RS-68 Engines Considered for the (not so SD)HLV

[john_316]

Well unless something new and radical comes along and is tested at the manufactures expense then we have to rely on either the RS-25 or the RS-68 engines.<br /><br />I mean if I had a billion to use I'd test a myriad of motors but I dont have a billion dollars or for that matter a rocket company to design and build them.<br /><br /><img src="/images/icons/smile.gif" /><br /><br />

 

[mattblack]

Actually, I'm wrong: I looked at the ESAS documents again and found that the 5-segments actually have 2 seconds LESS ISP than the 4-Segments, but the performance increase comes from their higher thrust and longer burntime. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[gunsandrockets]

Perhaps I wasn't clear. The main point I was trying to make was that the RS-68 is perfectly suited for the SDHLV. <br /><br />Even though the RS-68 has less ISP than the SSME, it's higher thrust can compensate. An RS-68 powered HLV could have a larger liquid-propellant-core-stage and smaller SRBs. I believe that would make an RS-68 version of the HLV have less GLOW, the same liftoff thrust, and the same payload as the original ESAS plan HLV which is powered by SSME and five-segment SRB.<br /><br />My secondary point was that since the RS-68 and existing four-segment SRB are adequate for the HLV job, it isn't even neccessary to develop the five-segment SRB. (As to whether the five-segment SRB is neccessary for the CLV, that is another story)

 

[propforce]

<font color="yellow"><i>"As to whether the five-segment SRB is neccessary for the CLV, that is another story) " </i><br /><br />It is. </font><br /><br />Because of the take off T/W needed? <div class="Discussion_UserSignature"> </div>

 

[mattblack]

The 4-segment SRBs are NOT adequate for the HLV. If you use the 4-segments with the RS-68s, the HLV will lift not much more than 91 tons (nett) to LEO. The 5-segment, 5-SSME design (LV-27.3) is optimised for 125 tons to LEO, which is why it was chosen. Using the less capable solid booster would only save you a few million bucks for TONS less capability. Why would you want to do that? Also, Shuttle_guy is right about the 5-segment being necessary for the CLV: using the J-2S for the upper stage instead of the more powerful SSME (RS-25) makes it compulsory to use the 5-segment. Nasa knows that getting the 5-segment SRM and J-2S into production saves development money on the CaLV. Not going with the air-startable RS-25(e) saves tens if not hundreds of millions of dollars developing it. This is a decison I agree with. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[vulture2]

>>Throwing away 15-18 RS-68s with five to six three CBC Delta IV heavies is more expensive in the long run. <br /><br />The majority of long-run costs for any launch vehicle except those with the highest flight rates are due to fixed overhead and development amortization. This is the main reason the Shuttle is so expensive.

 

[gunsandrockets]

"The 4-segment SRBs are NOT adequate for the HLV. If you use the 4-segments with the RS-68s, the HLV will lift not much more than 91 tons (nett) to LEO."<br /><br />Ah, now I see. The problem wasn't the clarity of what I posted, the problem was you didn't pay attention to what I wrote, even though I posted the same information twice.<br /><br />The vehicle I propose does not resemble the 91 tonne payload example you linked to.<br /><br />What I propose is a Shuttle-derived in-line vehicle almost exactly like the final ESAS SDHLV proposal. The ESAS SDHLV uses 2 x 5-segment SRB and 5 x SSME. My proposal uses 2 x 4-segment SRB and 5 x RS-68. (That's five, count 'em again, FIVE RS-68.)<br /><br />The point of my proposed vehicle is to provide the same payload performance as the ESAS vehicle, but with reduced development and operational costs. With my vehicle no expensive SSME are needed, no new 5-segment SRB are needed.<br /><br />My vehicle would have the same liftoff thrust as the ESAS vehicle, because even though the SRB on my vehicle are smaller, the core-stage liquid propellent engines are more powerful. The essential difference between my vehicle and the ESAS vehicle is that mine trades some of the solid-propellent-rocket thrust and propellant-mass, for liquid-propellent-rocket thrust and propellent-mass.

