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.