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The Planetary Society BlogBy Emily LakdawallaHit Me With Your Rocket StickOct. 6, 2006 | 11:57 PDT | 18:57 UTC
by Doug Ellison
Tom Williams was first up and talked about the first stage -- the upgraded solid rocket motor called the RSRMV, Reusable Solid Rocket Motor V, the V standing for the five segments it has instead of four. He highlighted the changes from the current Space Shuttle Solid motor. One would automatically think that the extra segment would be at the top (well, I did) but of course, the top contains all the avionics and the hardware to bring the stage back down to the ground, such as the drogue chutes and the main chutes etc. The new segment is actually the middle one. Getting a thrust profile (i.e., a variation in the amount of thrust that rocket produces over time) is fairly easy with liquid fuel rockets -- you just open the taps a little more, or close them off a little more. A solid motor just burns though, so what you have to do is change the shape of the solid propellant inside the motor. It is ignited at the top and burns from the top to the bottom. The four segment version has 11 fins in the top segment, giving a large surface area and thus a lot of thrust as a lot of propellant burns very quickly. The five segment design will have 12 fins for even more thrust. To allow for more thrust, the nozzle at the aft end will had a slightly larger throat to stop the exhaust choking inside the motor. Not long into flight however, the vehicle will be going very quickly, with still a reasonable atmosphere, and there is a point where the dynamic pressure reaches the limit set by the designers (aviators would know this as indicated air speed, but it's the point where the vehicle is getting the most force from the air it's travelling through). That limit is 800 psf (pounds per square foot), and the rocket engineers want to get as close as they can to that figure to get the most performance from the vehicle without exceeding it and stressing the vehicle too much. To stop then exceeding this limit the middle part of the motor is designed to burn a little slower and provide a little less thrust. Once past this point, the limiting factor is the squishy organic matter at the top of the rocket -- the astronauts on board. They don't want the vehicle to exceed 3.8 G of acceleration, but by this point, the rocket has burned a lot of its propellant and is thus quite a lot lighter than when it was sitting on the pad. If it still had the high thrust from the beginning of the launch, then it would exceed that 3.8 G figure and be very uncomfortable for the crew. To combat this, the fuel burns slower, producing less and less thrust all the way to the point at which it burns out. At this point, I feel the need to apologise for the series of NASA engineers who used feet, pounds, and inches all morning. At a major international engineering orientated conference, I would consider that more than a little embarrassing, but I am just quoting the figures that we have been given. You would think that a new vehicle would be the perfect opportunity for the engineers to bring themselves kicking and screaming into the 21st century, but it seems they want to stick to these so called 'English' Units (which no English engineer at this conference would ever dream of using today). I'm trusting that you're a bright bunch and can figure these numbers out for yourselves. As a guide, Space Ship One reached an altitude of just over 100 km and as its registration number (N328KF) suggests -- 100 km is about 328,000 ft. At separation, the vehicle will be at 194,300 ft some 130 seconds after launch and travelling at 6,640 ft/sec. They then have to get all that energy of a fairly hefty upper stage at a high altitude and a high speed to land gently on the ocean below. This process will start with the firing of some tumble motors designed to introduce the same sort of tumbling that the current motors experience by virtue of the fact that their separation motors fire sideways. 12 seconds later, already at 232,000 ft, the motor will have rotated 180 degrees (facing backwards) and the frustum and interstage that connected the first stage to the second stage will be separated. Travelling under its own kinetic energy, the stage will reach a height of 325,000 ft (just shy of 100km) and then settle into an oscillating plummet to a height of 15,000 ft where the thick atmosphere will have slowed it to 638 ft/sec and the small pilot chute will fire. By 4,400 ft and travelling under the drogue chute, it will be doing only 350 ft/sec and the impact, 7 mins 45 sec after ignition, will be at only 69 ft/sec -- slower than current shuttle SRBs but a speed that carries the same total kinetic energy of the current SRB impact. Lawrence D. Huebner has been working as part of a team looking at the aerodynamics during launch, the aerodynamic loads on the first stage during decent, and the aerodynamic loads on the upper stage during its entry and break-up. So far they have run more than 1500 wind tunnel tests with seven different models in four different facilities across the U.S., and also lots of CFD (Computation Fluid Dynamics) simulations to calculate measurements that would be hard to take via traditional methods. They have been trying to establish the aerodynamics of the vehicle in the Mach 0.5 to Mach 5 range.
He presented charts that compared the wind tunnel data to the CFD data, and the three different CFD codes they have been using to one another. They all matched exceedingly well, so they have high confidence in the fidelity of the CFD simulations. One anomaly was detected in some wind-tunnel models however. The Launch Abort System sits above the Orion vehicle and its motors stand proud on the mast on which they sit. Behind this component and in front of the Crew module, they noticed a flickering in some wind-tunnel images of "flow popping" -- a collapsing and expanding shockwave at about Mach 1.6. This only occurred at an angle of incident to the oncoming air of a few degrees, and it is unclear if it would occur at full scale (these models were between 1 and 1.5% scale) but they have been looking at the issue for the last few months and a redesign of the LAS geometry may be required. Presenting on behalf of an absent author (Jim Snoddy), Huebner then talked about the J2-X motor that will be used on the upper stage and also the Earth departure stage for lunar missions.
This whole stack need to have the ability to "roll" to get on its proper trajectory, and the first stage doesn't have that capacity so it is being included within the second stage. The original J2 was from the 1960s as part of the upper stages for the Saturn 1B and Saturn V and has been iterated just about continuously since then with various components including turbo machinery from the development of the X-33 linear aerospike engine. The requirements for the modern derivative have moved on. It has to be able to loiter on orbit for three months in case of technical or weather delays when waiting for the second half of the 1.5 launch mission design. At first, there were plans to use a space shuttle main engine for this upper stage but for reasons of in-flight restart, the J2-X was seen as a better option. Huebner said that the upper stage engine team have gone and visited "Apollo-Era 'grey beards'" to learn about the design challenges from back in the 60s. Two versions of the J-2X will be developed -- a 294,000 lb thrust version and a J-2XD 274,000 lb thrust version for risk reduction. A first fire of a full up test engine is expected in 2010. Now time for Doug's final thoughts: NASA did something good by funding SpaceX and Rocketplane Kistler for development of launch vehicles and cargo/crew carriers to the ISS in the COTS program (and I'm not saying that just because I bumped into someone from RPK at a Tapas bar last night). I look at this great big launch vehicle, the huge amount of work going into clinging with a white-knuckle grip on to Shuttle heritage hardware -- hardware which will be more than 30 years old come the first launch of Ares 1, I see all this work going into a new upper stage being developed, lengthy and expensive development processes, a very expensive series of flight tests, and I can't help thinking...wouldn't it just be easier to use a human rated heavy variant Atlas V or Delta IV? Is NASA going about the VSE in the best possible way or in the way that pleases the most politicians?
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