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SLS and Orion

NASA Now Has "Deep Space" RS-25 Engines

By Keith Cowing
NASA Watch
March 10, 2016
Filed under ,

Keith’s note: These RS-25 engines will end up on the bottom of the Atlantic ocean. They will not go into “deep space” – or anywhere in space. At least Shuttle missions took them for a ride into low Earth orbit. Deep space? No. Deep Atlantic? Yes. More goofy exaggerated NASA hype on the #JourneyToMars

NASA Watch founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.

27 responses to “NASA Now Has "Deep Space" RS-25 Engines”

  1. Jafafa Hots says:
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    The ocean is pretty deep.

  2. Daniel Woodard says:
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    The RS-25 is a relatively expensive engine developed over 40 years ago specifically to be reusable. The only real advance in that time is to throw it away? Why not develop a simpler, more powerful version designed to be expendable? Wait a minute, that would be the RS-68.

    The real problem is that it doesn’t make sense to use LH2 in the first (or core) stage, because it produces less thrust than kerosene and requires a much larger fuel tank. That’s why, more than half a century ago, it was not used in the first stage of the Saturn V. Somehow we forgot that.

    • EtOH says:
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      “The real problem is that it doesn’t make sense to use LH2 in the first (or core) stage”

      This. Save the expensive, low density, high Isp propulsion systems for the upper stages.

      • Jeff2Space says:
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        Lack of a suitable LOX/kerosene engine made in the US is the problem. That said, if we did have a very high thrust LOX/kerosene engine available, SRBs would not be attractive either, which would make certain Congressmen and Senators upset if they weren’t included in the design.

        The combination of SRBs (high thrust, but terrible ISP) with a LOX/hydrogen core stage is a horrible combination in general. This combination was originally chosen because development costs for large SRBs was lower than large liquid boosters. But in the long run, liquids have many advantages, especially if you can recover and reuse them.

        • Vladislaw says:
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          President Obama proposed funding new engines… along with 6 billion of new funding over 5 years to fully fund commercial, if congress would have approved that funding the new engine and commmercial crew would be coming online now.

    • John Thomas says:
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      Actually the LH2 engines produce more thrust. The RS-68 engine generates more than 4 times the thrust of the Merlin engine. An the LH2 first stage weighs less than a kerosene stage. The primary disadvantages are higher drag because of the larger stage and dealing with the smaller and colder H2 molecule.

      • SpaceMunkie says:
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        Merlin is much smaller engine. The RS 68 is more than 10 times its weight.

      • MattW2 says:
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        You’re comparing two engines that are different sizes. The F-1 is more than twice as powerful as the RS-68. Is that because it burns kerosene instead of LH2? Well, pretty much yeah. It’s not all that much bigger. Merlin has an even better TWR.

        LH2 requires excessive tankage and efficient but heavy engines. Without magic technology it cannot compete with denser propellants for the first stage.

      • Daniel Woodard says:
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        On thrust to weight ratio, and on thrust to cost, the Merlin wins by a wide margin:

        Merlin FT: 756KN/470Kg = 1.61N/g

        RS-68A 3137KN/6740Kg = .47N/g

        The RS-68 has a higher Isp but this is of limited benefit in a first stage or strap-on engine, which are the roles in which it was applied in the Delta IV. Even the SLS is a full two stage vehicle with a second stage and strap-on boosters.

        LH2 is more expensive to handle than RP-1, methane, or LOX. RP-1 is neither toxic nor cryogenic so the handling cost is minimal. The Falcon does not use LH2 in the upper stage, so the GSE is not present. This increases the required thrust of the booster, as the second stage is heavier than an LH2 stage would be, but on balance the Falcon appears to have a cost advantage.

        • John Thomas says:
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          Comparing thrust to weight of just the engine of 2 different technologies (LH2 vs Kerosene) is not valid. For a realistic comparison you would need to compare 2 fully fueled stages since LH2 is lighter than Kerosene.

          • EtOH says:
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            LH2 being lighter than kerosene is a bad thing, not a good thing. It’s the mass of propellent being expended that matters, and the extremely low density means you need giant tanks, which means more structural mass.

            Yes, on the whole, a LH2 stage will be lighter than a RP1 stage of equivalent capability, but this is due to higher Isp, not the density of propellant. Furthermore, as Daniel Woodward points out, the hydrogen stage will be more expensive. This is partly because hydrogen is more difficult to work with, and partly because the engines and tank structure of the hydrogen stage are actually larger/heavier, even though the fueled mass is lighter overall.

            On upper stages, these weight savings can pay off, in that they allow you to use less capable (and cheaper) stages underneath. But for first stages, it’s hard to argue for why the extra cost of LH2 is worth it.

          • Jeff Havens says:
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            So how does the upcoming Methane/Natural Gas +LOX engines figure into this debate?

