# Northrop HL-10

The Northrop HL-10 is one of five US heavyweight lifting body designs flown at NASA's Flight Research Center (FRC—later Dryden Flight Research Center) in Edwards, California, from July 1966 to November 1975 to study and validate the concept of safely maneuvering and landing a low lift-over-drag vehicle designed for reentry from space. It was a NASA design and was built to evaluate "inverted airfoil" lifting body and delta planform. It currently is on display at the entrance to the Armstrong Flight Research Center at Edwards Air Force Base.

## Development

Northrop Corporation built the HL-10 and Northrop M2-F2, the first two of the fleet of "heavy" lifting bodies flown by the NASA Flight Research Center. The contract for construction of the HL-10 and the M2-F2 was $1.8 million. "HL" stands for horizontal landing, and "10" refers to the tenth design studied by engineers at NASA's Langley Research Center, Hampton, Virginia. Main gear was a modified T-38 system retracted manually, and lowered by nitrogen pressure. Nose gear was a modified T-39 unit, retracted manually and lowered with nitrogen pressure. Pilot Ejection System was a modified F-106 system. Silver zinc batteries provided electrical power for the control system, flight instruments, radios, cockpit heat, and stability augmentation system. To assist in pre-landing flare, four throttleable hydrogen peroxide rockets provided up to 400 lbf (1.8 kN) of thrust.

## Operational history

After delivery to NASA in January 1966, the HL-10 made its first flight on December 22, 1966, with research pilot Bruce Peterson in the cockpit. Although the XLR-11 rocket engine (same type used in the Bell X-1) was installed, the first 11 drops from the B-52 launch aircraft were unpowered glide flights to assess handling qualities, stability, and control. In the end, the HL-10 was judged to be the best handling of the three original heavy-weight lifting bodies (M2-F2/F3, HL-10, X-24A).
The HL-10 was flown 37 times during the lifting body research program and logged the highest altitude and fastest speed in the lifting body program. On February 18, 1970, Air Force test pilot Peter Hoag piloted the HL-10 to Mach 1.86 (1,228 mph or 1,976 km/h). Nine days later, NASA pilot William H. "Bill" Dana flew the vehicle to 90,030 feet (27,440 m), which became the highest altitude reached in the program.
During a typical lifting body flight, the B-52—with the research vehicle attached to the pylon mount on the right wing between the fuselage and inboard engine pod—flew to a height of about 45,000 feet (14,000 m) and a launch speed of about 450 mph (720 km/h).
Moments after being dropped, the XLR-11 was lit by the pilot. Speed and altitude increased until the engine was shut down by choice or fuel exhaustion, depending upon the individual mission profile. The lifting bodies normally carried enough fuel for about 100 seconds of powered flight and routinely reached from 50,000 to 80,000 feet (15,000 to 24,000 m) and speeds above Mach 1.
Following engine shutdown, the pilot maneuvered the vehicle through a simulated return-from-space corridor into a pre-planned approach for a landing on one of the lakebed runways on Rogers Dry Lake at Edwards. A circular approach was used to lose altitude during the landing phase. On the final approach leg, the pilot increased his rate of descent to build up energy. At about 100 feet (30 m) altitude, a "flare out" maneuver dropped air speed to about 200 mph (320 km/h) for the landing.
Unusual and valuable lessons were learned through the successful flight testing of the HL-10. During the early phases of the Space Shuttle development program, lifting bodies patterned on the HL-10 shape were one of three major types of proposals. These were later rejected as it proved difficult to fit cylindrical fuel tanks into the always-curving fuselage, and from then on most designs focused on more conventional delta wing craft.
* HL-10 pilots
** John A. Manke — 10 flights, 7 powered flights
** William H. Dana — 9 flights, 8 powered flights
** Jerauld R. Gentry — 9 flights, 2 powered flights
** Peter C. Hoag — 8 flights, 7 powered flights
** Bruce Peterson — 1 flight, 0 powered flights

