Ten years after the Challenger accident, there is still argument over a phenomenon called dynamic overshoot.

Tim Furniss/Washington DC

IF NASA ERRED IN DESIGNING the Space Shuttle, its hypersensitive reaction to the term "dynamic overshoot" and to the name of Ali AbuTaha, who conducted an independent analysis of the phenomenon, would be understandable. NASA did make an error in the 1970s, and failed to recognise it until 1990, but it then began redesigning Space Shuttle components to take into account a 100% dynamic overshoot experienced by the vehicle at launch. Using NASA Marshall Space Flight Center, Space Shuttle SRB [Solid-Rocket Booster] Design Loads [4 April 1990], Dynamic Load Factor of 2.0 specifications, the work was completed in 1992 when the design was "frozen" (Flight International, 29 November-5 December, 1995).


Overshoot phenomenon

The agency need not be so hypersensitive, the error was not exclusive to the Shuttle, because the little-understood dynamic overshoot phenomenon applies "...to any rapidly ramped-up pressure-activated system and has been largely overlooked worldwide", says aerospace engineer AbuTaha, of Herndon, Virginia, who highlighted the phenomenon in 1986 after his own investigation into the Challenger accident.

A former senior structural-dynamics officer with Comsat, who designed interface structures between Intelsat satellites and launchers, AbuTaha says: "The concept is simple, but also misunderstood, and has led to gigantic errors." (see box).

The Shuttle was designed without consideration of dynamic overshoot, which "...has affected both it and many modern systems, including rockets, aircraft, and powerplants", causing damage, premature wear and tear, fatigue and structural defects. AbuTaha says: "During my years with Comsat, dynamic overshoot was completely overlooked - by me - and satellite contractors. When I found the memorable errors in the Shuttle, NASA claimed the work to be their own." NASA has categorically and vehemently denied AbuTaha's work for years, but the evidence contradicts the agency.

This includes NASA's introduction of the new 100%-increased "Dynamic Load Factor of 2.0" criterion, in 1990 and comments made in 1992 by NASA's own Bob Sieck, the former Shuttle launch director, who highlighted how the phenomenon occurs when any load is applied rapidly.

The Space Shuttle presented NASA with a unique lift-off environment, which magnified the adverse effects of dynamic overshoot on the system. The Shuttle assembly is asymmetric. The thrust line of the main engines (SSMEs), as they ignited before lift-off, is 9m (32ft) from the centre of the launch-vehicle structure, exerting a tremendous torque. The bolts holding the Shuttle on the pad withstand enormous forces, which are transmitted from the Orbiter through its support struts to the external tank (ET) and through the tank mountings to the SRBs.

In the 1970s, the Shuttle was designed to lift-off with SRB ignition at T+3.832s, at 90% SSME thrust, with the base bending force at the bottom of the aft SRB segments estimated at 350 million in lb - NASA's chosen unit of bending - (39.5 million Nm), as the vehicle lurched laterally - a phenomenon predicted, but not understood fully, caused by the effect of the angular positioning of the main engines. The Shuttle system was therefore designed within an allowable limit of 347 million in lb.

It was discovered that the horizontal movement of the Shuttle immediately after lift-off (the momentum of the lateral lurch) could cause a collision with the launch pad. At launch, torque build-up would not have disappeared. It would have been relieved by pushing the Shuttle sideways like a released spring, hitting some pad structures. The vehicle could have been tilted on the pad or have been lifted off at a lower thrust to avoid this.

In 1980, however, a lift-off delay of 2.5s was introduced instead, to give the vehicle time to rebound from the lateral movement. The base bending moment at this lift-off point at T+6.332s - 100% thrust rating - would actually be reduced to 150 million in lb. NASA failed to realise, however, that, to reach the new bending moment, the system had to go through a new peak bending-moment load of 580 million in lb - dynamic overshoot - for which the Shuttle was not designed (see diagram). It exceeded even NASA's design-plus safety margins.

When the Shuttle lurches horizontally at main-engine start and, as thrust builds up, it springs back to its original position for lift-off at 100% thrust because the thrust has ramped up rapidly - causing dynamic overshoot. If there were no dynamic overshoot, the craft would stay in the lurched position for lift-off at 100% thrust. To lift off at 100% thrust in its original, near-vertical, position, it must have exceeded something to spring back.

A NASA Marshall Space Flight Center Systems Dynamics Laboratory transient-load measurement showing the 5,005kN thrust of the main engines overshooting by 72% in milliseconds to 8,620kN, was also overlooked.

The original Shuttle-loads specification (from Rockwell, dated June 1973) states plainly that the design loads for main-engine ignition included "dynamic factors of 1.1 axial and 1.4 transverse". That is, the dynamic condition was taken to be 110% for axial loads and 140% for transverse loads, but a dynamic overshoot was never considered. "All of us in the space industry used these factors in the 1960s and 1970s, instead of the actual 70-100% overshoot," says AbuTaha.

