Quote from: guckyfan on 09/14/2014 08:13 amWhy use an ACES? A hydogen upper stage would make integration hugely complex and expensive. An expendable BFR upper stage cannot be that expensive in comparison. But with refuellling in LEO I guess it would be possible to send a spacecraft into a high energy orbit and still return the upper stage. Maybe with a simple storable fuel booster that provides extra kick and/or orbit insertion at the destination.There I go, optimizing for performance instead of cost. Habits die hard, it seems. A Refueled BFR upper stage would probably be a better option (even if expended), especially since refueling seems to be the logical architecture for MCT anyway.
Why use an ACES? A hydogen upper stage would make integration hugely complex and expensive. An expendable BFR upper stage cannot be that expensive in comparison. But with refuellling in LEO I guess it would be possible to send a spacecraft into a high energy orbit and still return the upper stage. Maybe with a simple storable fuel booster that provides extra kick and/or orbit insertion at the destination.
I've seen some posts like these, this one in response to someone proposing that SpaceX could put a scaled-up version of an Advanced Cryogenic Evolved Stage (a "super-ACES") on the Big Friggin' Rocket:Quote from: Owlon on 09/14/2014 08:59 amQuote from: guckyfan on 09/14/2014 08:13 amWhy use an ACES? A hydogen upper stage would make integration hugely complex and expensive. An expendable BFR upper stage cannot be that expensive in comparison. But with refuellling in LEO I guess it would be possible to send a spacecraft into a high energy orbit and still return the upper stage. Maybe with a simple storable fuel booster that provides extra kick and/or orbit insertion at the destination.There I go, optimizing for performance instead of cost. Habits die hard, it seems. A Refueled BFR upper stage would probably be a better option (even if expended), especially since refueling seems to be the logical architecture for MCT anyway.There's an idea going around that hydrogen is overrated as a rocket fuel: "it's a pain to handle because of its cryogenic nature," "it makes ground handling and manufacturing more complex and expensive," "it's an optimization for performance instead of cost." So why are Blue Origin & ULA going to use BE-3s?
Why are Arianespace and Mitsubishi still going to use hydrolox on their next generation rockets? Are they continuing to fall for the "siren song" of specific impulse or are they somehow making a wise decision?
Because BE-4 is too big for a space tug engine. For LEO, they are not going to use BE-3, only BE-4s.
Mostly because they have only hydrolox and solid engines, they have no methane or kerosine engines.
Sure, they'd have to develop those engines. But ultimately they should transition into all-kerolox or all-methalox architectures. You know, to avoid the cost and complexity of hydrogen, isn't that right?
I've seen some posts like these, this one in response to someone proposing that SpaceX could put a scaled-up version of an Advanced Cryogenic Evolved Stage (a "super-ACES") on the Big Friggin' Rocket:Quote from: Owlon on 09/14/2014 08:59 amQuote from: guckyfan on 09/14/2014 08:13 amWhy use an ACES? A hydogen upper stage would make integration hugely complex and expensive. An expendable BFR upper stage cannot be that expensive in comparison. But with refuellling in LEO I guess it would be possible to send a spacecraft into a high energy orbit and still return the upper stage. Maybe with a simple storable fuel booster that provides extra kick and/or orbit insertion at the destination.There I go, optimizing for performance instead of cost. Habits die hard, it seems. A Refueled BFR upper stage would probably be a better option (even if expended), especially since refueling seems to be the logical architecture for MCT anyway.There's an idea going around that hydrogen is overrated as a rocket fuel: "it's a pain to handle because of its cryogenic nature," "it makes ground handling and manufacturing more complex and expensive," "it's an optimization for performance instead of cost." So why are Blue Origin & ULA going to use BE-3s? Why are Arianespace and Mitsubishi still going to use hydrolox on their next generation rockets? Are they continuing to fall for the "siren song" of specific impulse or are they somehow making a wise decision?
Yes. Methalox could probably always have a common bulkhead in the propellant tankage to reduce stage mass and volume.
