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Doug Stanley
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« Reply #4 on: 01/14/2007 12:21 AM » |
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I will also take this opportunity to address on the record some of the alleged "issues" with the ARES 1 vehicle from "anonymous sources" that have been discussed in this forum and certain NASA-Related-Personal-Axe-to-Grind-Single-Source-is-Good-Enough-Blog sites.
An entire section of the Direct proposal is devoted to alleged "Flaws With the Ares Launch Vehicle Family". Very little that was written in the section concerning "problems" with the current Ares Program is correct. The premise that the current (or original ESAS) Ares 1 approach is "broken" and needs to be "fixed" by something like what is being proposing is simply not correct! I will attempt to address some of them in this section for the record in one place. All of the data in these responses come directly from the knowledgable NASA people in the responsible engineering or program office...
DIRECT Assertion: “The original design, of 4-segment SRB with Space Shuttle Main Engine Upper Stage, would probably have lived up to expectations - if the SSME could have been air-started. It can not. NASA is left with a compromise which attempts to fulfill the same requirements, but which fails to.”
NASA Response: This is not true. NASA was confident in its plan to air-start the SSME and no showstoppers were identified at the time NASA elected to change the Ares I baseline. NASA switched to the 5 segment/J-2X approach to achieve greater commonality with the Ares V, reducing the number of developments required - resulting in significant development and recurring costs savings (billions). This included moving from 2 SRB’s (4 and 5 segment) to one (5 segment), 2 upperstage engine developments (alt start SSME and J-2X) to one (J-2X), and moving to a low cost, commercially developed core stage engine flying on the Delta IV today (RS-68) vs. an engine unique to NASA needs (SSME derivative).
DIRECT Assertion: “The “Stick” Crew LV's biggest selling point was its high safety figures. However, the difficulties the design is suffering from today are continually whittling those away, with each 'fix' causing ever larger penalties to the performance.
NASA Response: This is not true. NASA currently projects a loss of crew of 1 in 2,150 - a robust vehicle when compared to STS and with any other alternatives evaluated and consistent with ESAS projections.
DIRECT Assertion: “The new 5-segment SRB's and J-2X engines are both completely unproven.”
NASA Response: This is not true. A 5 segment ground test motor was fired in October, 2003. The J-2X is a derivative of the Saturn J-2 and J-2S engines, elements of which (turbopumps) were recently utilized on the X-33.
DIRECT Assertion: “Together their performance is so desperately low that other parts of the vehicle are having to be designed down to dangerously minimal weight, in order just to get the system to fly at all.” Performance of just 22mT -30x100nm 28.5deg is at best, mediocre, at worst, anæmic. This poor performance is causing detrimental domino effect throughout every phase of the development of the new vehicle.”
NASA Response: This is not true. The original ESAS baseline CLV delivered ~27mT (without performance margin) to LEO/28.5°. This was with a much lighter launch abort system and before wind tunnel data was available. Using much more detailed models, the current Ares I is projected to deliver ~26mT (without performance margin) to LEO/28.5° - equivalent to ESAS. Orion is being designed to weigh no more than 22mT (in ESAS, this was 23mT, but was a 5.5m diameter capsule). Ares I will be the largest heavy lift capability in the U.S. until Ares V is developed.
DIRECT Assertion: “A normal rocket is naturally stabilized throughout its flight by having the Center of Gravity (CofG) ahead of the Center of Pressure (CofP). Like a thrown dart, the rocket will naturally fly nose-first. But the Ares-I's CofG is behind the CofP - which causes the rocket to want to flip around in mid-air. Only with very precisely applied Thrust Vector Control, can the rocket be kept on track without applying very high stress loads to the structure. The first stage has a very slow Thrust Vectoring system, simply because it is a Solid Rocket Booster. This is causing concern during the first minute after launch, before speed builds and aerodynamics affect the ascent. It is the job of the SRB's Thrust Vectoring system to keep the very tall and ungainly rocket stable and pointing in the right direction as it lifts from the Pad. It is a problem often equated to balancing a pencil, on end, using your finger. The nozzle at the bottom of the SRB is proving to be a very slow ‘finger’ performing the balancing act. If the rocket becomes unbalanced, perhaps due to crosswinds, the nozzle may be too slow, and be forced to apply very high bending moment forces on the structure in order to try to re-stabilize.”
NASA Response: This is not true and shows a lack of understanding of large rocket design. Typically, large, orbital capable rockets have a C.G. aft of the C.P., hence you utilize a TVC system. NASA has conducted over 1,500 wind tunnel tests of the Ares I configuration, and conducted analyses on the flight control system design. While Ares is a long and slender vehicle, it is within the control dynamics experience base of previous programs, most notably the Saturn V. 6DOF simulation results indicate a ~2x margin on first stage thrust vector control (angle and rate) and an ~8x margin on the vehicle structural response to control frequency ratio.
