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Space Forum / Shuttle / July 2007Speaking of the J-2/J-2X
I'd like to hear some opinions from some the experts. How much is NASA gaining by re-using and updating the Saturn era J-2 to the J-2X for the Constellation Program? If one extreme was just using the original J-2 as is (which would involve moving the design drawings to a modern CAD system and recreating the necessary tooling) and the opposite extreme was doing a completely new design of similar characteristics, where would the proposed J-2X stand. Lets call the first option (using the old J-2 as is) a "0" and lets call doing a completely new design of a similar engine from scratch a "10". Where would the J-2X fall on this scale. I guess I'm curious has to how much sense this makes. I actually *really* like the idea of using a proven, pedigreed design, but I guess I'm afraid that after all the work is done to update the design and build the J-2X it is going to bear little in common with the original J-2 and hence the positive aspect of using a proven design may be lost. Looking at NASA versus the Soviet/Russian approaches to building launch vehicles and spacecraft I think one advantage that the Russians have had is they seem to have little inclination to re-invent new and improved hardware every generation or so. It's definitely not as sexy from an engineering point of view, but slow and conservative evolution of existing engines and spacecraft as opposed to throwing everything out the window and starting with a blank sheet of paper every 20 to 30 years does have some advantages. I guess NASA is attempting to do it the other way with the Constellation program by using Shuttle components but I'm not sure that they wouldn't have been better served picking other existing hardware (perhaps pieces of the EELVs) instead of using Shuttle technology which is relatively costly. What do you guys think? > I'd like to hear some opinions from some the experts. How much is NASA > gaining by re-using and updating the Saturn era J-2 to the J-2X for the [quoted text clipped - 27 lines] > > What do you guys think? My understanding is NASA is basing the current J-2 on a version that was built and tested but not flown before. I have read in two different places the turbo pump design was used in the X-33 project. I do know it is over twice the thrust of the J-2 used on Apollo and there was talk of going even higher to aid the performance problems both Ares 1 and Ares V are having. Maybe someone can confirm this. Danny Deger > My understanding is NASA is basing the current J-2 on a version that > was built and tested but not flown before. I have read in two [quoted text clipped - 4 lines] > > Danny Deger The version of the J-2 that was tested but not flown was the J-2S. It did not provide 2x the thrust of the J-2 but roughly an additional 30% increase (265,000lbs versus 200,000lbs). There was also a J-2T program as well but I don't think that was ever built and I believe that the thrust was in the same range as the J-2S. A 2x increase in thrust would be a noteworthy improvement in rocket technology but unfortunately I just don't see it happening any time soon (but I'd love to be proven wrong). JC >> My understanding is NASA is basing the current J-2 on a version that >> was built and tested but not flown before. I have read in two [quoted text clipped - 15 lines] > > JC Is thrust really the key to the Ares I/V performance questions, or is it ISP?  SignatureFear is the path to the dark side... Fear leads to anger... Anger leads to hate... Hate leads to banjos... Banjos lead to suffering! >> The version of the J-2 that was tested but not flown was the J-2S. It >> did not provide 2x the thrust of the J-2 but roughly an additional 30% [quoted text clipped - 7 lines] >Is thrust really the key to the Ares I/V performance questions, or is it >ISP? In a vehicle that has to make it off the surface of the Earth into orbit, thrust can be as important as specific impulse, because it reduces the necessary delta vee. In theory, a rocket whose thrust was exactly equal to its weight would have an infinite delta vee to orbit. One whose thrust was infinite would have a delta vee exactly equal to the theoretical minimum of eight kilometers per second or so. In practice, the speed change that has to be achieved is not infinite, but can be several hundred meters per second more than the minimum, severely affecting the payload capacity. A lower thrust could be partly compensated for by improving the Isp, or vice versa, of course. SignatureDel Cotter NB Personal replies to this post will send email to del@branta.demon.co.uk, which goes to a spam folder-- please send your email to del3 instead. >>> My understanding is NASA is basing the current J-2 on a version that >>> was built and tested but not flown before. I have read in two [quoted text clipped - 18 lines] > Is thrust really the key to the Ares I/V performance questions, or is it > ISP? Danny mentioned that he believe that the thrust of the J-2X was twice that of the J-2. Based on what I had read I did not believe that to be true and pointed that out. That said, specific impulse is generally a more important figure of merit than raw thrust. The numbers I have seen on the J-2 versus the J-2X (which admittedly is not a finalized design) put