The HL-20 Spaceplane Design

A presentation on the American HL-20 Spaceplane from the 1980s

By Andy Hill (Feb 2005)

Introduction• One of the most promising design

concepts for a manned spacecraft in recent years produced by NASA was the HL-20.

• Although much work was done on the design and testing which included production of a full scale vehicle mock-up, the program was cancelled before a real spacecraft was ever built at an estimated total cost of $2Bn.

• The craft was designed as a back up crew vehicle to the Shuttle for transporting American astronauts to and from Earth orbit and would have prevented America’s current reliance on Russian Soyuz craft during the Shuttle’s grounding. 1

HL-20 Body Mock up - NASA

Early Beginnings• The origins of America’s HL-20

spaceplane had its roots in a Soviet design called Bor-4. The Soviet Union had been developing small space gliders to test heat shield materials on re-entry since the early 70s in support of their Spiral and Buran programs.

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• Bor-4 was placed in orbit using an expendable rocket and allowed to de-orbit after 1.25 revolutions of the Earth to make a splash landing in the Indian Ocean where it was recovered by the Soviet Navy. It came to the attention of the US when the recovery was photographed by the Australian Air Force.

Bor-4 on display in Berlin

Bor-4 being recovered - NASA

Early Beginnings• In 1983 NASA began the

investigation of the Bor-4 vehicle that would eventually lead to the HL-20 concept. Using the photographic evidence obtained by the Australian Airforce and the HL-10 lifting body design it had worked on in the 1960s it came up with the HL-20.

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• NASA used estimations of size, shape and weight from the photographic evidence of the Bor-4 as a baseline for its design. The design was then optimised using knowledge gained through its earlier lifting body program. NASA was able to use the same engineers that had previously worked on lifting bodies to create the HL-20.

HL-10 Lifting Body - NASA

HL-10 Lifting Body with B-52 Bomber - NASA

HL-20 Concept• Also known as ACRV (Assured

Crew Return Vehicle), CERV (Crew Emergency Return Vehicle) and PLS (Personnel Launch System), one of the HL-20’s primary roles was toact as a crew lifeboat for theproposed US Freedom Space Station.

• In normal configuration it hada flight crew of 2 and was ableto carry 8 passengers. It would have remained docked to the space station ready to evacuate the crew in the event of a problem (this role is now fulfilled by a Soyuz craft on the ISS which has a 3 man crew).

• The HL-20 could be launched with a Titan IV booster in to orbit and the Titan’s fairing would have been equiped with solid rocket motors with a thrust of 154,000kgf to enable a launch abort should it have become necessary, parachutes would then have allowed a tail-down water landing.

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HL-20 used as CERV - NASA

HL-20 Concept• Designed to land like a glider

on a runway, its small sizecompared to the Shuttle meantthat most aircraft runwayswere long enough (the Shuttle requires an extra long runwaywhich limits the locations at which it can land). This gave it much more flexibility than theshuttle when choosing re-entrywindows.

• Interest in the HL-20 increased significantly after the 1986 Challenger Shuttle disaster and the HL-20 was designed for low operating cost and improved flight safety.

• The proposed Personnel Launch System (PLS), would have utilised the HL-20 design and an expendable launch system to provide manned access complementing the Space Shuttle should future problems develop with it. 5

HL-20 mock up on runway - NASA

HL-20 Concept• To minimise costs and

increase turn around times the HL-20 was designed to operate much more like a conventional aeroplane than a spacecraft. Most of its flight controls were borrowed from the aircraft industry and its cockpit closely resembled a commercial jet.

• As a small vehicle designed with available technologies, the PLS version of the HL-20 was forecast to have a low development cost. Subsystem simplification and an aircraft approach to ground and flight operations would also greatly lower the operating costs (about a 10th of the Shuttle’s costs) once it entered service.

• The craft was designed with easily removable panels and hatches to allow easier access for maintenance tasks. 6

HL-20 body panel construction - NASA

HL-20 Testing• A series of models ranging in size from 6

inches to 5 foot were tested in wind tunnels to compile data on how the HL-20 would handle over its entire Mach range (up to Mach 20).

