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JF Mezei wrote:
On 2017-05-14 17:45, Fred J. McCall wrote: If you do almost a total rebuild after every flight, things last a lot longer. But then the engines are not truly reusable. IF SSMEs *really* need a total rebuild between flights when operated commercially (not NASA), then they are worthless engines for today's environment. They're not as bad now as they originally were, but they still require substantial work after every flight. So the question becomes: if SSMEs/RS25 were operated by an outfit such as SpaceX, how much work would then need between flights ? It doesn't matter who operates them. The engines are what they are. They don't magically get better because they change owners. No, that's not why it 'won'. It 'won' because it costs less to OPERATE, not because it costs less to BUY. Exactly. My point was that just because SSME/RS25 may be more expensive to buy does not mean that overall cost of ownership has to be higher. Then you stated your point poorly, which I assume is why you removed your original statement about it having to do with cost of engines. **IF** the engines are truly re-usable, needing fewer to launch stuff may translate into competitive total cost of ownership due to lower maintenance. **IF** there is magic pixie dust dropping out of unicorn asses... Having said that: Sea level: Merlin: 845.2 kN 470kg SSME: 1859 kN 3500kg So to get the same thrust as 1 SSME, you need 2.199 Merlin engines which would weight 1,033kg instead of 3500kg. That is basically 2 tonnes of extra weight for each SSME engine's thrust. Would LH2/LOX fuel needed for each SSME engine running for 8 minutes end up being at least 2 tonnes lighter than equivalent Kerosene/LOX ? Thrust is proportional to mass out the ass, but ISP matters. If at the end of the day, the SSMEs end up being far heavier for amount of thrust they generate, then I guess it really is a no brainer that they have no place in modern rocketry. SSMEs have significantly higher ISP than a Merlin engine. That means they require less mass flow rate to get the same thrust. However, you have to worry about tank size and drag because of the lower density of LH2 as a fuel. Then there's the cost and manufacturing complexity. SpaceX can build 200+ Merlin engines a year. RS-25 takes a lot longer and making it really reusable with no rebuild required would be somewhere between really expensive and impossible, given how hard it pushes performance. I think Merlin engines cost less than $2 million each. A fully reusable RS-25 (let's say 10 flights with nothing but inspection and minor repair) is going to cost well upwards of $60 million each. Maintenance per flight will be some percentage of the cost of the engine, so SSME will always be significantly more expensive to fly. You could probably throw away a Merlin engine after a couple of flights and still be competitive with a 'reusable' RS-25. -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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JF Mezei wrote:
On 2017-05-15 04:48, Fred J. McCall wrote: SSMEs have significantly higher ISP than a Merlin engine. That means they require less mass flow rate to get the same thrust. How significant is the savings in fuel mass ? Since, for equal thrust, the SSME is 2 tonnes heavier than equivalent thrust on Merlins, does this mean that the LH2/LOX needed to accelerate to same speed at MECO would be at least 2 tonnes less per SSME engine ? Or put it this way: do SSMEs truly have a performance advantage over Merlins or are they just bigger engines? LH2/LOX engines like SSME have a performance advantage over RP-1/LOX engines. The Merlin has higher thrust/weight but the SSME can burn a lot longer for a given amount of fuel. Back to the 777/747 comparison: GE 90-115B: thrust: 513kN weight: 8,762kg * 2 = 1026kN at 17,524kg CF6-50 : thrust: 240kN weight: 4104kg * 4 = 960kN at 16,416 Interestingly, the thrust to weight ratio is the same for the twin engine vs quad. But maintenance costs and purchase cost (buying 2 more expensive instead of 4 less expensive) make the twin engine far more attractive. Again, that's not it. The purchase price of the jets has nothing to do with aircraft choices. It's all about operating costs and the 777 costs about 70% of what a 747 costs to operate. So, in an era where re-use becomes common, the maintenance cost between flights starts to matter if you need fewer engines to launch same payload. So, no. the big driver for aircraft operating costs is fuel consumption and the 777 is that much better than the 747. So what starts to matter now is whether SSMEs could ever become commercially competitive if it was unhindered by anything "NASA" and was actually re-usable with little turn around costs. In an alternate universe where unicorns **** magic pixie dust anything is possible. However, in the real world a higher performance engine like RS-25 is always going to require more maintenance than an engine that doesn't push performance so hard like the Merlin. They will cost more to build because they push performance so hard and they will cost more to maintain because that maintenance is going to