Rocking and Rolling on the Launch Pad

Difficult as it is to believe, when the Space Shuttle rolls out from the Vehicle Assembly Building, it is kept steady by only eight hold-down posts. Each Solid Rocket Booster (SRB) has four hold-down posts that fit into corresponding support bases on the Mobile Launcher Platform (MLP). Hold-down bolts secure the SRB to the launcher platform. These bolts are so strong and so thick, that they require explosive charges to fracture the nut on each bolt for separation at liftoff. The Ares I-X vehicle, using the same type of posts, will have only four of them to keep its "single stick" steady on its slow roll out to Launch Complex 39B ( The hold-down posts are fine for keeping the rocket steady for its six-hour journey rollout, but it will need something a little stronger once it is out on the pad.

When Ares I-X gets out to the pad, ground crews will need to be able to enter the rocket to prepare or service the avionics. Anyone who has been near the top of a very tall building knows how disconcerting it can be to feel the structure sway in the wind. Such a sway, combined with people moving around inside the rocket, would interfere with the ground staff’s ability to do precision work -- or worse, cause the rocket to topple over. That's where the Vehicle Stabilization System, or VSS, takes over.

Two vehicle stabilization concepts

Figure 2: Two vehicle stabilization concepts—a tower added to the Mobile Launch Platform (left) and a smaller mechanism attached to the existing launch tower (right). Image Credit: NASA

The VSS, designed by United Space Alliance (USA) for NASA, went through several different concepts before the final hardware and design were selected. One of the original ideas included a new tower attached to the top of the Mobile Launch Platform (Figure 2). In addition to being tall, labor-intensive, and expensive, the tower construction posed a significant schedule risk that could have prevented Ares I-X from being launched in time to supply flight data for the Ares I program.

As vehicle requirements matured, the design was finalized. A smaller framework attached to the existing Space Shuttle Fixed Service Structure the need to fabricate a new tower, which is less expensive to build -- and saved several months of construction time. The primary ingredients for this stabilizer include hundreds of structural steel members, two vehicle attachment interfaces (latches), and four spring/damper assemblies -- similar to your car or truck suspension system, only much, much larger! The system is made up of four sets of stabilizers. There are two sets of stabilizers for north-south motions and two for east-west motions. The east-west stabilizers (called Y-dampers, because it limits vehicle motion in its y-axis) connect directly to the vehicle by hydraulically actuated latches and are pinned to the north-south stabilizers (called Z-dampers), so that the lateral north-south vehicle motions can be transmitted to z-axis stabilizers.

Closeups of the hydraulic shock system for the Ares I X vehicle stabilization system

Figure 3: Close-ups of the hydraulic “shock” system for the Ares I-X vehicle stabilization system. Image Credit: NASA

The stabilizers employ four large spring/damper assemblies to reduce vehicle displacements due to wind forces and wind-induced oscillation motions (Figure 3). A rigid system would have imparted too large a load into the vehicle’s 5th segment simulator, so springs allow the vehicle to move - but not too much! The loads induced to the vehicle by the VSS are reduced by sharing some of the load with the Aft skirt as the vehicle moves. The dampers, or shocks, remove the energy added to the system by the wind. Without the dampers, the vehicle could swing back and forth with ever-increasing amplitudes until something broke.

A shock-absorber of the size and characteristics required for proper operation of the VSS damper assemblies was quoted by vendors to be a 72 week lead time, posing a significant threat to the Ares I-X launch schedule. Enter Roger Lenard, a former Air Force Colonel and fighter pilot working for Lee & Associates, who commutes from one troubleshooting spot to another in his private plane. Lenard is one of 700 civil servants and contractors putting together Ares I-X on a limited budget, so every dollar saved matters. Roger began investigating Commercial Off-The Shelf (COTS) shock absorbers that could be utilized, from those used on C-130 landing gear to those used on trains, in an effort to drive down costs and schedule. He found his damper candidate at the Dellner division in Sweden, owned by Monroe shocks. Dellner was able to utilize one of its shocks from the European high speed rail system to meet the requirements stipulated by the VSS design. The United Space Alliance design team estimates that the Monroe shock-absorber solution saved the project several hundred thousand dollars and reduced the VSS implementation time by 32 weeks.

Of course it's one thing to come up with a bright idea, quite another to make it a reality. Once Lenard's Monroe shock absorber solution was accepted, actual fabrication was turned over to Monroe’s Dellner division in Sweden. The team plans to test the VSS in component form before installing it on the launch tower this summer.

0 Response to "Rocking and Rolling on the Launch Pad"