### Unrealized space flight

According to the book "Wingless Flight", by project engineer R. Dale Reed, the HL-10 was considered to fly into space in the early to mid-1970s. Following the cancellation of the Apollo moon project, Reed realized that there would be substantial Apollo hardware left over, including several flight-rated command service modules (CSM) and Saturn V rockets.
The proposal was to add an ablative heat shield, reaction controls, and other additional subsystems needed for crewed spaceflight to the HL-10. The now space-rated vehicle would have then been launched in the space for the Lunar Module on a Saturn V launch vehicle with an Apollo CSM. Once in Earth orbit, it was planned that a robotic extraction arm would remove the HL-10 from the rocket's third stage and place it adjacent to the crewed Apollo CSM spacecraft. One of the astronauts would then spacewalk from the Apollo and board the lifting body to perform a pre-reentry check on its systems.
It was planned that there would be two flights in this program. In the first, the lifting body pilot would return to the Apollo and send the HL-10 back to earth uncrewed. If this flight was successful, the second launch would be involve a piloted landing at Edwards AFB. Reportedly, Wernher von Braun was enthusiastic about the mission, offering to prepare two Saturn Vs and Apollo Command Service Modules. However, he was overridden by the Flight Research Center director, and nothing came of the proposal. Launching a Saturn V to low Earth orbit with a light payload would not be an efficient use of capability, and the Apollo program was ended mainly on cost grounds.

## HL-10 flights

| Vehicle Flight # | Date | Pilot | Mach | Velocity /km/h | Altitude /m | Duration | Comments |
| HL-10 #1 | December 22, 1966 | Peterson | 0.693 | 735 | 13,716 | 00:03:07 | First HL-10 Flight Unpowered glide |
| HL-10 #2 | March 15, 1968 | Gentry | 0.609 | 684 | 13,716 | 00:04:03 | Unpowered glide |
| HL-10 #3 | April 3, 1968 | Gentry | 0.690 | 732 | 13,716 | 00:04:02 | Unpowered glide |
| HL-10 #4 | April 25, 1968 | Gentry | 0.697 | 739 | 13,716 | 00:04:18 | Unpowered glide |
| HL-10 #5 | May 3, 1968 | Gentry | 0.688 | 731 | 13,716 | 00:04:05 | Unpowered glide |
| HL-10 #6 | May 16, 1968 | Gentry | 0.678 | 719 | 13,716 | 00:04:25 | Unpowered glide |
| HL-10 #7 | May 28, 1968 | Manke | 0.657 | 698 | 13,716 | 00:04:05 | Unpowered glide |
| HL-10 #8 | June 11, 1968 | Manke | 0.635 | 697 | 13,716 | 00:04:06 | Unpowered glide |
| HL-10 #9 | June 21, 1968 | Gentry | 0.637 | 700 | 13,716 | 00:04:31 | Unpowered glide |
| HL-10 #10 | September 24, 1968 | Gentry | 0.682 | 723 | 13,716 | 00:04:05 | Unpowered glide XLR-11 installed |
| HL-10 #11 | October 3, 1968 | Manke | 0.714 | 758 | 13,716 | 00:04:03 | Unpowered glide |
| HL-10 #12 | October 23, 1968 | Gentry | 0.666 | 723 | 12,101 | 00:03:09 | 1st powered flight engine malfunction landed Rosamond |
| HL-10 #13 | November 13, 1968 | Manke | 0.840 | 843 | 13,000 | 00:06:25 | 3 tries to light engine |
| HL-10 #14 | December 9, 1968 | Gentry | 0.870 | 872 | 14,454 | 00:06:34 | - |
| HL-10 #15 | April 17, 1969 | Manke | 0.994 | 974 | 16,075 | 00:06:40 | - |
| HL-10 #16 | April 25, 1969 | Dana | 0.701 | 744 | 13,716 | 00:04:12 | Unpowered glide |
| HL-10 #17 | May 9, 1969 | Manke | 1.127 | 1,197 | 16,246 | 00:06:50 | 1st lifting body supersonic flight |
| HL-10 #18 | May 20, 1969 | Dana | 0.904 | 959 | 14,966 | 00:06:54 | - |
| HL-10 #19 | May 28, 1969 | Manke | 1.236 | 1,312 | 18,959 | 00:06:38 | - |
| HL-10 #20 | June 6, 1969 | Hoag | 0.665 | 727 | 13,716 | 00:03:51 | Unpowered glide |
| HL-10 #21 | June 19, 1969 | Manke | 1.398 | 1,484 | 19,538 | 00:06:18 | - |
| HL-10 #22 | July 23, 1969 | Dana | 1.444 | 1,350 | 19,446 | 00:06:13 | - |
| HL-10 #23 | August 6, 1969 | Manke | 1.540 | 1,656 | 23,195 | 00:06:12 | 1st four- chambered flight |
| HL-10 #24 | September 3, 1969 | Dana | 1.446 | 1,542 | 23,762 | 00:06:54 | - |
| HL-10 #25 | September 18, 1969 | Manke | 1.256 | 1,341 | 24,137 | 00:07:06 | - |
| HL-10 #26 | September 30, 1969 | Hoag | 0.924 | 980 | 16,383 | 00:07:16 | - |
| HL-10 #27 | October 27, 1969 | Dana | 1.577 | 1,675 | 18,474 | 00:06:57 | - |
| HL-10 #28 | November 3, 1969 | Hoag | 1.396 | 1,482 | 19,544 | 00:07:19 | - |
| HL-10 #29 | November 17, 1969 | Dana | 1.594 | 1,693 | 19,687 | 00:06:48 | - |
| HL-10 #30 | November 21, 1969 | Hoag | 1.432 | 1,532 | 24,165 | 00:06:18 | - |
| HL-10 #31 | December 12, 1969 | Dana | 1.310 | 1,402 | 24,372 | 00:07:08 | - |
| HL-10 #32 | January 19, 1970 | Hoag | 1.310 | 1,399 | 26,414 | 00:06:50 | - |
| HL-10 #33 | January 26, 1970 | Dana | 1.351 | 1,444 | 26,726 | 00:06:51 | - |
| HL-10 #34 | February 18, 1970 | Hoag | 1.861 | 1,976 | 20,516 | 00:06:20 | Fastest lifting body flight |
| HL-10 #35 | February 27, 1970 | Dana | 1.314 | 1,400 | 27,524 | 00:06:56 | Highest lifting body flight |
| HL-10 #36 | June 11, 1970 | Hoag | 0.744 | 810 | 13,716 | 00:03:22 | Lift/Drag powered approach |
| HL-10 #37 | July 17, 1970 | Hoag | 0.733 | 803 | 13,716 | 00:04:12 | Last flight |