be some percentage of the cost basis of the engine. For rockets the cost of fuel is almost irrelevant to the cost of operation, so trying to compare to aircraft is comparing apples and aardvarks. In my mind, just because NASA required major engine refurb between flights doesn't mean that it actually needed it. Don't be an idiot. They wouldn't have done it if it didn't need doing. If that's how things look in your mind, you're out of it. In the history of jet planes, it took quite a while for engine performance/reliability to allow the drop from 4 to 3 and then 2 engines on wide body jets. Apples and aardvarks. I can see it being easier for SpaceX to go with "many" smaller/simpler engines first just as early jets had many engines smaller engines. But that does not mean that eventually, fewer larger rocket engines will end up being more economical, just as it happened with jet aircraft. Apples and aardvarks. To this end, some real R&D effort to turn the SSMEs into truly re-usable low maintenance engines would be interesting especially if those engines offer performance advantage despite their heavier weight. And who do you think is going to pay for that? The entire life of the RS-25 engine program was about reducing maintenance requirements and they apparently have decided that that is a mug's game and are now going in the other direction to lower the cost of the engines. You just want to go that one step further and invoke magic and unobtainium. High performance engines like RS-25 will ALWAYS be 'less reusable' than lower performance engines like Merlin or Raptor. Just look at chamber pressures. RS-25 runs twice the chamber pressures of something like Merlin, with all that implies for pumps, wear, etc. -- "Some people get lost in thought because it's such unfamiliar territory." --G. Behn |
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ON May/15/2017 at 4:39 PM, Jeff Findley wrote :
In article om, says... On 2017-05-15 13:43, Jeff Findley wrote: stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid hydrogen is a p.i.t.a. to work with since it's deeply cryogenic. Is LH2 a show stopper argument for any modern engine? It's not a show stopper, but it's going to make for more expensive engines (bigger turbopump needed for its lower density), a more expensive stage (bigger tanks which need cryogenic insulation), and more expensive operations (due to its cryogenic nature). LH2 is best reserved for upper stages of launch vehicles which need a "high energy upper stage". For a lower stage, it's just not worth the added cost. Just to expand a little on that. It is good to have light fuel for upper stages. For lower stages, what is more important is to have high thrust. Once you are in low Earth orbit, if you want to escape Earth, you can use a low thrust engine and burn for a long time, so you don't need high thrust. If you aren't quite in LEO yet but are close to orbital speed, you can get away with not too much thrust and somewhat long burn times. If you are on the launch pad and you have a low thrust engine, you aren't going anywhere. You can have a high thrust LOX/LH2 engine, but it will cost you one way or another. It is much easier/cheaper to have a high thrust engine with a higher density fuel than LH2. And the added weight of the fuel is a much milder problem for the first stage because that weight will in large part be burnt before reaching too high a speed. For the first stage of a rocket, higher density fuels are clearly more favourable. For the second stage, SpaceX chose to stick with high density. I think that is a wise choice, but the advantage is much smaller. It isn't impossible that someone would someday develop technology for using LH2 that would make that more favourable for a second stage. Alain Fournier |
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JF Mezei wrote:
On 2017-05-15 13:43, Jeff Findley wrote: stage it's doubtful since SSMEs burn LOX/liquid hydrogen. Liquid hydrogen is a p.i.t.a. to work with since it's deeply cryogenic. Is LH2 a show stopper argument for any modern engine? For a first stage it isn't a great choice. It can make sense for upper stages. The reason I ask is that until the last couple of years, there has never been any reason for outfits such as Rocketdyne or Boeing to work to lower costs since NASA contnued to hand them pork money, and satellite launch market was served only by expensive rockets (with Soyuz starting to make a dent). Since then, SpaceX has been born and "old" companies would/should fear for their survival with their old expensive launch services. Most of them are and you will note that they're all moving away from LH2 first stages. So, in my mind, it is possible that the SSMEs are currently uncompetitive simply because Rocketdyne never really tried. So the question is whether it would be possible to turn SSMEs into something competive if they realised their life depeneded on it. You need to put some knowledge in your mind so you don't have space for such silly notions. The RS-25D is the most complex and sophisticated bleeding edge performance engine ever built by anyone anywhere. The fallout of that is high costs and high maintenance if you're