• A thermographic phosphor technique has been used to study the heat transfer characteristics of a HL-20 model in high-speed wind tunnel tests. The model, coated with a phosphor, radiates at varying color intensities as a function of temperature during test when illuminated by ultra-violet light.

• Computational fluid dynamics (CFD) codes, which mathematically simulate the flow field in the vicinity of the HL-20, were used in conjunction with wind tunnel tests to study patterns of flow field phenomena, shock waves, stability and control and heating on the windward and leeward surfaces of the vehicle.

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HL-20 wind tunnel model (Above) and CFD simulation

(Below) - NASA

HL-20 Testing• In addition to computer

modelling of vehicle flight patterns during entry, a flight simulator has been set up to permit pilots to study the finallanding phase of flight.

• Starting at an altitude of 15,000 feet, the simulation gives the pilot a realistic view of the approach to a runway landing. Using a side-stick controller, pilots have demonstrated this configuration to be controllable and capable of pinpoint landings.

• Results have shown that the HL-20 can be controlled through the hypersonic entry using only 30 pounds of reaction control thruster fuel in nominal cases, or less than 200 pounds of fuel in cases where the vehicle centre of gravity is offset and the upper atmosphere density and wind profiles are off-nominal. 8

HL-20 flight simulator - NASA

HL-20 Testing• A cooperative agreement

between NASA, North Carolina State University and North Carolina A&T University led to the construction of a full-scale model of the HL-20 PLS for further human factors research on this concept.

• Students at the universities, with requirements furnished by Langley and guidance from university instructors, designed the full size research model during their spring 1990 semester with construction following during the summer.

• The craft measured 8.93m in length with a maximum diameter of 7.16m.

9A student working on the full scale research model of the HL-20 - NASA

HL-20 Testing

• Getting in and out of the HL-20 (Launch orientation), required climbing through a hatch and up or down a ladder. In the horizontal mode, crew members walked along an aisle leading through the tail, which would be the exit-entry path at a space station or on the ground after a runway landing.

• Partial-pressure suits, borrowed from the Johnson Space Center in Houston, were used for part of the study. Participants noticed less head room and restricted movement with the bulkier and heavier suits, particularly in the last row of seats which were found to be restrictive for individuals over 1.7m in height.

10Egress Simulation on the full scale research model of the HL-20 - NASA

HL-20 Testing

• The human factors research objectives, using this model, were to assess crew ingress and egress operations, assess crew volume and habitability arrangements and determine whether visibility requirements for the crew during critical docking and landing operationswere adequate.

• The results from these studieshave shown where improvements in the baseline HL-20 design are desirable. These improvements will have little impact on overall vehicle shape or aerodynamic performance.

• The testing, using Langley Research Center volunteers as subjects, was completed on the HL-20 model in December 1991.

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Interior view on the full scale research model of the HL-20 - NASA

HL-20 Conclusion

• Work on the HL-20 design was eventually stopped when NASA, looking for a higher-capacity alternative (the HL-20 had a payload capacity of only 545kg in addition to the 10 man crew of 1,270kg) and concerned about reliable availability of the Soyuz in the future, began development of the ill-fated X-38 CERV in 1997 which was also cancelled.

• Perhaps one of the reasons that the HL-20 was cancelled was that it was always envisaged as a space vehicle to compliment the Shuttle not as an eventual replacement. In the final analysis America was dependant on the Shuttle to build the ISS and could not afford the luxury of a less capable crew vehicle. In the end the HL-20 was a victim of spiralling costs and NASA budget cuts, it could have given America the guaranteed access to space that it has been lacking for the past 2 years but was instead another cancelled program.

• It is interesting to note that America’s current policy is to retire the Shuttle and separate cargo from crew. It is even looking at commercial companies providing access to space for its astronauts aboard craft that might resemble the HL-20, perhaps all that money and effort will not go to waste after all.

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Acknowledgements

• This Presentation would not have been possible without the help of the following organisations or sources:

• National Aeronautics and Space Administration (NASA)www.nasa.gov

• Molniya Research & Industrial Corporationwww.buran.ru

• Encyclopedia Astronauticawww.astronautix.com

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EncyclopediaAstronautica