NASA evaluates Launch Abort System options for Orion

by Chris Bergin

A new NASA presentation has revealed a fascinating insight into Constellation’s evaluations of the Launch Abort System (LAS) that will be used to save the crew of Orion, in the event of a serious failure of the Ares I launch vehicle.

With alternative concepts, including a hand drawn Service Module Abort Motor concept by NASA head Mike Griffin, the presentation gives the most comprehensive overview of the system to date, including the option of using the LAS on every flight to assist the vehicle’s performance on ascent.
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NASA engineers have been looking at three LAS systems, namely the Multiple External (x4) Service Module (SM) Abort Motor concept, the Crew Module Strap On Motors (x4) concept, and the In-Line Tandem Tractor (Tower) concept – with the latter currently baselined into Ares I/Orion design.

As defined by NASA, the ‘LAS Design Key Driving Requirements’ tasked engineers with ensuring the risk of losing the crew in an abort scenario was no greater than 1-10. Not the greatest odds, but the system is self-described as the final option open to saving the crew – and highly favorable when compared to the Shuttle, which lacks any form of a realistic escape for the crew on a failing vehicle during ascent.

‘CEV Ascent and Pad Abort Reliability Allocation. The CEV shall provide a risk of loss of crew during a pad or ascent abort no greater than 1 in 10. Rationale: The abort situation is the final step in assuring crew survival,’ outlined the presentation.

‘It is imperative that the system be reliable. This reliability number includes survival of the initial event leading to the abort, the completion of the abort itself, and successful recovery of the crew. Current assumptions show that the LAS itself will be highly reliable and the most difficult part of meeting this requirement with be the initial survival of the abort-initiating event.’

The favored Tandem Tractor (Tower) LAS design comprises of a Nose Cone, Attitude Control Motor (Eight Nozzles), Canard Section (Stowed Configuration), Jettison Motor (Four Aft, Scarfed Nozzles), Interstage, Abort Motor (Four Exposed, Reverse Flow Nozzles), Adapter Cone, and Boost Protective Cover (BPC). This is the system that is favored in the overall Ares I baseline.

The primary role of the LAS is to save the crew during the ‘three stages’ of an abort, the first involving the firing of the Orion away from a failing vehicle on the launch pad to a safe distance, before deploying the parachutes on the Orion, for a landing in the Atlantic ocean within a 3450 ft radius due east of the launch pad.

‘LAS integrated impulse defined by Pad Abort: 30 g-s. Minimum altitude to ensure successful parachute deployment. Minimum downrange required to ensure that winds do not blow you towards the pad/populated areas. Reorientation executed quickly to ensure adequate time for chute deployment events’ noted the presentation, while noting the concern of ‘High heat, as from a pad fire, compromises integrity of the parachute system.’

While the Ares I pad will have an option of another form of escape, via the Rollercoaster Emergency Egress System (EES), such an option would be undesirable during a serious failure. The pad abort sequences are also included for a failure soon after lift-off, below 25,000 ft.

The second stage of an abort is noted as ‘mid altitude’ – which has a different characteristic when compared to pad abort. This stage of abort works for up to 150,000 ft, involving the LAS remaining on the vehicle after firing the Orion safely away from the failing vehicle until the point of drogue chute deployment, which become necessary at that altitude.

The final stage of abort, which would still require the LAS, is at an altitude of between 150,000 ft and 300,000 ft – the latter being the point the LAS would be jettisoned on a nominal flight.

‘Canards ineffective at high altitudes, reorientation performed using ACM (Attitude Control Motor),’ added the presentation on this abort scenario. ‘Reorientation begins after abort motor burnout with ACM.’

Interestingly, current evaluations are on-going as to the possibility of using the LAS even when an abort isn’t called. These evaluations are looking at using the LAS to assist Orion’s ascent targets.

This was touched upon in the presentation, which noted a normal firing of the LAS for this purpose would be detrimental, as the vehicle would require its structural strengthening, which would ultimately increase the weight past anything gained by the LAS firing.

‘LAS Abort Impulse used for Ascent Assist: Theoretically can increase mass to orbit by 1000 lb. However, additional tension loading on the Command Module requires additional structure that leads to overall decrease in mass to orbit.’

However, what is under evaluation is to use nozzle inserts in the LAS motors, which would reduce the thrust and thus structural loadings on the vehicle. This option would mitigate any concerns of Command Module (CM) mass penalties.

‘An Alternate Option Using Nozzle Inserts: Reduces Abort Motor Thrust, Increases burn time. Relieves Command Module Compressive load – no tension loads. Increases the Payload Mass-to-Orbit by ~650 lb,’ added the presentation.

New information this week noted that Constellation engineers are looking at refining the option yet further, from the 15G for two seconds experienced on a normal firing of the LAS, to 7G for three seconds, which is a further reduction on the presentation’s second option, increasing payload mass-to-orbit to 400lbs. This option is still being evaluated.

Also presented in the LAS document were the other options that NASA/Constellation evaluated as potential LAS concepts, which opened with the requirements of the system.

‘The CEV Launch Abort System shall provide an axial thrust, as measured at vacuum and 70 degrees F propellant mean bulk temperature conditions, of not less than 15 times the combined weight of the CM+LAS for a duration of not less than 2 seconds.