going to reuse them. But if the use of LH2 precludes competitive low maintenance engines, then I guess there is no point in trying to make SSMEs competitive. Tankage, deeply cryogenic fuel, high chamber pressures (which means high wear on the engine and pumps) are all going to make RS-25 uncompetitive with 'milder' engines using less cryogenic and denser fuels. In a context where NASA is supposed to do R&D/science, *IF* NASA developped new materials or construction techniques to make LH2 engines truly reliable and re-usable, could LH2 engines ever be competitive? (thrust/weight/ISP, cost of fuel, tanks etc) ? Anything that would do that for LH2 engines could equally be applied to 'milder' engines and so they would still maintain their wear and maintenance advantages. Or has kerosene or even methane zipped by LH2 are are so far ahead that there is absolutely no point in researching LH2 engines anymore ? It's not a matter of 'zipped by'. It's a matter of those both being less cryogenic and denser fuels. Also, how come LH2 was chosen for the Shuttle if it already presented serious challenges for re-usability and kerosene was already used by others? Because the Shuttle isn't a staged vehicle and you kind of want the higher energy LH2 engine once you're up in near vacuum. But you can't carry enough tankage to make that work all the way from the ground with just LH2, so you get strap ons... -- "The reasonable man adapts himself to the world; the unreasonable man persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man." --George Bernard Shaw |
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JF Mezei wrote:
On 2017-05-15 14:09, Fred J. McCall wrote: percentage of the cost basis of the engine. For rockets the cost of fuel is almost irrelevant to the cost of operation, so trying to compare to aircraft is comparing apples and aardvarks. The numbers given by Elon Murk show cost of fuel is minimal compared to cost of new rocket. Cost of fuel is minimal compared to almost anything you do with a rocket, new or reused. However, in an environment where re-use becomes common, cost of turn around (incl engine check/maintennce) and cost of fuel become the big ticket items, just as is the case for commercial aircraft. Cost of fuel is still going to be literally in the noise. Hardware turn around and launch costs are going to drive the numbers. And who do you think is going to pay for that? The entire life of the RS-25 engine program was about reducing maintenance requirements But NASA has lacked aggressive plans to "finish" the Shuttle by making the SSMEs truly reusable. Not much point in spending mega money in R&D to make SSMEs truly re-usable if at the end of the day, NASA management will still require total teardown between flights. You need to get off this nutty idea that NASA was tearing them down and refurbishing them just for the **** of it. They made it as reusable as they could (through four versions of the engine). When SLS is put out of its misery, and Rocketdyne has delivered those 6 SSMEs that potentially go to museums, what is to happen to Rocketdyne if it has no engine that is competitive? Shouldn't the company be worry about its future with SpaceX revolutiuonalizing launch services ? Of course it should but what do you propose they do about it? Throwing billions of dollars into an engine that has no market (RS-25) is just insane behaviour. They're making R-1 (RP-1/LOX) to be as competitively priced as they can and it can't compete with BE-4 (methane/LOX) which has similar performance. But if LH2 engines have no future, then I guess they are correct in not spendng any more money than NASA forces them to on the SSMEs. Big of you. going in the other direction to lower the cost of the engines. You just want to go that one step further and invoke magic and unobtainium. No, I am merely exploring whether something that was never tried before could be possible. Or whether the SSMEs are truly hopeless in terms of conceptually being competitive. It WAS tried. They just couldn't bloody do it and they can't do it now. High performance engines like RS-25 will ALWAYS be 'less reusable' than lower performance engines like Merlin or Raptor. Hand waving doesn't comvince me, especially since there was never a serious attempt to make SSMEs cheaper to make with modern manufacturing and more re-usable wth moderns materials knowledge. Well **** you, then. One of us is right and the other one is you. I'm fine with that. Educate your ignorant ass. Just as a clue, 'cheaper' means 'less reusable'. You can't do both. On any given engine, making it 'more reusable' will mean making it more expensive. And the idea that they just didn't bother to try to make the RS-25 reusable is, well, STUPID. The SLS pork to Ropcketdyne COULD have been such an attempt to turn those expenseive engines into something that might have been commercially viable. (Unless , again, LH2 engines are just never going to cut it). Unicorns and pixie dust... -- "Ignorance is preferable to error, and he is less remote from the truth who believes nothing than he who believes what is wrong." -- Thomas Jefferson |
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