 

[gunsandrockets]

"No, for the extra payload capability."<br /><br />It was not my intention to get into CLV issues in this thread, but since you want to...<br /><br />Yes, of course the 5 segment SRB is neccessary for the CLV. That is driven by by both the use of the J-2S in the second stage and the payload requirement for the CLV.<br /><br />I would point out though, that the payload requirement for the CLV is driven by an oversized CEV. There is no reason why the CEV must mass 23 or more tonnes to do all the jobs that the ESAS plan details (even NASA has already shrunk the original CEV from a 5.5m diameter capsule to one of 5m diameter) If the CEV was smaller the payload requirement of the CLV could be smaller too, small enough to the point that a 4 segment SRB would suffice for the CLV. (hence my previous comment that the neccessity of a 5 segment SRB for the CLV was another story)

 

[mattblack]

According to Dr Doug Stanley, shrinking the CEV to 5 meters will save (conservatively) 1500 pounds in structural weight. The real proof of the pudding will be the choice of the CEV's main engine propellant and whether the Service Module will have 1 main engine or 2. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[gunsandrockets]

"According to Dr Doug Stanley, shrinking the CEV to 5 meters will save (conservatively) 1500 pounds in structural weight. "<br /><br />Hopefully that estimate is very very conservative, and refers just to the Crew Module, since the complete CEV savings should be much greater. The ESAS report gives the mass of the 5.5m CM as 9,506 kg. 1,500 lbs is only 7% reduction in weight, that's not much savings for a diameter reduction of 9%. But even a savings as little as 7% would mean 1.6 tonnes saved from the original 23 tonne mass of the CEV, since the Service Module could be smaller too. <br /><br /><br />Mission mass savings snowball rapidly just from shaving down the mass of the Crew Module. The launch escape system can be smaller, the service module can be smaller, the crew launch vehicle can be smaller, the earth departure stage can be smaller, etc. <br /><br />"The real proof of the pudding will be the choice of the CEV's main engine propellant and whether the Service Module will have 1 main engine or 2."<br /><br />Alternatively to shrinking mission mass, with a smaller CM other mission components could afford to employ less efficient propulsion without increasing overall mission mass. The crew launch vehicle could use the J-2S instead of the SSME, or the CEV service module could use storables instead of LOX/CH4.<br /> <br /><br />If the CM could shrink from the original mass of 9.5 tonnes to a mass of 8 tonnes, than the total mass of the CEV could be as little as 19.5 tonnes. Not only would that help solve the current SRB and upperstage engine selection for the CLV, it could potentially open the field to many other different existing launch vehicles which would then have adequate payload capacity to launch the CEV. Obviously NASA wants it's own vehicle for launching the CEV, but it would be nice if other launchers were also available just in case.

 

[mattblack]

I think the CEV mass margin they should aim for is 21 metric tons with LOX/Ethanol (my preference) or LOX/Kerosene, which would allow for reasonable margins for growth. Still much better than the Apollo 17 CSM figure of 29 tons, bearing in mind that the Apollo CSM had to do LOI, plane-change and TEI burns. CEV will only be doing a plane-change and the TEI burn. I really, really hope they don't go for hypergolics, unless they use some sort of Shuttle OMS or Apollo-derived engine for the CEV. And even if they do go for storables, they'd be struggling to bring the weight under 20 tons. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[mattblack]