the specific impulse in the same ballpark. I'll look around and see if I can find some reliable numbers and if so I'll post them. Likewise, if anyone has a good source for specific impulse data on the J-2 and J-2X please post. My recollection is that both were in the 400-ish range which should line up pretty well with the SSMEs. If I am wrong please correct me. > Danny mentioned that he believe that the thrust of the J-2X was twice > that of the J-2. Based on what I had read I did not believe that to [quoted text clipped - 7 lines] > both were in the 400-ish range which should line up pretty well with > the SSMEs. If I am wrong please correct me. http://tinyurl.com/2lsr5j (news release on J-2X contract, with specs) http://en.wikipedia.org/wiki/J-2_%28rocket_engine%29 (general background on the J-2 family) J-2(original): Thrust: 232,000 lb/f Isp: 421 seconds J-2X: Thrust: 294,000 lb/f Isp: 448 seconds Those are target figures, especially for the Isp. Calling this engine a J-2(variant) hardly makes any sense. It's essentially a new engine from the bottom up with major design and manufacturing changes. Ought to be called a J-3... The J-2X on the Saturn V would have been kick-a.s. One of the S-II stage's five engines could have been entirely eliminated and still have improved performance. Pratt & Whitney had a planned 300,000 lb/f class expander cycle engine with probably even better Isp, but development would have been more expensive. That or a new staged-combustion engine would have been nice, but expensive. So we'll be going with an improved gas generator cycle engine. --Damon > J-2(original): > Thrust: 232,000 lb/f [quoted text clipped - 12 lines] > stage's five engines could have been entirely eliminated and still have > improved performance. Combine that with the upgrade F-1A and a 2nd or 3rd generation Saturn V would have kicked even more serious butt than the original did. And that would have been w/o upgrades like the SRBs and nuclear upper stage. Oh well. > Pratt & Whitney had a planned 300,000 lb/f class expander cycle engine > with probably even better Isp, but development would have been more [quoted text clipped - 3 lines] > > --Damon SignatureGreg Moore SQL Server DBA Consulting Remote and Onsite available! Email: sql (at) greenms.com http://www.greenms.com/sqlserver.html "Greg D. Moore \(Strider\)" <mooregr_deleteth1s@greenms.com> wrote: >Combine that with the upgrade F-1A and a 2nd or 3rd generation Saturn V >would have kicked even more serious butt than the original did. And that >would have been w/o upgrades like the SRBs and nuclear upper stage. And it _still_ would have been hideously expensive and had little demand. D. SignatureTouch-twice life. Eat. Drink. Laugh. http://derekl1963.livejournal.com/ -Resolved: To be more temperate in my postings. Oct 5th, 2004 JDL > "Greg D. Moore \(Strider\)" <mooregr_deleteth1s@greenms.com> wrote: > [quoted text clipped - 4 lines] > And it _still_ would have been hideously expensive and had little > demand. begin fanboy voice But Derek, it would have been so cool! end fanboy voice Upgraded Saturn V's look great on paper, but you're absolutely right, they all ignored the political and economic realities which meant that Saturn V production was capped at a very small number due to high costs (and an incredibly shrinking NASA budget). Jeff Signature"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety" - B. Franklin, Bartlett's Familiar Quotations (1919) >> "Greg D. Moore \(Strider\)" <mooregr_deleteth1s@greenms.com> wrote: >> [quoted text clipped - 10 lines] > >end fanboy voice I forgot to add the "But DAMM I'd have loved to see it fly" part! :) (Especially one of +SRB variants!) Even us cynical supporters still have a bit of fanboy in us. D. SignatureTouch-twice life. Eat. Drink. Laugh. http://derekl1963.livejournal.com/ -Resolved: To be more temperate in my postings. Oct 5th, 2004 JDL >>> "Greg D. Moore \(Strider\)" <mooregr_deleteth1s@greenms.com> wrote: >>> [quoted text clipped - 14 lines] > > Even us cynical supporters still have a bit of fanboy in us. It's nice to see you haven't gotten TOTALLY bitter in your old age, D. :-) (Says the guy who's personal domain is "AngryHerb.net" LOL!) It would've been something else, wouldn't it? Imagine probes sent on direct-trajectories to outer planets rather than requiring nearly-decades long gravity-assist trajectories, landing four Vikings at a time (*) on Mars, a space station made up of SIV-sized modules plugged into one another like Legos . . . the mind boggles. (*) Yeah, I'm just talking out of my a.s here - it's MY daydream, don't spoil it with facts! > D. SignatureFear is the path to the dark side... Fear leads to anger... Anger leads to hate... Hate leads to banjos... Banjos lead to suffering! >>> begin fanboy voice >>> [quoted text clipped - 20 lines] > (*) Yeah, I'm just talking out of my a.s here - it's MY daydream, don't > spoil it with facts! Damn tooting!!! Ever watch Mythbusters, when they blow up the concrete truck? Why... because they can. ;-) >> D. The sentence least likely to be heard in the English Language: "That's the banjo player's porsche." SignatureGreg Moore SQL Server DBA Consulting Remote and Onsite available! Email: sql (at) greenms.com http://www.greenms.com/sqlserver.html |
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