He exposed oversights by NASA and the official Rogers Commission into the Challenger disaster, which concluded that the base bending moment experienced by STS 51L was 291 million in lb, "within the allowable design limit of 347 million in lb". AbuTaha revealed that the 291 million in lb figure was too low for two boosters and was the figure for one. For two boosters, it was 582 million in lb.

Further investigations also revealed that the original 347 million in lb limit continued to be used for the whole system. "The load estimate had been doubled as a consequence of the delayed lift-off time, which nearly doubled the loads, forces and torques on the SRBs, ET, orbiter and payloads - causing damage and premature wear and tear," AbuTaha says.

The bending moment and dynamic loads were causing extraordinary strains on the aft segments and on the external-tank attach ring above the aft segment, in particular, and on other parts of the Shuttle system and payloads, which were being "mysteriously" damaged, requiring repair, redesign and strengthening.

The effects of dynamic overshoot were clearly being seen by NASA before the Challenger accident, but the cause was not recognised as such. The highly publicised thermal tiles failed at half strength. Their strength was doubled. The number of pins in the forward segments of the SRBs was doubled. The design of the SRB separation motors was changed to allow for a 100% dynamic load, and the Shuttle's payload has been reduced by almost half.

NASA strengthened the aft segments of the SRBs and other Shuttle components throughout the programme until, by 1992, it had fully compensated for the effects of dynamic overshoot.

The first lightweight SRBs, introduced to increase payload performance and flown on STS 6 in 1983, were "buckled, bent, broken and damaged", reported NASA. The agency put this along with a damaged ET attach ring and other regular SRB damage down to splashdown effects. Dynamic overshoot also resulted in mirror-image damage to the pad, including 1m-long cracks on the launch platform.

Correcting for dynamic overshoot has cost payload capability, which, for the Shuttle to 28¡ low-Earth orbit, has been reduced from 29,500kg to 17,690kg (a difference of 11,810kg) for the Columbia. Typically, a 13,600kg payload can use up the ascent capability of the Shuttle. The heaviest payload carried by the Shuttle weighed, 22,150kg, on the STS41, and the weight of its SRBs was reduced dramatically.

Another factor, however, has been the "insidious" 11,000kg increase in the weight of the combined external tank-SRB (ET/SRB) "stack" to strengthen it against dynamic overshoot.

The weight of the orbiter and payload has varied from mission-to-mission, but the supposed "constant" weight of the SRB/ET "stack" has varied (see diagram). From 1992-3, as the Shuttle's design was being frozen, this figure has steadied, representing an average 11,450kg rise over that of the beginning of the programme.

This increase has been borne by the twin SRBs. The ET weighs about 750,970kg when full of liquid oxygen and hydrogen, but this can vary by about 1,360kg depending on loading conditions, temperatures and atmospheric pressure. The official figure for the combined SRB weight is still given by NASA as 1,172,275kg, but calculations show it to be about 1,183,540kg, making the 11,000kg difference between the 1981 and current "stack" weights.

While SRB weight variations are explained by "different manufacturing batches of segments and solid-propellant grain composition, tailored for a mission", says NASA, weight increases have also resulted from strengthening of the SRBs. NASA not only redesigned the field joints after the Challenger accident - as instructed by the Rogers Commission - but also immediately reinforced the attach ring and has been strengthening the system, as a result of AbuTaha's findings.


Mysterious damage

AbuTaha spent weeks visiting NASA centres, attended meetings with Shuttle engineers and hierarchy after 1986 and made his investigations into the Challenger accident public, but not the dynamic overshoot. He thought, wrongly, that NASA engineers understood what he was telling them and that it would work on this internally.

He attended another hearing at NASA headquarters in October 1989 to discuss his work and realised then that NASA had not understood the dynamic-overshoot principle. NASA still believed that the base bending moment increased by 10% only when it increased the lift-off thrust from 90% to 100% in the 1980 redesign. The agency was fixing parts of the system, but not the whole, trying to understand what was happening to the "mysterious" damage and wear and tear to the system.

AbuTaha only went public with the dynamic-overshoot theory in January 1990. He has always officially been persona non grata at NASA, although, privately, many NASA engineers have appreciated his work.

NASA refuses to respond to specific questions about AbuTaha's work. Rod Wallace, flight-dynamics officer at the Johnson Space Center, Houston, Texas, says: "Our knowledge of ascent-flight regimes has matured, not just lift-off dynamics. No changes were made to Space Shuttle hardware because of lift-off dynamics".

Officially, NASA says: "We consider this matter closed. AbuTaha says: "100% dynamic overshoot and similar transient magnifications were never mentioned by NASA and the Rogers Commission. None of the many loads and deflection curves presented included the overshoot. The evidence shows that corrections were made subsequent to my work."

Source: Flight International