With all the talk of SpaceX and Blue Origin sending rockets to orbit and vertically landing part or all of them back on Earth for reuse you’d think that they were the first to try it. Nothing can be further from the truth. Back in the 1990s, a small team backed by McDonnell Douglas and the US government vertically launched and landed versions of a rocket called the Delta Clipper. It didn’t go to orbit but it did perform some extraordinary feats.

Origin Of The Delta Clipper

The Delta Clipper was an unmanned demonstrator launch vehicle flown from 1993 to 1996 for testing vertical takeoff and landing (VTOL) single-stage to orbit (SSTO) technology. For anyone who watched SpaceX testing VTOL with its Grasshopper vehicle in 2012/13, the Delta Clipper’s maneuvers would look very familiar.

Initially, it was funded by the Strategic Defence Initiative Organization (SDIO). Many may remember SDI as “Star Wars”, the proposed defence system against ballistic missiles which had political traction during the 1980s up to the end of the Cold War.

Ultimately, the SDIO wanted a suborbital recoverable rocket capable of carrying a 3,000 lb payload to an altitude of 284 miles (457 km), which is around the altitude of the International Space Station. It also had to return with a soft landing to a precise location and be able to fly again in three to seven days. Part of the goal was to have a means of rapidly replacing military satellites should there be a national emergency.

The plan was to start with an “X” subscale vehicle which would demonstrate vertical takeoff and landing and do so again in three to seven days. A “Y” orbital prototype would follow that. In August 1991, McDonnell-Douglas won the contract for the “X” version and the possible future “Y” one. The following is the story of that vehicle and its amazing feats.

Handling Abort-Once-Around

For military purposes, it was to have “abort-once-around” (AOA) capability. This meant that it should be able to land after only a single orbit, something which was a contingency for the Space Shuttle, but unlike for the Shuttle, the landing was to be at the launch site (the Shuttle could AOA to either Kennedy, Edwards, or White Sands). If takeoff is to the east then there’s no a problem. But military flights sometimes use polar orbits, i.e. a north-south path. The Earth is rotating east-west and if the orbit is north-south then after a single orbit, the Earth will have rotated the launch site around 1,500 miles (2,400 km) to the east.

To cross that distance an orbital Delta Clipper would have flat sides and large control flaps. It would re-enter the atmosphere nose-first and use those control surfaces to maneuver. It would then rotate so that the tail is facing down to land vertically. These design requirements were reflected in the experimental versions.

The Delta Clipper – Experimental (DC-X)

The “X” version was given the name Delta Clipper Experimental, or DC-X. The “Delta” in Delta Clipper was a nod to the expertise lent from the Thor-Delta rocket program for which McDonnell-Douglas was a manufacturer. The “Clipper” refers to the fast Yankee clipper ships of the 1800s. The two words were selected to make up the initials DC, a reference to the Douglas DC-3 aircraft of the 1930s and 1940s which revolutionized air transport.

It was 40 feet (12 m) high and 13.3 feet (4 m) in diameter at the tail. It used four RL10A5 rocket engines built by Pratt & Whitney Rocketdyne, now a part of Aerojet Rocketdyne. Reaction controls consisted of four gaseous oxygen-hydrogen thrusters. The outer aeroshell was made by Scaled Composites, the same company which in 2004 flew the first privately funded suborbital spaceflight with SpaceShipOne.

The DC-X was designed to be simply built from commercial off-the-shelf parts at a cost of $60 million ($118 million in 2018 dollars). It even used the F-15’s flight navigation system. Simple also meant it was to run as autonomously as possible, in the end requiring just three people in the control center: two for flight operations and one for ground support. Apollo astronaut Pete Conrad sat at the controls for some flights.

Flying The DC-X

Between 1993 and 1995 the DC-X flew 8 times. Here are the highlights.

Flights 1 and 2: August 18, 1993 and September 11, 1993

These flights rose vertically 150 and 300 feet, respectively, then translated 300 feet horizontally, and finally flew straight down for a soft landing.

Flight 3: September 30, 1993

Helium bubbles formed in the propellant lines resulting in unequal thrust from the engines at takeoff. It rose at an angle, but the onboard controls compensated and it continued its flight, this time to 1200 feet.

