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Bender222

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why not design the ares v/ direct jupiter 232 to use 4 srbs iinstead of just 2. I read somewhere that the srbs provide 80 % of the total thrust onthe shuttle. the two srbs take up the same amount of space as the fuel for the SSMEs yet provide alot more thrust. Why use SSMEs at lift-off at all and just make the engines airstart anda start them after the srb seperation?
 
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mjennings

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Air start and ground start liquid engines some key differences. The Ares 1 was originally going to have an air start SSME, but converting the SSME to an air start was almost a 100% redesign thus the J-2X, but I won't get into that

As for the 4 SRB idea it would probably necessitate a complete rebuild of the MLP (Mobile Launch Platforms) instead of just a mod, and perhaps the crawler couldn't take the weight. Thus adding more cost. see the ares 1X thread http://www.rocketryforum.com/showthread.php?t=6773&highlight=ares there is some more details in it on the air start complications
 

Bender222

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of course the modifications would cost alot but it would seem like it would still be less than ares and provide the capability without the time gap
 

Zeus-cat

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One reason they use liquid fuel motors is that the thrust can be controlled throughout the flight. Once you light a solid fuel rocket it burns all it's fuel at the predetermined thrust curve. You can throttle up or throttle down liquid fuel motors at any time.

Pay attention during a shuttle launch and you can hear Ground Control tell the astronauts to throttle down as the shuttle aproaches Max Q (maximum aerodynamic pressure). After the shuttle passes through this point they are are instructed to throttle up to 104%. The ratings on the liquid fuel motors was increased at some point during the shuttle program and they are now allowed to run at 104% of the original max thrust. The shuttle carries different payloads and doesn't weigh the same for each launch. My guess is that they use the liquid fuel motors to compensate for this to some extent as the solid fuel motors are essentially dumb and will provide whatever thrust they provide.
 

luke strawwalker

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why not design the ares v/ direct jupiter 232 to use 4 srbs iinstead of just 2. I read somewhere that the srbs provide 80 % of the total thrust onthe shuttle. the two srbs take up the same amount of space as the fuel for the SSMEs yet provide alot more thrust. Why use SSMEs at lift-off at all and just make the engines airstart anda start them after the srb seperation?
Several reasons...

First, SRB's are ENORMOUSLY heavy on the launcher-- when you see a shuttle on the crawler heading for the pad, most of the weight is the SRB's alone- the ET is actually a featherweight despite it's size (it's unfuelled until ready for liftoff) and the orbiter is about 100 tons. SRB's must be moved FULLY FUELLED and that makes them mondo heavy. The shuttle stack weighs considerably more than a Saturn V did during the rollout, because the liquid fuelled Saturn V was just an empty shell during rollout. In fact, just adding a single segment to the SRB's for Ares V will overload the crawler and crawlerway so badly that a new six-truck crawler will have to be built, and the crawlerway may have to be dug out and strengthened/refurbished to support the enormous weight of the LUT and Ares V with it's two 5.5 segment SRB's. Going to FOUR SRB's (16 segments) would require an ENTIRELY NEW super-monster crawler that would make the present crawler look like a tinker-toy, assuming it would work at all. It's just TOO MUCH weight. Ares V is already stressing the capabilities of the VAB floor and concrete at the pad because it's going to be SO heavy. Basically at that point you'd do better to discard the VAB/crawler concept and go back to stacking the entire thing on the launch pad, like Titan III did it and the EELV's do it today, and Saturn IB did through Apollo 7 at Pad 34. That would negate having to move super-heavy SRB's, core stages, ET's, orbiters, and pads on top of the crawlers and remove the weight constraint for the move, since the segments could be hauled out individually and stacked at the pad, as the Titan III SRB's were.

Secondly, the 2 5.5 segment SRB's for the Ares V, firing with the RS-68 main engines for Ares V, is already close to the 11 million pound (IIRC) thrust limits that the flame trenches and pad can withstand. Adding another pair of SRB's would blow the pad all to heck at liftoff.

Thirdly, the SRB's are connected to the tank through a thrust beam that goes through the intertank of the ET, between the upper LOX tank and the lower LH2 tank. The SRB's transmit their thrust into the stack through two large connecting points at the top of this huge "I-beam" which absorbs the shocks of flight. It's not as simple as bolting two additional SRB's onto the tank, as the connection beam would have to be redesigned into an "X" shape to add two additional connection points, and it would have to be flight certified and tested (all of which is HUGE expense!)

Fourthly, using four SRB's would make for WAY too fast of a takeoff and VERY high G-forces and max-Q in flight, and the entire rocket would have to be redesigned and recertified to take this stress. Basically it would be no cheaper to just design a whole new rocket.

Fifthly, SSME's and RS-68's are NOT designed to be airstarted. Ares I was originally supposed to use an SSME for it's second stage, which had been modified for airstart capability. When PWR (Pratt and Whitney Rocketdyne, the SSME engine manufacturer) really got into the nitty-gritty of actually figuring out how to make it work, it was found to be a non-starter. The SSME would basically have had to been redesigned from the ground up to make it airstart. It's staged combustion cycle is VERY complicated to start, and requires a MASSIVE amount of GSE to get the engines started, all of which is EXTREMELY difficult to downsize and integrate into an air-startable system which can be made into flight hardware and fit onto the rocket. This would be EXTREMELY expensive to design, build, test, and certify, so the J-2S was chosen instead, later upgraded to the J-2X since basically the SSME would have been twice as powerful (the J-2S was approximately half the thrust of the SSME.) The RS-68 is not an air startable engine either, and owing to it's massive helium use at startup, isn't likely to ever be feasible. It too would require an almost complete redevelopment program to make an airstart engine. They basically have to be designed with that capability in mind from the get-go: Adding it later is a VERY expensive and time consuming and difficult job! Also, it's a lot easier with simpler combustion cycle engines like J-2 than with complicated staged combustion cycle engines like SSME, though the simpler cycle reduces the specific impulse (ISP) and therefore 'fuel mileage'-- the SSME's have VERY high ISP because of their complicated cycle, for engines as large as they are. That also makes them very expensive (that and reuse, which COULD be cut out and a simplification program applied to make them cheaper to produce for expendable rockets).

SO, basically, this is a TOTAL non-starter for all the reasons above and probably a lot of others I haven't thought of. This has been discussed NUMEROUS times over at www.nasaspaceflight.com/forums in various threads.

Later and hope this helps! OL JR :)
 
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