‘Rationale: The launch abort system needs to provide as much acceleration as is tolerable by the astronauts. For the pad abort case, the acceleration sizing must provide substantial margin in distance from an on-pad explosion. For other flight regimes, the LAS needs to provide equally substantial margins; however, these are very difficult to quantify.

‘A variable acceleration design with an early maximum acceleration provides more distance and allows an LAS to weigh about the same as a constant acceleration design. Further analysis is required to determine the desired thrust profile.

‘The CEV system shall be sized such that a pad abort/low altitude abort will achieve a water landing, with full LRS deployment and functionality, for the pad abort design winds.

‘Rationale: The LAS must ensure LAV abort trajectories will result in: 1) water landing to increase crew survivability, and 2) an altitude sufficient to ensure safe deployment of the LRS, and 3) sufficient range from thermal radiation sources (fireball) following a catastrophic CLV event.

‘During CEV abort where the LAS is used, the LAS thrust profile shall be sufficient to achieve positive separation for all abort conditions. Rationale: During the high drag abort, the LAS must provide sufficient thrust to overcome a thrusting CLV and all aerodynamic drag including: free stream, proximity effects, and jet plume interaction.’

Past the previously noted In-Line Tandem Tractor LAS, the Service Module Strap-on LAS option is a Multi-Stage LAS, which has four motors placed around the SM. It also includes a mini-tower on top of the Orion.

This option appears to have come directly out of the mind of NASA administrator Mike Griffin, with the presentation including a hand written sketch of the system, which is dated March 22, 2006.

The presentation also noted positives surrounding this concept, which included ascent assist potential – as under evaluation on the In-Line Tandem Tractor design – as a positive element of the system

‘Multi-Stage LAS has several desirable attributes: Additional Operation Options. Ascent Assist. Reduces ACM Requirements,’ noted the NESC Smart Buyer Side Mounted LAS Synopsis, included in the presentation – which added: ‘Four motors with fixed cant angle nozzles mounted on Service Module. Mini-tower with Attitude Control and Jettison Motor. No Boast Protective Cover (BPC).’

‘Pros: Operational Flexibilities. Potential to perform Ascent Assist. Without stressing the structure. Reduces ACM Requirements.

‘Cons: Reduced Mass-to-Orbit. Higher Atmospheric Drag. Higher Radial Structural Loads -> Increased Mass. Asymmetric Thrust Control Challenges. Increases CLV Negative Static margin. Greater Complexity (Separation).

Unlike the third option of the Crew Module Strap-on concept – which appears to have gotten no further than the drawing board – the Service Module Strap-on concept appears to have gone through wind tunnel tests, which also resulted in cementing the In-Line Tandem Tractor (Tower) LAS as the favored – and thus baselined – system for Orion.

‘Side Mounted Multi-Stage LAS Cons: SM LAS has Increased Atmospheric Drag. SM LAS reduces CLV longitudinal aerodynamic stability. Aerodynamic center of pressure moves forward during max Q. May require more CLV TVC and higher gimble rates. SM LAS induces larger CLV aerodynamic normal force loads.

‘Higher associated structural loading. Decreased Mass-to-Orbit. Canted Nozzles Induces Higher Structural Loads in Service Module. LAS Control Challenging due to Asymmetric Thrust. Motor-to-motor variability. Motor temperature differentials (sun/shade). Motor tail off. Greater Complexity (Separation).’

The presentation also reveals highly detailed drawings of the LAS system and related data, once again concentrating on the In Line Tandem Tractor (Tower) System, as it continues to solidify its position in the baseline for Ares I/Orion.

L2 Resources For Ares I, V and Constellation:
Orion Launch Abort System (LAS) overview presentation – Jan 16. Major changes to Ares I Upper Stage – expansive details and data. Ares I/Orion CxP 72031 Requirements Validation Matrix Information. Saturn Twang Test Video for use with Ares I-1R. CLV Umbilical Trade Matrix XLS.

Vehicle interfaces for the DAC 1C version of Orion Ares – Jan 3. Ares I-1R Test Flight Plan (full outline) Presentation. Ares I-1 timeline and modification expanded info. Ares I troubleshooting latest. Ares I Reference Trajectory. Boeing’s STS to Ares – Lessons Learned Presentation. Latest Ares I and Ares V baseline Configuration image and data. CLV DAC-1C (Changes to CLV Upper Stage).

Ares I-1: Four Seg+Dummy ‘Tuna Can’ stage. Ascent Developmental Flight Test Presentation. CLV Pad 39B Handover Info and Latest. New images of CLV on top of new MLP and LUT. Lockheed Martin CEV/Orion Updates. Constellation news updates. ATK figures on the 5-Seg Booster weight for CLV.

90 Minute Video of Constellation all hands meeting. CLV TIM Meeting Information. CLV/CaLV Infrastructure, Timelines and Information. Escape System Trade Study Presentation.

CEV-CLV Design Analysis Cycle Review (DAC-2) Presentation. Constellation SRR updates. CLV Stick – Troubleshooting/Alternatives/Updates. New CEV Images (include abort mode). Flight Design and Dynamics Division CEV update. CLV Mono-propellant RCS system. CEV pressurisation system review. CLV/CEV Configuration Images. The 2×3 Seg SRB Crew Launch Vehicle Option Presentation…plus more.

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