### Aircraft serial number

* Northrop HL-10 — NASA 804, 37 flights

### Status

The HL-10 is currently on display at the entrance of Armstrong Flight Research Center at Edwards, CA.

## Specifications (Northrop HL-10)
### General characteristics

* Crew: one pilot
* Length: 21 ft 2 in (6.45 m)
* Wingspan: 13 ft 7 in (4.15 m)
* Height: 9 ft 7 in (2.92 m)
* Wing area: 160 ft$^2$ (14.9 m$^2$)
* Empty: 5,285 lb (2,397 kg)
* Loaded: 6,000 lb (2,721 kg)
* Maximum takeoff: 10,009 lb (4,540 kg) (propellant wt 3,536 lb - 1,604 kg)
* Powerplant: 1 x Reaction Motors XLR-11 four-chamber rocket engine. 8,000 lbf (35.7 kN) thrust

### Performance

* Maximum speed: 1,228 mph (1,976 km/h)
* Range: 45 miles (72 km)
* Service ceiling: 90,303 ft (27,524 m)
* Rate of climb: ft/min ( m/min)
* Wing loading: 62.5 lb/ft$^2$ (304.7 kg/m$^2$)
* Thrust-to-weight: 1:0.99

## Fictional references

In the pilot movie, and an episode of The Six Million Dollar Man series, titled "The Deadly Replay", the HL-10 serial number 804 is identified as the aircraft flown by Col. Steve Austin when he crashed, leading to his transformation into a bionic man, and the HL-10 is also featured in this episode. Other episodes and Martin Caidin's original novel, Cyborg, contradict this, however, by identifying Austin's aircraft as a fictional cousin of the HL-10, the M3-F5. Further confusion is added by the fact that both the HL-10 and the M2-F2 are featured in the opening credits of the television show.