The Space Reviewin association with SpaceNews

STS-122 launch
The space shuttle is capable of airplane-like operations—if one is imagining a 1910-era airplane. (credit: NASA/KSC)

Space myths 2

Simplicity equals reliability

You don’t have to be either a rocket scientist or a reliability expert to be able to see that more parts in a vehicle translates into more opportunities for failure. It’s been a generally accepted principle that the more complex the vehicle, the greater the opportunities for failures. It’s a reasonable theory—and one that is demonstrably wrong.

First, there is the fact that the traditional approach to corrective action after launch failures was to add features to a vehicle, not take existing ones away. As experience was gained, vehicles tended to become more complex, not less so. Some of this increased complexity was merely added instrumentation, but more complex critical systems added to correct failures were common as well.

It’s been a generally accepted principle that the more complex the vehicle, the greater the opportunities for failures. It’s a reasonable theory—and one that is demonstrably wrong.

Second, there is the experience with solid-motor-powered vehicles. Solid motors are far less complex than liquid engines, having virtually no moving parts, except for that required for attitude control provisions, which liquid engines also require. This simplicity does not translate into reliability. The average probability of a mission failure for all types of launch vehicles over the last 30 years has been about 6%. For vehicles powered primarily by solid rocket motors the demonstrated failure probability is nearly twice the overall average, almost 12%.

Also, there is the case of the Space Shuttle. Easily the most complex vehicle now flying—and probably the most complex vehicle every built—the Shuttle is actually a bit more reliable than the average. With 121 flights to date and only 5 mission failures, the Shuttle has a mission success reliability of almost 96%—two-thirds better than the overall average.

Then, perhaps most importantly of all, are the vehicles with strap-on stages. Take an existing booster, add strap-on rockets to the side, and the reliability just has to go down, doesn’t it? In reality, looking at data for the last 30 years, vehicles with strap-on motors have proven to be even a bit more reliable than ones without.

Many factors influence a launch vehicle’s reliability—but the number of parts isn’t one that by itself has much impact.

RLVs should be like airplanes

Back in the heyday of the X-33 and its follow-on the Lockheed Martin Venturestar RLV, it was repeatedly emphasized that RLVs must display “airplane-like” operation. The X-33 was a bust, but today many continue that same refrain, or at least seem to assume that mode of operation will be the norm.

So, what is “airplane-like” operation? Unfortunately, that depends on which airplane you are talking about and what era it was in. Back in 1910, heavier-than-air vehicles had been flying for almost seven years. There were multiple companies in multiple countries producing airplanes and engines; they were available to just about anyone who had more money than good sense. Air rallies or races were held in a few countries, and the procedures for each of them were similar. The airplanes would be trucked in, assembled, and flown for exhibition purposes. The winner typically was the one that managed to complete the course, or most nearly so, without a fatal crash. Then the airplanes were dismantled and hauled off. The engines almost invariably were torn down completely and examined, with at least a few parts replaced as result. That was “airplane-like” operation in 1910; few would choose to emulate it today. The main difference between RLVs of today and airplanes of that era was that there actually were some airplanes flying back then.

Asserting that RLVs must display “airplane-like” operating characteristics denies the simple fact that they are not airplanes; they are space boosters.

Airplane-like operation also must presume that the vehicle has been developed and tested to the same degree and in a similar manner to airplanes. That is a tough row to hoe; airplanes undergo quite an incredible amount of testing. For even an amateur-built light aircraft, the FAA requires that it be operated at least 40 hours in the vicinity of its home airfield before engaging in longer trips. For more complex aircraft the testing is far more extensive and much more rigorous. For example, the original test plan for the C-17 cargo aircraft called for the prototype to be flown every three days and that was to be for a period of years. Later test models were added to the test schedule of the first example, as were the first few production models of the airplane. And as impressive as all this sounds, the C-17’s pace of testing came to be considered as both too slow and inadequately progressive. For more technologically challenging aircraft the test program is even more extensive. They only built a total of 116 B-58’s and some 90 of them were part of the test program at one time or another. Attempting to test RLVs to the same level is going to be very, very expensive and will take an awful lot of time—and RLVs capable of putting payloads into orbit will make a B-58 look like a light aircraft in comparison. If it took years to properly test a subsonic airlifter it will almost certainly require decades to properly test a true RLV “spacelifter.”

The Space Shuttle experience bears this all out. The first flights of the Shuttle showed it to require post-flight activity closer to that of a 1910-vintage airplane than that of a modern airliner. Despite this, the third flight of the Space Shuttle Columbia was declared to be an “operational” mission. Following another 104 space shuttle flights and the loss of two Shuttles with their crews—including the Columbia itself—the official investigation board would feel compelled to point out that launching a Shuttle was a process much more akin to flying a very high performance experimental aircraft than operating a typical operational airplane.

Asserting that RLVs must display “airplane-like” operating characteristics denies the simple fact that they are not airplanes; they are space boosters. Over a hundred years after the first successful manned flight we don’t claim that even light airplanes can be operated as casually as automobiles are. Nor did people give up in despair when airplanes proved to be as difficult to operate as they were in 1910. They kept plugging away, and today, after decades of incredible technological advancement, we are still not to the point where flight can be considered to be as casual as almost any form of ground transportation. People have been trying to develop “flying cars” or “aircraft for the masses” for over 50 years but they have not been successful—and very likely will never be—because airplanes are not cars.

The giant leap between airplanes and space launch RLVs will be much greater than that the significant steps between airplanes and ground vehicles. And RLVs will of necessity resemble airplanes in their operations even less than Cessna does a Chevy.