OpEd: Space Shuttle Past and future


posted: 29 August 2005

11:42 am ET

While recognizing the “compromises” noted in Brian Berger’s article “Discovery Opens Last Chapter for Space Shuttle Born of Compromise ” [Aug. 15, page 1], I would like to point out that there were several major state-of-the-art technology challenges which NASA took on in order to make the space shuttle system happen. These technology developments were key to achieving the space shuttle system that has flown so many unique space missions over the last two and one half decades, many of which no other vehicle can fly, such as satellite repair/retrieval, large cargo return to Earth and space station assembly .

From my perspective, particularly significant examples are:

– Development of the Space Shuttle Main Engines (SSME). The most advanced materials within reach at the time were brought together in a throttleable liquid oxygen/liquid hydrogen design that is able to operate at extremely high internal pressures and temperatures, enabling it to achieve unparalleled engine performance among rocket engines, which prevails to this day. I personally view the SSME as one of the most remarkable machines man has created.

– Development of the space shuttle four-string, digital, fly-by-wire, fail operational, fail safe avionics system.

At the time, such an avionics system did not exist in operational flight vehicles. The shuttle system was evolved from a development test program conducted at NASA Dryden Flight Research Facility using an F-8C Crusader as a flying test bed. I believe the shuttle was the first operational vehicle to fly using such a system, although the Air Force also was experimenting with one for use on the F-16, which began flying in the same time period.

This system has enabled the space shuttle to be two-fault tolerant for failures across many of its vital subsystems with fraction-of-a-second response times which are necessary during time-critical ascent flight phases. Of course today, many airplanes and other flight systems use multi-string, digital, fly-by-wire avionics systems that are second- and third-generation evolutions of the type of system that was developed for the shuttle.

– Development of the space shuttle orbiter thermal protection system. Previous spacecraft had flown with single-use ablative materials to dissipate the heat of re-entry. In addition to being reusable, the orbiter system had to be extremely light weight to meet vehicle performance goals, conform to the idiosyncrasies of the aerodynamic mold lines of the vehicle and fly successfully through the combined stresses of the flight profiles for both ascent and entry.

The thermal silica tiles (20,000-30,000 depending on which orbiter), together with their vehicle attachment system, while requiring significant between-flight maintenance, have proven to be highly effective and damage tolerant in providing safe entries for both the orbiter and the crew. (To great misfortune, however, the sensitivity to impact of the wing leading edge carbon-carbon panels, generally a harder material than the tiles, was somehow underappreciated during the events leading up to STS-107 and its tragic flight).

– Overall vehicle integration. C hallenges related to the design, understanding, design, balancing and optimizing of the space shuttle combined mechanical, propulsive, thermal, aerodynamic and acoustic loads for the side-mount shuttle launch configuration and for the orbiter for entry were extensive and in many cases unprecedented.

However, the NASA designs for these considerations were successful on the first flight and they were subsequently optimized using flight test data over a number of flights as the shuttle program progressed.

Most remarkable of all, John Young and Bob Crippen successfully flew this vehicle, with all this new technology and associated risks, on its first flight. To my knowledge, no other human space vehicle has been flown for the first time with crew onboard.

On another subject, as pointed out in the article, solid rocket boosters were chosen for the space shuttle as a less expensive option to liquid fuel boosters. One of the aspects of the solids, as they are implemented on the space shuttle, is that once they are ignited they cannot be shut down or separated throughout the course of their nominal two-minute burn as the first stage of the shuttle launch stack.

This essentially has precluded the potential for any sort of crew abort (other than ejection seats, which are impractical for full-size shuttle crews) during the first two minutes of shuttle ascent — once it’s lit, you ride it. Early on, Shuttle Solid Rocket Booster (SRB) thrust termination approaches were studied but it was concluded that simultaneous shutdown and balanced thrust tail-off of the two independent SRB s could not be consistently achieved and could potentially lead to hazardous events due to shuttle stack thrust unbalance.

The NASA space shuttle community repeatedly has longed for a liquid booster system, which could be reliably shut down in flight, to replace the solid rocket boosters. Such a system might have been configured in such a way that an abort option could have been available for Challenger or any other flight where a first-stage boost problem occurred.

I understand that liquid vs. solid was a significant debate during early shuttle design phases. During my tenure as space shuttle program manager and subsequently as space shuttle program director, converting to liquid-fuel boosters was studied in depth at least twice, only to be turned down on each occasion based upon the projected cost and time required for development. I believe it has been studied at least once again in more recent times.

To avoid any confusion, I would add that I am very comfortable with the SRB with an upper stage and an escape tower as a manned “single stick” launcher. With the help of Guy Stever and his outstanding committee, NASA and Thiokol made substantial and very adequate improvements to the SRB segment joints as well as to the nozzle and nozzle gimbal areas following Challenger, resulting in a very reliable launcher.

Finally, Brian’s article quotes John Logsdon as stating that NASA today appears “determined to propose an approach to the next generation system for carrying people to space that learns from Shuttle’s history.” It’s interesting to note that current thinking also appears to focus on using significant elements and technology from the space shuttle launch system as the basis for the new exploration program launchers for both crew and cargo.

Arnold D. Aldrich is a former NASA associate administrator for space systems development.