DIRECT Assertion: “The two issues above can cause forces which, quite literally, try to bend the vehicle in half. The SRB is a very strong structure. The pressurized Upper Stage tanking is also a very strong structure. But the Interstage between them is a hollow cylinder, 18ft (5.5m) wide, and 40ft (12m) long, with walls only 1.25" (3cm) thick - and complicated further by a conical structure changing diameter from 13ft (3.9m) to 18ft (5.5m). The Interstage will be the “weak point” if the vehicle suffers instability issues during flight. It is the structure which would fail first if the rocket goes off-course and takes too much time to be forced back on course. The Ares-I test vehicles’ Interstages are being specifically over-built to combat this problem in a bid to dissuade disparaging comment from the space community, who is already well aware of this concern. But the final flight versions of Ares-I must be built down to the lowest possible weight limits in order to keep performance high enough - which means this will be the weakest structural point in the final design. The SRB first stage is currently 18,000lb overweight because the seals around all of the segments need additional, unplanned, strengthening. This is because the in-line design, with the stage and payload located above the booster instead of beside it, are experiencing different loads during flight from the SRB’s intended design - so require additional strengthening at these joints to compensate.”
NASA Response: This is not true. While the Shuttle RSRM was not originally designed to have a second stage ride atop it on the way to orbit, this is a very robust stage which carries the entire load of the Shuttle External Tank and offset load of the Shuttle Orbiter. In addition, it was sized to carry the offset load of 3 Space Shuttle Main Engines firing at ignition (“twang load”) which Ares will not have due to its single engine, in-line first stage configuration. The NASA team is using proven, validated engineering tools and loads models and conservative margin factors at this stage of the design. Analyses performed to-date indicates that the existing Shuttle RSRM cases, joints and aft skirt have sufficient design capability to support the Ares in-line configuration and are not “overweight” as characterized above. In addition, the upperstage and interstage are being designed for the loads expected on the ground and in-flight. The upcoming Ares I-1 flight test in 2009 will give NASA important data early in the development cycle.
DIRECT Assertion: “The roll-control system was not predicted to be as considerable an issue as it is proving to be. It requires an extra system which was unplanned originally, which impacts the weight of the vehicle, and increases the number of systems which can cause an expensive Loss of Mission or, worst of all, a Loss of Crew contingency.”
NASA Response: This is not true. Characterizing and controlling roll torque has been a high priority since Ares’ inception. NASA has utilized what it believes are worst case roll torque predictions and then designed the control system to handle 1.7 times that torque using RCS thrusters. Our goal now is to further refine the roll torque predictions through ground test firings of the motors with calibrated sensors, analyzing similar launch systems (Athena, for example) and the Ares I-1 flight test in 2009. We believe we have utilized very conservative predictions and then used a conservative design approach.
DIRECT Assertion: “The original “Stick” launcher utilized the Upper Stage to reach an initial elliptical orbit of 60x160nm, then that Upper Stage to then perform the Circularization burn to achieve the stable 160x160nm orbit. The Orion is now required to perform a 1000ft/s high-Delta-V burn to reach an initial orbit of just -30x100nm - that means the low-point is 30 nautical miles under the Earth’s surface.”
NASA Response: This is not true. In ESAS and until last Spring, the Ares I injected the Orion into a 30x160nmi transfer orbit and the Orion then circularized itself, to avoid the complexity of deorbiting the large upperstage. Working with Constellation and CEV project teams, the program elected to change to a -30x100nmi orbit to move the ocean impact of the CLV upperstage to the Indian Ocean from the South Pacific to stay away from populated islands. This also allowed the impact point for both ISS and lunar missions to be in the same general vicinity. Appropriate performance was transferred from Ares to Orion so that the spacecraft was not penalized. Performing multiple OMS types burns is commonplace on STS today and does not increase risk. Also, Orion does not have to do a burn to reach -30x100nmi - Ares places it in that orbit. Orion carries 1,000 ft/s to perform all orbital maneuvers, including transferring from -30X100 to 160 circ, and on to 220 for ISS, rendezvous, prox ops and docking, and deorbit.
DIRECT Assertion: “Together, this reduces the original “Stick” concepts Loss of Crew (LOC) figures below the stated 1 in 1918. The Ares-I’s fundamental design requires that the Upper Stage engine be ignited at altitude, only after the SRB First Stage has burned-out. There is no guarantee that any engine will start correctly, or safely, let alone at altitude. If there is a problem, the mission would become an abort, requiring the use of the escape system. NASA has yet to publish new, independent, ‘apples-to-apples’ comparison safety figures between the original CLV and the current evolution. The figures will obviously be lower today. Loss of Crew (LOC) safety figures of between 1 in 1500 to 1600 are rumored for the current risk factor as this paper was compiled - so the gap to DIRECT’s 1 in 1355 LOC risk is now very narrow indeed.
NASA Response: This is not true. The current Ares probabilistic risk assessment, which is much more comprehensive than the PRA used in ESAS is predicting a loss of loss of crew of 1 in 2,150 (mean). This is ~1.6x the DIRECT claim of 1 in 1,355 LOC (which is not supported with any analysis - the best ESAS vehicle in this “direct” class had an LOC of 1 in 1,170).
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