Quote from: Pipcard on 12/14/2016 08:24 pmSure, they'd have to develop those engines. But ultimately they should transition into all-kerolox or all-methalox architectures. You know, to avoid the cost and complexity of hydrogen, isn't that right?It's not that simple. France and Japan have special interest to have big solids in their LVs. It has very little to do with minimum cost and a whole lot more with maintaining strategic capability to manufacture such solids for less civilian purposes. Hydrolox is there to offset solid subpar Isp.
Are there real costs associated with using LH2 in an upper stage? Sure. But there are also many benefits, and it's not at all clear that the costs outweigh the benefits. Especially when you start talking about high-energy upper stages and eventually in-space refueling. When I got a tour of SLC-3E last month, I asked some of the pad guys how hard LH2 was to handle, and while they agreed it was more annoying than LOX, they didn't seem to think it was that big of a deal. I wouldn't use it for a booster propellant (unless I was doing an SSTO or air-launched vehicle), but for an upper stage I think it isn't obvious that it is a bad choice, *even if you are designing for cost*. For ULA at least, I doubt they'd save anything by replacing Centaur with a LOX/Kero upper stage (and growing their first stage big enough to handle the much heavier upper stage). In fact, I'm almost positive it would make their system more expensive.~Jon
Look, it's perfectly clear hydrogen has more energy per kg than kerolox, and hence allows a lighter first stage for the same performance. That's simple physics and not in dispute. But hydrogen has drawbacks as well, and hence may not be the most economical choice. It's not a good first stage fuel (not dense enough). So now you need a two-fuel system. This implies different engines for the different stages, more specialists on your launch team, and now your second stage engine is produced in low volume. All of these can be solved, but it costs money. On the whole, is the hydrolox upper stage cheaper? Like all engineering, it's a question of tradeoffs.Take Ed's example of a hydrolox upper state for Falcon, then reducing the first stage to 7 Merlins. That's three less Merlins, which are rumored to cost about $1 million each. How much does a BE-3 cost? If it's more than 3 million you are already behind. Even if it's less than 3 million, hydrogen might still be a losing proposition once you add in the ground infrastructure and support, amortized over missions. And if they get re-use working, then the cost of that additional first stage mass may be smaller yet, reducing hydrogen's advantage still more.
Why is that? Do you have data on this assumption?
Musk says that overhead starts with how the launch vehicle is designed. The workhorse Atlas V, for example, used for everything from planetary probes to spy satellites, employs up to three kinds of rockets, each tailored to a specific phase of flight. The Russian-built RD-180 first- stage engines burn a highly refined form of kerosene called RP1. Optional solid-fuel strap-on boosters can provide additional thrust at liftoff, and a liquid hydrogen upper stage takes over in the final phase of flight. Using three kinds of rockets in the same vehicle may optimize its performance, but at a price: “To a first-order approximation, you’ve just tripled your factory costs and all your operational costs,” says Musk.
We don't even know whether it costs less, we only know it's being priced lower.
Quote from: jongoff on 12/15/2016 04:37 amAre there real costs associated with using LH2 in an upper stage? Sure. But there are also many benefits, and it's not at all clear that the costs outweigh the benefits. Especially when you start talking about high-energy upper stages and eventually in-space refueling. When I got a tour of SLC-3E last month, I asked some of the pad guys how hard LH2 was to handle, and while they agreed it was more annoying than LOX, they didn't seem to think it was that big of a deal. I wouldn't use it for a booster propellant (unless I was doing an SSTO or air-launched vehicle), but for an upper stage I think it isn't obvious that it is a bad choice, *even if you are designing for cost*. For ULA at least, I doubt they'd save anything by replacing Centaur with a LOX/Kero upper stage (and growing their first stage big enough to handle the much heavier upper stage). In fact, I'm almost positive it would make their system more expensive.~JonI thought it wasn't about mass optimization, it was about cost optimization, and that additional engine manufacturing lines and ground support equipment to handle different types of fuel, especially hydrogen (regardless of whether or not the workers were used to it), made things more expensive.
And if you really crank flight rate and rapid reuse up to 11, then price of propellant could start to matter (it does for ITS). Liquid methane is almost as cheap as liquid oxygen.
