because the Pogo would return intact, the fuel and oxidizer tanks it carried would become fully reusable, thereby reducing the operating cost of the total system. The Russian Zenit was considered a possible candidate for the upper stage in this configuration.

f. New reusable upper stage. A second stage developed specifically to work with the Pogo could provide consistent aerodynamics for the Pogo while allowing for efficient use of volume and weight of the second stage. The upper stage could be very similar to the proposed NASA/Orbital Sciences Corporation X-34. The development costs would be higher because of the second vehicle (unless a vehicle such as the X-34 had already been developed), but the operational costs would be reduced.

g. Addition of ramjets for enhanced propulsion. The F-100 engines, proposed for use on the Pogo, are turbofans. As such, these engines have high thrust/weight (T/W) ratios at low altitudes but lose thrust faster than turbojets or ramjets at the higher altitudes. Simple ramjets, such as those used on the Bomarc, could be added to improve performance without greatly adding weight. Similar engines have been used to propel the X-7A missile to Mach 4.3. Because of the added weight and complexity at lift-off, a trade-off study would be required to identify the potential benefits.

h. Zoom maneuver. It might be advantageous for some vehicle configuration/upper-stage combinations to perform a zoom maneuver to increase altitude and reduce velocity before separation. The MIG 25 achieved its world record altitude of 123,524 ft through such a maneuver. This could be enhanced further with reusable rocket engines on the Pogo. In this case the Pogo could achieve very high altitudes, allowing for upper stages to be released from a cargo bay similar to that of the Space Shuttle. The Pogo would then perform as a pop-up first stage.

i. Vertical flight without wings. This would allow for lower development costs because of the lack of wings and simplification of the flight profile. For the same number of engines the velocity and altitude achieved by the Pogo would be much reduced, thereby increasing the per-pound operating costs.

DISCUSSIONS ON IMPROVING THE PERFORMANCE OF THE POGO

Higher Velocities

Much higher velocities appear possible using ramjet and similar technologies, though these would not likely be included in a first generation Pogo because of cost and difficulty.

....a. Many historical references[1-2] were found on ramjet propelled vehicles being flown in the Mach 4 to 6 regime with predictions of Mach 6 to 10 within reach. One theorized that ramjets could be operated at Mach 18 to 20 if operated very fuel-rich to keep the engine combustion temperatures in the useable range[3].

....b. Rocket Ramjets (aka Integral Rocket/Ramjet) are still being developed that combine the booster rocket and the ramjet[4-7]. It may also be possible to start ramjets via Ram Accelerators[8].

....c. Both turbojets and ramjets might also be operated at higher speeds using active cooling such as water injection (as is done with the MIG-25 engine).

....d. Supersonic combustion ramjets (scramjets) have been studied for several decades and continue to be tested. While scramjets have for years promised velocities in excess of Mach 20, actual developmental progress has been very slow.

....e. A propulsion method that bears mention is the Pulse-jet. This first became widely known when it was used in the German "Buzz Bombs" of World War II. Pulse-jets are capable of achieving thrust at zero velocities and it may be possible to combine Pulse-jet features with ramjets. This combination could eliminate the need for turbojets or rockets for initial velocity. Pulse-jet/ramjets of the size needed for boosting large upper stages may be difficult to engineer but more manageable ones might be applicable to model rocketry, sounding rockets, and boosters for small satellites.

....f. According to one reference[9] it might be advantageous to use liquid hydrogen (LH2) as the propellant. This reference states that LH2 contains 49,888 Btu/lb compared to 18,400 Btu/lb for JP-4. This relates to an air-breathing specific impulse (Isp) of 4,000 for LH2 versus 1,600 for JP-4. Both of these compare very well to liquid oxygen/LH2 (LOX/LH2) rocket propulsion, which reaches a theoretical Isp limit of about 460. Another referencea identified testing that demonstrated a doubling of thrust on current jet engines using hydrogen as the fuel. Because of the high combustion temperature this would probably be used to increase altitude rather than lifting capacity.

Based on discussions with aeropropulsion engineers[a], and research into advanced jet and ramjet technology, it appears that a Pogo could be designed and built capable of operating in the Mach 4-6 regime. Technologies developed as part of the Integrated High Payoff Turbine Engine Technology (IHPTET) program or more exotic propulsion schemes could exceed these velocities, though aerodynamic heating would require the use of special materials for the engines and vehicle structure.
[a] Personal Communiques with Marvin Stibich, Wright Lboratories, Aeropropulsion and Power Directorate, Wright Patterson AFB, OH

Higher Altitudes

Increasing the Pogo velocity would allow higher altitudes because the thrust of jet engines is dependent on velocity and altitude.

Another way of increasing altitude is by using larger wings. The TR-1 has an operational ceiling of 100,000 ft[10] while flying subsonic. The difficulty and cost of such wings[11] is expected to be high compared to increasing velocity and the benefits would be minimal.

Increased Lift Capacity

Increased lift capacity could be achieved by adding more engines. Each F-100-PW-220 is expected to have an excess thrust capacity (after its own weight, fuel, cowling, and structural support) of 16,500 lb at lift-off. Because of the short operating times these engines are expected to see, it may be advantageous to remove unnecessary equipment (such as redundant generators) and/or operate them at higher than rated thrust.

Lower Costs

The optimum (lowest life cycle cost) design speed of the Pogo may be considerably lower than technology will allow because of practical considerations of vehicle recovery and reuse. Standard aircraft aluminum and fiberglass appear to be sufficient for short periods of Mach 3 to 3.5. Higher speeds, however, would require active cooling or special materials (i.e., stainless steel, titanium, ceramics) on the hot leading edges. These speeds could also exceed the capabilities of some current launch vehicles which would otherwise be suitable as upper stages.

On the other hand, the lowest life cycle cost will also depend on how much of the total launch system is recovered for reuse. Increasing the velocity of the Pogo reduces the size and cost of the upper stage needed to launch a given payload. This results in a trade-off between a) the cost to increase the velocity of the Pogo, b) the cost of the upper stage, c) the cost to recover and reuse each, and d) other factors.

REFERENCES

  1. Jaumotte, A.L., et al, Editors, Combustion and Propulsion, Fourth AGARD Colloquium, High Mach Number Air-Breathing Engines, Milan, Apr 4-8, 1960, multiple writings.

  2. Thomas, Arthur N., JR., "Exploding Ramjet Myths," National Defense, Sep 1983, pp 18-23.

  3. ARS Journal, Nov 1959, pg 77.

  4. Burson, William C., Jr., Ducted Rocket Ramjet Advanced Development, AFWAL/POPR, Wright-Patterson AFB, Jun 1987.

  5. Donaldson, Wayne Allen, Advances in Ducted Rocket Technology, WL/POPR, Wright-Patterson AFB, 1992.

  6. Covault, Craig, "French Flight Test Rocket-Ramjet Missile," AW&ST, Feb 27, 1995, pp 22-23.

  7. Covault, Craig, "Precision Weapons Give France New Flexibility," AW&ST, Feb 27, 1995, pp 52-53.

  8. Tomassian, Jack, George O'Connor, and Roger Campbell, High Speed Testing for Scramjets, Rockwell Threshold, Summer 1995, pp 4-12.

  9. Koelle, Heinz Hermann, Editor, Handbook of Astronautical Engineering, (McGraw Hill Books, 1961), pp 18-12 to 18-15.

  10. Janes' All the World's Aircraft, various editions.

  11. Smoot, George, and Keay Davidson, Wrinkles in Time, (Morrow Press, 1993), pp 129-131.


Opening Screen
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This Page Last Updated 27 May 97