FACT NUMBER #1: The RS-68 has 7% percent inferiority over the SSME in ISP.<br /><br />FACT NUMBER #2: An RS-68 is 86.5 % percent heavier than an SSME.<br /><br />FACT NUMBER #3: The RS-68 produces 74% percent more thrust than an SSME. GOOD!! However, add that fact to the lower ISP and you get a THIRSTY Mutha needing immense propellant tanks for a burn lasting the 6 or more minutes needed for a corestage. Also, the RS-68 nozzle expansion ratio is not optimized for the upper atmosphere like an RS-25. So it would be preferable to upgrade the RS-68 with a regeneratively-cooled and reshaped nozzle, plus improvements to the powerhead, pumps etc. (Expensive upgrades, most probably costing more than a disposable RS-25 program). <br /> <br />FACT NUMBER #4: There is a limit to the acceleration and dynamic loads you could place upon such a high-thrust launcher -- 5x RS-68+2x 5-segment SRBs = 10.3 million pounds takeoff thrust. Thrust aint everything: Fuel efficiency and Thrust-Over-Weight ratio IS!! Otherwise, you might as well use Kerosene fuelled RD-180s or RD-170s, or F-1As.<br /><br />Conclusion? You soon reach a point-of-no-return with development costs, infrastructure modification, structural weight etc, and then you start to throw out the baby with the bathwater and get a CLEAN SHEET design. And all the design studies done by Mike Griffin and his experts get tossed on the scrapheap. They are GOOD designs, for the most part. My money is firmly on Mike Griffin’s credibility and knowledge, not so much on yours or mine.<br /><br />Now all that wouldn’t matter if you threw away the E.T. tooling and devised a new corestage with a 35-foot diameter so as to have plenty of propellants for those awesome, but fuel-guzzling RS-68s. But with a fuel load and WIDE corestage structure adequate for 5x RS-68s, your GLOW would very likely creep over 7 million pounds, negating most of the benefits of increased thrust. Otherwise, to keep the ET diameter corestage at 27.5 feet and still retain enough <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[rocketman5000]

very nice visual

 

[mattblack]

Thanks! Amazing what a gluestick and scissors can do. I based the length on an 8% percent stretch of the corestage for the 4x RS-68 fuel load, pushing the CaLV height to more than 380 feet. Any higher and I'd bet you'd have a problem with the VAB doors and roof with the boosters' Service Tower. I estimate the resulting booster would get you 114 tons to LEO with standard RS-68s, and probably 120 with upgraded engines optimised for efficiency rather than thrust. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[gunsandrockets]

"Besides, only 4x RS-68s could fit on an ET-derived corestage, not 5, as there would be no room."<br /><br />I used to think that myself until SG pointed out that the RS-68 nozzle was the same size as the SSME nozzle.<br /><br />http://uplink.space.com/showthreaded.php?Cat=&Board=missions&Number=465750&page=&view=&sb=&o=<br /><br />"The RS-68 has 7% percent inferiority over the SSME in ISP." <br /><br />7% inferiority only in a vacuum. Much of the HLV fuel will be burned in higher pressure conditions than vacuum. At sea level the 362 ISP of the SSME is the same as the RS-68.<br /><br />http://www.boeing.com/defense-space/space/propul/RS68.html<br /><br />http://www.astronautix.com/engines/ssme.htm<br /><br />And don't forget that a cluster of five RS-68 allows for less reliance on SRB. So even though SSME have 7% better ISP in a vacuum than the RS-68, at sea level the RS-68 have 54% better ISP than the SRB.<br /><br />http://www.astronautix.com/stages/shulesrb.htm<br /><br />"There is a limit to the acceleration and dynamic loads you could place upon such a high-thrust launcher -- 5x RS-68+2x 5-segment SRBs = 10.3 million pounds takeoff thrust."<br /><br />And I point out (now for the fourth time!) I never proposed using the 5-segment SRB in combination with 5 x RS-68. My proposal uses the smaller lower thrust 4-segment SRB, balancing out the higher thrust of the 5 x RS-68, and thereby equalling the liftoff thrust of the original ESAS design for the shuttle derived heavy lift vehicle.<br /><br />It's all quite simple really.<br /><br />Five RS-68 can fit on the bottom of the HLV core stage

 

[gunsandrockets]

