Gas Strut Separation Alternative for Ares I  Brian Floyd* and James Owens  Abstract     This paper presents a design alternative and the rationale for a stage separation system based on Metering Adiabatic Gas Struts (MAG Struts) for the Ares 1 launch vehicle. The MAG Strut separation system was proposed as an alternative to the current Ares 1 separation system, which relies on small solid rocket motors to provide the main separation force. This paper will describe technical issues that were addressed during the trade study and present a conceptual design of the strut system that best resolved the issues. Needed development testing and programmatic considerations will be addressed as  part of the paper.  Introduction 

Gas struts show promise as an efficient way to prov ide the separation force for launch vehicle staging.  Strut systems are currently in use on a number of vehicles, but so far all have been unmanned. Several factors make the MAG Strut system unique. The struts are entirely self-contained. They are themselves pressure vessels, which are pre-charged with gas prior to launch. They require no additional actuation, but simply act as springs when the physical connection between stages is severed. Due to the mass  properties of the separating sta ges, this system provides excellent no zzle clearance during fly-out in offnominal conditions. Consequently, safety and mission success objectives are enhanced. Since the struts are light weight relative to other separation systems capable of applying the same force, the separation timing can be adjusted to separate earlier during the assent trajectory, increasing payload lift capability. The proposed struts apply the separation force smoothly du ring release in order to minimize disturbance  of the Upper Stage propellant and reduce the buckling loads applied to the upper stage aft skirt. The trade study also predicts significantly lower life-cycle-cost. Since the MAG Strut system is not in flight operation on any launch vehicle, development testing and system-qualification introduce some risk into the Ares program, which is a barrier to adopting the system.   Background   The Ares I launch vehicle will lift the Orion crew vehicle to low-earth orbit for manned missions to the International Space Station and to the moon. Ares I consists of two stages. The first stage is a modified Space Shuttle Solid Rocket Booster (SRB) with 5 solid motor segments instead of the 4 segments currently used for shuttle. The Ares I upper stage is a LOx / LH2 stage powered by a J-2X engine. The stages are connected by a cylindrical interstage and a conical frustum. The J-2X engine is housed in the compartment formed by the interstage and frustum.  

Figure 1 - Ares I   In the current flight trajectory baseline, the first stage ascent phase ends when the first stage reaches  178 kN of residual thrust. Eight Booster Deceleration Motors (BDMs) fire to push the first stage aft. Eight Ullage Settling Motors (USMs) thrust forward to maintain positive acceleration on the upper stage. Once  * NASA Marshall Space Flight Center, Huntsville, AL 

Proceedings of the 39th Aerospace Mechanisms Symposium, NASA Marshall Space Flight Center, May 7-9, 2008  331