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Indeed, as I say it's very much harder to work with, but it's worth a revisit with 2012 technology, I think. I also am involved with oxygen+hydrogen rocket prop

Indeed, as I say it's very much harder to work with, but it's worth a revisit with 2012 technology, I think. I also am involved with oxygen+hydrogen rocket propulsion research but can't really discuss it much here. But things have come on a bit in the last 30 years :) Re: 'BFR', well I have a few hypotheses about 'MCT' in a similar vein but I reasoned they might be a little bit blue for HN! Edit: A better reason for first stage hydrogen occured to me over a cup of tea: > "SpaceX has always made the (correct, IMHO) decision to optimise the cost-efficiency of their rockets, rather than the propulsion efficiency, and hopefully they won't stray from that path." I think the economics change when you try and go reusable in the way that they are. Lox Hydrogen engines and tankage might be more expensive, which is obviously a factor in an expendable vehicle, but if you reuse the vehicle, and have to keep enough fuel back to land, as SpaceX plan, I think actually the equation goes far more back towards optimising for propulsion efficiency in order to get the cost efficiency. The launcher cost metric is basically dollars per kg into orbit, and for a given reusable rocket, lox+hydrogen will get many more kgs into orbit than lox+kerosene - you need to keep much less fuel back in reserve to land the various rocket stages, and the extra fuel you can use on the ascent is also a lot more thrusty per unit mass. I wouldn't be surprised, assuming you sell at a dollar/kg price, if lox+hydrogen ended up being much more profitable in reusables than lox+kerosene, everything else being equal. I have not done the sums though.

