Starting as an air-launched missile engine and finishing as a multi-mission general propulsion for the space age, the basic design went through a series of iterations and versions that enabled it to have a long and productive career.
Bell Model 117:USAF designationXLR81.[6] Also known as theBell Hustler Rocket Engine. The engine was developed for theB-58 Hustler Powered Disposable Bomb Pod. It reached a maturity of development where its performance was confirmed through a Performance Flight Rating Test. However, the project was cancelled before it could be flight-tested. It burnedJP-4 aircraft kerosene as fuel and usedred fuming nitric acid (RFNA) as oxidant, to supply a thrust of 67 kN (15,000 lbf).[5]
Bell Model 8001: USAF designationXLR81-BA-3.[2] It was used on theAgena-A prototype. It was based on the Bell Model 117. It only needed agimbal mount to providethrust vectoring, relocating the gas generator exhaust port to enable the gimbal movement and addition of a nozzle closure as major modifications. As its predecessor, it burned RFNA and JP-4 propellant and had a thrust of 67 kN (15,000 lbf) with anIsp of 265.5 s (2.604 km/s) with its 15:1 expansion ratio. Its rated duration was 100 seconds and only launched twice. The first flight was on 28 February 1959.[5][7][8]
Bell Model 8048: Also known as theXLR81-BA-5.[2] Used on the Agena-A, it switched propellants to thehypergolic RFNA andUDMH. Since the mixture self-ignites on contact, the engine could be greatly simplified. For example, the combustion chamber ignition system was eliminated. The most important system was the passive thrust regulating system. The use of a series ofventuri holes in the gas generator allowed it to supply a 67 kN (15,000 lbf) with just a 1.6 kN (350 lbf) variability without moving parts. Also, the expansion ratio was increased to 20:1 which enabled it to achieve anIsp of 276 s (2.71 km/s). First flew on January 21, 1959 and last flew on January 31, 1961. It was used for the first American experience on vacuum starting an engine, since it was believed at the time that engines would need atmospheric pressure for start-up.[2][5][7][8]
Bell Model 8081: This version was the first designed to have two restart capability, by use of three ignition cartridges, and extensive validation of vacuum starting behavior. Thrust was increased to 71 kN (16,000 lbf) and expansion ratio to 45:1 for anIsp of 293 s (2.87 km/s). USAF designationXLR81-BA-7. Used on theAgena-B, it first flew on December 20, 1960 and last flight was on May 15, 1966.[9]
Bell Model 8096: USAF designationXLR81-BA-11 and later,YLR81-BA-11.[4][5] Main production version, used on theAgena-D. It added to the 8081 atitanium withmolybdenum reinforcementsnozzle extension, which enabled it to reach anIsp of 280 s (2.7 km/s). It also added inducers to the turbopumps, reducing the pressurization requirements on the tanks. In 1968, restart capability was increased to three restarts.[2][10][5][7]
Bell Model 8096-39: This was a version that switched oxidizer to MIL-P-7254F Nitric Acid Type IV — known as HDA (High Density Acid) — mixture of 55%nitric acid and 44%N2O4 with somehydrogen fluoride as a corrosion inhibitor, providing more performance than standardIRFNA.[11] It achieved a thrust of 76 kN (17,000 lbf) with anIsp of 300 s (2.9 km/s).[7]
Bell Model 8096A: A proposed improvement over the 8096-39 that would increase the size of the nozzle extension at an expansion ratio of 75:1, achieving anIsp of 312 s (3.06 km/s).[7]