          • Skinny_Lu says:
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            A big advantage of RP-1 engines: the fuel is also used as hydraulic fluid for the nozzles’ thrust vector control system. Without RP-1, a methane stage needs a different TVC system. Maybe it will evolve into an electric motor TVC??? But it would need a lot of electrical “juice” to power it… Maybe Musk has some high performance batteries (Tesla?) that could do the job?

          • Daniel Woodard says:
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            The Falcon has more than twice the thrust of the larger Delta common booster core, which allows it to launch without solid fuel strap-on boosters. The lift-off T/W ratio for the Falcon first stage is about 1.6 vs about 1.4 for the Delta.

      • fcrary says:
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        You forgot the mass of the fuel tank. That, and the associated support structure, are most of the first stage’s dry mass. The tank mass per kilo of fuel, is much higher for liquid hydrogen.

    • SpaceMunkie says:
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      on a 1.5 stage vehicle like the Shuttle, it makes perfect sense to use LH2LOX engines. On a two or more stage, it becomes a tradeoff on ground support complexity vs the efficiency gain. You already have to mess around with LH2 for the second and maybe third stage, so why not just do it for the first stage instead of adding the additional RP1 infrastructure and headaches.

    • james w barnard says:
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      The reason that the RS-25 engines used on the Shuttle are being use for the SLS Block 1 core is because they are available (16 IIRC… four per first stage). Later, a non-reusable version, with an improved power head and some 3D manufactured parts will be available. When deciding the propellant combination or solid vs. liquid propellants, you have to consider Isp, bulk density, ease of handling and cost. Considering the low frequency of flights of the SLS, recovering the first stage or even the engines probably doesn’t make economic sense. For other launch vehicles used commercially, where flight frequency and mission will be a factor, recovery of the first stage or at least the engines may make economic sense in terms of overall development costs. SpaceX and Blue Origin are showing these factors. As in most aerospace engineering programs, trade-offs must always be considered.

  3. Tritium3H says:
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    “Deep space? No. Deep Atlantic? Yes”.
    LOL! Loved your comment, Keith.

  4. BeanCounterFromDownUnder says:
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    More pork to the usual suspects.
    Cheers

  5. Ben Russell-Gough says:
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    So, there are two possibilities that I can see:

    1) The PAO intern who updates @NASA is so badly briefed that he or she doesn’t know what the RS-25s on the SLS will do;

    2) NASA is rather ham-fistedly trying to cover up the fact that, after the hundreds of millions spent on J-2X, SLS still doesn’t have a confirmed upper stage engine.

  6. Michael Spencer says:
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    The disposable mindset is just nuts.

    • fcrary says:
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      In this case, I agree. But I have doubts about making it an universal statement. Toilet paper is disposable, and I am not convinced that trying to reuse or recycle it would be efficient. In some cases, disposable items can be the best option. I’d.prefer to look at it on a case-by-case basis. Reuse is clearly better for a Falcon 9 core, if they can make it work. Or the Merlin engines,.if that’s all they can practically reuse. I’m not as certain about things like reentry heat shields. Building a reusable one might be less efficient than using disposable ones.

      I think one of the mistakes of the Shuttle program was a fixation on reuse. I’d call reuse a good idea in general, but not a hard requirement.

      • Jafafa Hots says:
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        Toilet paper might not be the best example. It’s not always a decision between using a disposable product or reusing the same kind of product.

        A huge amount of the “super soft” toilet paper that Americans use is made from virgin, old growth rain forest timber. The reason for that is because longer fibers make for softer paper, and recycled paper or quick-growing trees have shorter, hence scratchier fibers.

        Wiping our butts with Amazon rain forest is not really a great idea, but that doesn’t mean we should recycle toilet paper.

        Most people in the world don’t use toilet paper. They clean themselves via other means, many of which not only produce no (additional) waste but are also healthier for… um… sensitive tissues.

        Point being, think outside the roll.

      • Michael Spencer says:
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        Toilet paper, no.
        Interplanetary space vessels, yes.

        The American colonies depended on sailing ships making return trips, not crashed upon the shoals of the New World after somehow tossing out the goods.

        Throwing away very expensive machines time and again simply will not work. In fact, it’s a policy that indicts not only the planning mindset but the underlying tech base (which is very far from even thinking about a trip to Mars).

        • fcrary says:
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          OK, the toilet paper remark didn’t work. Using an extreme example to make a point is risky rhetoric; it can backfire if people misunderstand and think you take the example seriously.

          But heat shields are a serious example, as is a first stage where you only reuse the engines. Others include the number of cycles you design for. (Did SpaceX design the Merlin for 10, 100 or 1000 cycles?)

          My point was that, while throwing everything away after one use is clearly moronic, over engineering _everything_ for an infinite number of reuse cycles is going way too far in the other direction. I think the high level goal should be efficiency and effectiveness. Reusability flows from that when it makes sense. But if you make reuse the primary goal, you get, well, the Space Shuttle.

          • Michael Spencer says:
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            I didn’t miss your point about toilet paper.

            But wondering about the number of design cycles for Merlin is surely off-topic. I objected to the mindset.