The Flying Traffic Cone: Delta Clipper Experimental (DC-X)

Delta Clipper Experimental (DC-X) was an uncrewed prototype of a reusable single-stage-to-orbit (SSTO) vertical take-off and landing (VTOL) launch vehicle. It was developed by McDonnell Douglas in conjunction with the Department of Defense before being transferred to NASA. The project ran from 1991 to 1996 with flights taking place from 1993 until its cancellation.

DC-X during flight (McDonnell Douglas)

The DC-X was inspired by former McDonnell engineer Phillip Bono. Bono believed SSTO and VTOL vehicles were the future of space travel. His Saturn-Application-Single-Stage-to-Orbit concept from 1967 influenced the DC-X the most. Up until the very first manned spaceflights, most rockets and spacecraft depicted in science fiction landed vertically, including in the Academy Award winning film Destination Moon from 1950. The only VTOL spacecraft prior to the DC-X was the Apollo Lunar Module.

Thor-Delta rocket (NASA)

The DC-X was initially funded by the US Department of Defense's Strategic Defense Initiative Organization (SDIO), which was known in the 1980s as "Star Wars". The organization wanted a suborbital recoverable launch vehicle that could that could carry 3,000 pounds (1.36 metric tons) of payload to an altitude of 284 miles (457 km) and perform a soft landing to a precise location before launching again anywhere from three to seven days later. That specific altitude is the altitude of the International Space Station. The name "Delta" was a nod to McDonnell's Thor-Delta rockets of the early 1960s while "Clipper" referred to clipper ships of the 1800s. The first letters of both words, "DC", alluded to the Douglas DC-3 aircraft of the 1930s and '40s which revolutionized air travel just as the new spacecraft was going to revolutionize space travel. The first flights were made with a sub-scale vehicle called "X" to test VTOL capability. "X" is the Air Force designation for experimental craft. They would be followed by an orbital prototype "Y", which is the Air Force's designation for pre-production test aircraft and prototypes. If all went well, the final operational vehicle would be "1". The contract for "X" was awarded to McDonnell Douglas in August of 1991 and was the only type of vehicle ever constructed and flown.

The DC-X had a height of 40 feet (12 m) and a diameter of 13.3 feet (4 m) at its tail. It was pyramid-shaped with four RL10A5 engines built by Pratt & Whitney Rocketdyne on the bottom. It used liquid oxygen and liquid hydrogen for propellant. Its aeroshell, or outer layer, was made by the company Scaled Composites, who in 2004 would fly the first privately funded suborbital spaceflight with SpaceShipOne. Its simple design cost $60 million (without inflation) and was built with commercial off-the-shelf parts, including the F-15's navigation system. The DC-X also ran almost completely autonomously with only three people needing to be in the control center— two for flight operations and one for ground support. Former astronaut and commander of Apollo spacecraft Yankee Clipper Pete Conrad was at the controls for some flights.

Although the DC-1 was meant to be suborbital, it was also required to have abort-once-around (AOA) capability for military purposes. AOA was a Space Shuttle abort mode in which the spacecraft performed one orbit around the Earth before reentering the atmosphere and landing [1]. Unlike the Shuttle, which could land at either the Kennedy Space Center in Florida, Edwards Air Force Base in California, or White Sands Missile Range in New Mexico, the DC-1 was to land back at its launch site [2]. This could easily be done if the vehicle was launching to the east. However, some military flights use polar orbits, meaning after the 90 minutes it takes for the vehicle to orbit from north to south, the Earth would have rotated the launch site around 1,500 miles (2,400 km) to the east. Fortunately, its flat sides and control flaps gave it maneuverability. It would reenter the atmosphere nose first and use its control surfaces to maneuver similar to a Space Shuttle orbiter. Then it would pitch up so its tail was facing down and land vertically. This procedure was never tested.

Cutaway sketch of the DC-X and its flight path (NASA)

There were a total of twelve flights during DC-X testing, all of which occurred at White Sands, New Mexico. It first flew on August 18, 1993 and made the news as the first rocket to land vertically on Earth. Two more flights followed on September 11 and September 30. After its third flight, funding began to run out due to the SDIO coming to its official end. Fortunately, NASA and ARPA, the DOD's Advanced Research Projects Agency, provided further funding and the program continued. The DC-X's fourth flight was on June 20, 1994. During the next flight on June 27, the vehicle suffered a minor explosion and successfully executed an abort to make a safe landing. Two more flights followed on May 16 and June 22, 1995 after the damage was repaired. The last flight of the vehicle took place on July 7. After flying to a record-setting altitude of 1.5 miles (2500 m), a hard landing cracked the vehicle's aeroshell. By this time, the program had stopped being financed by ARPA.

DC-X's second flight on September 11, 1993 (New Mexico Museum of Space History)

NASA agreed to continue funding of the DC-X and it was transferred to the agency completely in 1996. The next vehicle was called the DC-XA, "A" standing for "Advanced". Some of its upgrades included replacing the oxygen tank with one made of lithium-aluminum produced by Energia in Russia, replacing the hydrogen tank with a graphite-epoxy composite made by McDonnell Douglas, and improving the reaction control system from Aerojet. The first flight was on May 18, 1996 which resulted in a damaged aeroshell due to heating from performing a slow landing. Two more flights occurred on June 7 and 8 with a turnaround time of only 26 hours, proving such a complex vehicle like a rocket could be ready to go in a short amount of time like a jet aircraft. The flight on the 8th set an altitude record of 10,300 feet (3.14 km). The last flight took place on July 31 and went out with a bang— a landing strut failed to deploy and the vehicle tipped over as it touched the ground. Furthermore, a liquid oxygen tank that had cracked during testing caused a fire that resulted in more damage than could be repaired.

NASA decided to call it quits after the failed flight primarily due to budget constraints; the agency had been working on Lockheed Martin's SSTO VentureStar before it adopted the DC-X. Not counting the DC-X's very final flight, the vehicle and the project were a success: They fulfilled their objectives of demonstrating VTOL capabilities and a fast turnaround.

DC-XA's inaugural flight (NASA)

25 years after its final flight, the technologies developed during the DC-X program play pivotal roles in modern-day spaceflight. Most commonplace are SpaceX's Falcon 9 rockets which launch, land on recovery pads or on drone ships, and are refurbished to be flown again. The company also drew inspiration from DC-X for its Starship rocket, which is expected to launch from Boca Chica, Texas in the coming months. Blue Origin's New Shepard rocket also lands vertically on the ground and is reusable.

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[1] AOA would have been instigated if the spacecraft could not enter a stable orbit but had an adequate velocity to still complete a single orbit

[2] There was a Shuttle abort mode called "return to launch site" but it was highly complex and dangerous

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Bibliography

• Dufresne, Steven. “Delta Clipper: A 1990s Reusable Single-Stage to Orbit Spaceship Prototype.” Hackaday, 19 July 2018, hackaday.com/2018/07/19/delta-clipper-a-1990s-reusable-single-stage-to-orbit-spaceship-prototype/.

• Gannon, Megan. “20 Years Ago: Novel DC-X Reusable Rocket Launched Into History.” Space.com, Space, 16 Aug. 2013, www.space.com/22391-reusable-rocket-nasa-dc-x-anniversary.html.

• McLaughlin, Hailey Rose. “DC-X: The NASA Rocket That Inspired SpaceX and Blue Origin.” Discover Magazine, Discover Magazine, 19 Nov. 2019, www.discovermagazine.com/the-sciences/dc-x-the-nasa-rocket-that-inspired-spacex-and-blue-origin.