Bell Model 8096B: Proposed version for use with anAgena based reusable upper stage for theSpace Shuttle. It would switch propellant toMMH plushexamethyldisilazone (HMZ) andN2O4 on a 1.78 mixture ratio and add aniobium nozzle with a 100:1 expansion ratio for an increase inIsp to 327 s (3.21 km/s), or 330 s (3.2 km/s) with a 150:1 nozzle. The propellant change would require modification of the gas generatorventuri holes to achieve power balance with the new performance without redesigning the turbopump. The chamber pressure would be reduced to 3.35 MPa (486 psi). Within the same actuators, it enable to increase the gimbal angle to 3 degrees, change engine clocking to reduce oil leakage. It would decrease the coolant passage diameter, since the new oxidizer could stay within specification at a higher flow velocity. The injector would change from flat to a 5-legged baffle, the pump seals would be improved and the oxidizer valve would change to a torque motor design. It would also implement some material changes in the turbopump bearings that would eliminate the oxidizer boiling that prevented a restart in the 15 minute to 3 hours period after an ignition. The multi start capabilities of the 8247 would have been ported. This would enable up to 200 starts. Also, single burn life was expanded to 1200 seconds.[12][13]
Bell Model 8096L: Since the 8096B would require expensive changes in propellant handling, a middle step was proposed. It would switch fuel toMMH plushexamethyldisilazone (HMZ), while keeping the same oxidant as the 8096-39, and change the mixture ratio at 2.03. The rest of the changes were the same as the 8096B, except that it would keep the same cooling channel diameter as the 8096, the chamber pressure would be reduced to 3.34 MPa (484 psi), and theniobium nozzle would have a 150:1 expansion ratio. The restart capabilities would be 10 to 100 starts depending on certification effort.[3][12][14]
Bell Model 8247: USAF designationXLR81-BA-13. Used on theAgena target vehicle and as a pure upper stage in the form of theAscent Agena. It added a new system that allowed multiple restarts. The system replaced the start up cartridges for two metallic bellows on the oxidizer and fuel tank, which could supply enough pressure for start up. Once the turbopump reached its peak power, the outlet pressure was used to refill those bellows, and thus it recharged itself. While it was rated at 15 restarts, in practice it never did more than 8, which were performed during theGemini XI mission.[5][7][15]
Bell Model 8533: A program to develop an upgraded version of the 8247. It switched propellants toUDMH andN2O4 and had general performance improvements. The propellant switch not only enabled better performance, but also allowed it to stay fueled on the pad for periods of time longer than 15 days.[5][16]
^abCarter, W. K.; Piper, J. E.; Douglass, D. A.; Waller, E. W.; Hopkins, C. V.; Fitzgerald, E. T.; Sagawa, S. S.; Carter, S. A.; Jensen, H. L. (1974-03-15). "Section 3.2.3".Reusable Agena Study Final Report (Technical Volume II)(PDF). pp. 3–8. Retrieved2015-06-17.
^ab"Section II - Agena and Support Systems".Athena Payloads User Handbook(pdf). Lockheed Missile & Space Company. 1971-03-01. pp. 2–4. Retrieved2015-06-17.
^abCarter, W. K.; Piper, J. E.; Douglass, D. A.; Waller, E. W.; Hopkins, C. V.; Fitzgerald, E. T.; Sagawa, S. S.; Carter, S. A.; Jensen, H. L. (1974-03-15). "3.3.2 Propulsion Systems".Reusable Agena Study Final Report (Technical Volume II)(PDF). pp. 3–37. Retrieved2015-06-17.
^Carter, W. K.; Piper, J. E.; Douglass, D. A.; Waller, E. W.; Hopkins, C. V.; Fitzgerald, E. T.; Sagawa, S. S.; Carter, S. A.; Jensen, H. L. (1974-03-15). "4.5 Alternative Concepts".Reusable Agena Study Final Report (Technical Volume II)(PDF). pp. 4–20. Retrieved2015-06-17.
^Carter, W. K.; Piper, J. E.; Douglass, D. A.; Waller, E. W.; Hopkins, C. V.; Fitzgerald, E. T.; Sagawa, S. S.; Carter, S. A.; Jensen, H. L. (1974-03-15). "2.3 NOMINAL SHUTTLE/AGENA UPPER STAGE CONCEPT".Reusable Agena Study Final Report (Technical Volume II)(PDF). pp. 2–4. Retrieved2015-06-17.
^"Bell 8247". Encyclopedia Astronautica. Archived fromthe original on 2017-02-05. Retrieved2015-06-17.
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