Fig. 1 - The DC-XA on its second flight.

How can we believe an Apollo lunar module landed flawlessly six times on the lunar surface when NASA still cannot produce a rocket that can land and take off again? The Delta Clipper crashed attempting to land.

There were actually two versions of the Delta Clipper jointly developed by McDonnell Douglas and NASA. The DC-X flew eight times successfully, taking off and landing each time. On its eighth flight (which took it to a height of 8,200 feet) it landed a bit too hard and cracked its hull.

The replacement craft, DC-XA (Fig. 1), made four flights -- each time taking off and landing under rocket power. On the fourth landing one of its landing struts failed to extend and the vehicle toppled over (after landing successfully) and ruptured the fuel tank. Later investigation showed that the ground crew had failed to connect a helium line to the landing strut.

The DC-X and DC-XA were test beds. They were built to explore technology, not to develop a reliable reusable launch vehicle. As typical with test beds, they were used until they were damaged in testing. In 1996 NASA withdrew funding for the project; they had simply learned what they wanted to learn. Project manager Gary Payton said, "The way the budget is now, we cannot afford to rebuild the Clipper Graham and will not be able to continue with that landing technique, so we will declare victory with the DC-XA".

The flight record of the Delta Clipper series clearly indicates that the technology to take off and land again under rocket power was something NASA could indeed accomplish. But the Apollo lunar module had a much easier task than the Clipper. Here are some important differences between the Apollo lunar module and the Delta Clipper:

Fig. 2 - Combined time-lapse photograph of the DC-X taking off, flying horizontally, and landing safely.

• The lunar module had an onboard human pilot, whereas the DC was flown by automation and remote control. This is because part of its mission was to see how to automate rocket flight.
• The DC flew in earth's atmosphere and had to correct for wind gusts. The lunar module flew in vacuum.
• The DC had to fly in earth's gravity, requiring a much larger engine. This increased the rotational effect of off-axis thrust, requiring a much more agile guidance system to keep it on course.
• The lunar module used two separate propulsion systems: one for descent and one for ascent. The DC attempted to use the same propulsion system and fuel supply for both takeoff and landing.