Funding ran out after the 3rd flight when the SDIO program wound down but further funding was found from NASA and DARPA (called ARPA at the time) and flying soon resumed.

Flight 4: June 20, 1994

The DC-X flew vertically to 1,500 feet, rose further while following a curve to 2,600 feet, and then flew laterally 1,050 feet from where it took off. It next reversed direction and climbed to 2,850 feet to over the landing pad wherein it descended.

Flight 5: June 27, 1994

A minor explosion dominated this flight. Prior to launch, some of the hydrogen gas is vented while the propellant lines are chilled. During engine start, some of the vented hydrogen gas ignited, causing a fast burn and an overpressure which blew away a sizeable section of the aeroshell. Despite this, the DC-X continued to fly perfectly. After seventeen seconds, flight manager Pete Conrad commanded the DC-X to go into the autoland sequence. The DC-X then stopped all upward and forward flight and soft-landed.

Flight 6: May 16, 1995

Primarily, this flight tested flying at a constant angle of attack, rising at a constant angle to 4,350 feet. It also did pitch over maneuvers and included the first to use of GPS data and differential throttling.

Flight 7: June 12, 1995

This flight was most notable for carrying out a 75° reverse pitch over maneuver, basically tilting the vehicle to an almost horizontal orientation. The side flaps, engine throttling and the four gaseous oxygen-hydrogen thrusters were also tested. Once again, the altitude record was raised, this time to 5,700 feet.

Flight 8: July 7, 1995

This one tested the maneuvers it would have to do after atmospheric re-entry during an “abort-once-around”, though during an actual re-entry the velocity would be far greater. At 8,200 feet, the DC-X pitched the nose over to ten degrees below the horizon and moved nose-first back toward the landing pad. It then rotated 138° to a tail-first attitude.

Unfortunately, the flight ended with a hard 14 feet/second landing which cracked the aeroshell. Funding had already been cut by the time Flight 8 went up and so there was no money for repairs.

The DC-XA: Delta Clipper Experimental Advanced

After flight 8, NASA agreed to take on the Delta Clipper program. A new, upgraded vehicle was built designated the DC-XA, the “A” standing for Advanced.

Upgrades included:

replacing the oxygen tank with a lightweight lithium-aluminum one made by Energia in Russia,

replacing the hydrogen tank with a graphite-epoxy composite design made by McDonnell-Douglas, and

an improved reaction control system from Aerojet.

The DC-XA flew for only four flights.

In the first one on May 18, 1996, heating due to a planned slow landing damaged the aeroshell. It was repaired and two flights were made in quick succession on June 7th and 8th with only 26 hours between them, proving that a complex launch vehicle could have a turnaround similar to that of a jet aircraft. The June 8th flight also set records with a height of 10,300 feet and a 142 second flight time.

The July 31st flight was the fourth and final one. A helium pressurant line which was supposed to supply hydraulic pressure for one of the landing struts was not connected and the strut failed to deploy. As a result, and as the end of the video below shows, the vehicle tipped over during landing, falling to the ground on its side. Unfortunately, one of the liquid oxygen tanks had cracked during testing. The structural damage due to the fall would have been fixable but liquid oxygen from the cracked tank worsened a fire which resulted in more damage than could be repaired.

End Of The Delta Clipper Project

There is speculation that NASA had been shamed into taking on the Delta Clipper project due to its public success under the SDIO. But NASA already had its own SSTO project, the Lockheed Martin VentureStar, and so it cancelled the Delta Clipper, citing budgetary constraints.

Despite having ended in a crash, the Delta Clipper “X” vehicles were a success, demonstrating VTOL, quick turnaround, and the types of maneuvers necessary for an “abort once around” back to launch site. And they looked every bit as amazing at the time as watching SpaceX’s and Blue Origin’s VTOL flights do now.

Banner image via [Space.com].

The images of DC-X’s Flights 5 and 8 were taken from this Flight for Cheap Access to Space video.

Thanks to [Daniel Matthews] for the idea for this article by reminding us about the Delta Clipper in a recent comment to our post about doing yaw control using a model of a Falcon 9.