"No matter what prop is used they are struggling to keep the weight around 24 tons. There is no way to get to 20 with the current mission requirements."<br /><br />Yes, such vital mission requirements as found on page 38 of the ESAS report executive summary, "The CEV was scaled to maximize vehicle size while maintaining adequate performance margins on the CLV." In other words they scaled the CEV up to fit the expected payload of the CLV, not scaled the CEV down to fit the requirements for ISS access, lunar missions and mars missions.<br /><br />The original 5.5m diameter, 9.5 tonne Crew Module is ridiculous. It has three times the volume of the Apollo CM! The 5 meter scaled down Crew Module isn't much better though a step in the right direction.<br /><br />The lunar mission should be the main driver of the Crew Module design. Under the 1.5 launch EOR-LOR plan there is no reason for the Crew Module to be as big as it is. During a two week mission the crew will either be inside the LSAM or the LSAM will be docked to the Crew Module for all but three days, making the (quite generous) space of the LSAM ascent module available to the crew. Even the old 3.9 meter diameter Apollo capsule moldline would be big enough for the proposed lunar mission.<br /><br />With an appropriately sized Crew Module of 8 tonnes or less, the mass of the CEV would shrink almost in proportion, since most of the mass of the service module is propellant. Such a shrinkage easily puts the mass of the CEV under 20 tonnes.

 

[mattblack]

>>And I point out (now for the fourth time!) I never proposed using the 5-segment SRB in combination with 5 x RS-68.<<<br /><br />(Sorry!) Yeah, but why not?! They have to develop it for the CLV, so why pass up it's capability and develop two parallel booster programs? Also, the RS-68 nozzle diameter is not the sole driver of size: you'd have it's length, different mounting/gimballing systems, diameter of prop lines and clearances compared to an RS-25. The RS-68 IS bigger, however way you slice it. I'll concede that the use of the RS-68 is now probably a done deal, though while it slices away some more 'pork' it also sacrifices some performance. However, I'd bet a dozen beers that the most you'll ever see on an ET-diameter corestage will be 4x RS-68s, as THAT quantity is the most I've ever seen on any booster trade studies using a Shuttle-derived tank as reference. And let's face it: trade studies are the most many "space geeks" like me(us) get to see.<br /><br />Also, regarding the CEV diameter: I largely agree with you, as in earlier posts I've said that the 5.5 meter diameter was a bit bloated. In fact, if you wanted to optimally size the Crew Module for 4 Astronauts in a row comfortably: 4.5 meters should be sufficient. But you've GOTTA have something bigger than an Apollo CM, which was always fairly cramped, especially for a crew of four on the longer coming missions. However, 5 meters is a good compromise for the future with 6 Astros: Four in a row and two more "stacked" maybe at their heads and/or feet. But I'm sure most folk agree the LSAM is the more important vehicle, mission-wise, to preserve the capability and size of. <div class="Discussion_UserSignature"> <p> </p><p>One Percent of Federal Funding For Space: America <strong><em><u>CAN</u></em></strong> Afford it!!  LEO is a <strong><em>Prison</em></strong> -- It's time for a <em><strong>JAILBREAK</strong></em>!!</p> </div>

 

[barrykirk]

I understand that their are a huge number of talented and good people working at Morton Thiokol, but I have to ask the question. What would be the implication of replacing the SRB's with side boosters using F-1 or F-1A engines?<br /><br />In other words, something approximating what NASA wanted to do originally with the shuttle.<br /><br />From what I understand, it is easier and cheaper to refuel a liquid rocket than a solid rocket.<br /><br />If the thrust is too low, what about using 4 or 6 strap on boosters?<br /><br />How easy or difficult would it be to stretch a booster tank if a longer burn time is required compared to adding a segment for a SRB?<br /><br />Just questions.

 

[nibb31]

Just a quick question: <br /><br />What kind of docking mechanism is the CEV going to have? APAS for the ISS missions? Something else for lunar missions? Will it be a mission-specific interchangeable adapter ?

 

[nibb31]

Thanks shuttle_guy. I suppose a new mechanism makes sense, especially for the LSAM missions later on. <br /><br />Does that mean that a new PMA-like adapter will have to be brought up by a shuttle before it's retired? Maybe attached to Node 2?

 

[nacnud]

They were looking at LIDS for docking but the need for a new PMA scuppered that plan. IIRC the spec reverted to the APAS, whether that has changed since the ESAS I dunno.