nkoren 14y ago
I admit to having a bit of an attitude about LH2; for 25 years after they developed the SSME, NASA maintained an attitude that if "it ain't hydrogen, it's crap". No non-hydrogen developments were funded during that time, and not coincidentally, no new first-stage engines ever reached flight status. Until SpaceX came along and developed its own hydrocarbon engines, we were reduced to buying hydrocarbon engines from the Russians. During that time I developed a heck of a bias towards hydrocarbons -- but I have to grant that it's certainly possible that advances in LH2 have balanced out the negatives since then. You're correct that as you go reusable, the economics shift away from capital costs to operating costs. However, it seems to me that when you consider first stages from a systemic perspective, it's not obvious that it's any cheaper. Yes, Hydrogen gives you much more impulse, and so for a given degree of propulsion, you can use less fuel by weight. However, LH2: 1. Costs 4x as much as kerosene by weight 2. Is only 11% as dense as kerosene, requiring much larger and heavier (and highly insulated) tankage to hold it, and larger and heavier turbopumps, combustion chambers, and everything else that puts weight on a rocket. So while your wet weight may still be less, your dry weight is generally quite a bit higher. T/W is always lower than for hydrocarbon systems. 3. This much greater bulk increases your drag losses if you're operating in an atmosphere; lower T/W increases your gravity losses. 4. Although hydrogen doesn't suffer from coking on the combustion surfaces the way that hydrocarbon engines do, it embrittelises (is that a word?) everything it touches, making between-flight inspections a much more crucial and delicate process. There are hydrocarbon engines which have been fired thousands of times between rebuilds, for hundreds of hours of firing time. Show me a hydrogen engine is even 10% as durable. Of course when you're in orbit, neither drag nor gravity loses matter, and the nature of the Rocket Equation dictates that every ounce of gross weight really matters. So LH2 makes undoubted sense there. But the exact inverse is true when you're launching off the ground. Unless some fundamental breakthroughs have occurred which I'm not aware of (always possible), I will remain very surprised if SpaceX is pursuing cryogenic first stages.
- ballooney 14y ago
  All valid and I'm enjoying the discussion but I would address the points as: 1) Sure at the moment, and it will be difficult to match economies of scale with the oil industry, but the amount of research money going into hydrogen production at scale is enormous. It's only getting cheaper, and while 'The Hydrogen Economy' has been promised since the 70s and hasn't yet arrived, it's definitely coming. I'm sort of thinking about this debate in a 10+ years out sense rather than right now, I admit. 2&3. Sure, but that only becomes a smaller problem as rockets get bigger (i.e. very heavy launchers such as this one being proposed) and the cube/square law works in your favour (i.e. tank volume increases faster than frontal area to which the drag is proportional). T/W is often limited anyway by payload constraints rather than rocket engineering constraints, especially in manned systems. Doesn't matter if the vehicle can do 5G off the tower, the people inside it can't. 4) 'Embrittels' I would guess? Anyway, it occurs in most metals but certainly not all metals and very certainly not "everything it touches". This is the clue for the way out of this problem. I obviously couldn't show you a hydrogen engine with the same track record as no operational hydrogen oxygen engines have been built with a clean sheet using what we know today. That said, a few are being designed precisely as we speak for very long life operation in reusable vehicles. I don't think the chickens have come in to roost yet.
  - nkoren 14y ago
    I'm enjoying the conversation too! Your point about the scaling properties making LH2 more favourable for larger vehicles is something I hadn't properly appreciated in those terms, but is certainly correct. As far as T/W goes, I think that you do need to be able to pull 3G off the tower, and moreover do it with one propulsion system (or cluster of identical systems). Again from a systems-engineering point of view, the point where a launch vehicle design jumps the shark is where you're strapping together completely disparate types of rockets to get the desired effect. I don't think that an LH2 engine has ever actually done this, although the the X-33 designs perhaps could have worked. (I always thought that the Venturestar was emblematic of everything that can go wrong with an LH2-driven design, since it basically turned into a (non-)flying kitchen sink exposition. But the DC-Y was elegant enough; I would have loved to see somebody make an honest attempt at that.) So basically, it sounds like there are three things required to make a cryogenic first stage/RLV viable: 1. A market for large payloads -- large enough to allow the scaling laws to work out in LH2's favour -- with sufficient frequency to pay off the development costs. As such, this doesn't exist yet, but it's certainly conceivable that someday it could. 2. New types of LH2 engine designs, which are much more durable than SSMEs (which I guess would be the current benchmark of LOX/LH2 durability? Or would that be RL-10s?) Presumably with some substantial revolutions in materials sciences. This may be well underway already (as you allude), but is not public info yet. 3. New, cheaper ways of generating hydrogen. As long as the lowest-cost method is steam reformation, then hydrocarbons will, by definition, be cheaper. If that's the case, then you'd end up in a curious situation where LH2 might only be economical in the middle of the spectrum. In an ELV which is primarily concerned with capital costs, hydrocarbons clearly are more more cost-effective, since they're much easier to develop and build. On the other hand, in an RLV which achieves true airline-like operations -- where the cost of fuel begins to become a meaningful concern, which it currently is not -- then hydrocarbons win again. That might only leave a thin band in the middle where LH2-based designs are the most cost-effective. On the other hand, a new low-cost hydrogen-generation technique (eg., bacteria that exhale it) could change the game. So yes, I guess I'd echo your sentiment that this is something that could be relevant in a decade, if all three of those items develop in the right way. Definitely promising enough to merit some R&D, but not yet assured or inevitable.
Roboprog 14y ago
Where does liquid methane CH4 fall in the spectrum between H2 and Kerosene?

nkoren 14y ago

I admit to having a bit of an attitude about LH2; for 25 years after they developed the SSME, NASA maintained an attitude that if "it ain't hydrogen, it's crap". No non-hydrogen developments were funded during that time, and not coincidentally, no new first-stage engines ever reached flight status. Until SpaceX came along and developed its own hydrocarbon engines, we were reduced to buying hydrocarbon engines from the Russians. During that time I developed a heck of a bias towards hydrocarbons -- but I have to grant that it's certainly possible that advances in LH2 have balanced out the negatives since then. You're correct that as you go reusable, the economics shift away from capital costs to operating costs. However, it seems to me that when you consider first stages from a systemic perspective, it's not obvious that it's any cheaper. Yes, Hydrogen gives you much more impulse, and so for a given degree of propulsion, you can use less fuel by weight. However, LH2: 1. Costs 4x as much as kerosene by weight 2. Is only 11% as dense as kerosene, requiring much larger and heavier (and highly insulated) tankage to hold it, and larger and heavier turbopumps, combustion chambers, and everything else that puts weight on a rocket. So while your wet weight may still be less, your dry weight is generally quite a bit higher. T/W is always lower than for hydrocarbon systems. 3. This much greater bulk increases your drag losses if you're operating in an atmosphere; lower T/W increases your gravity losses. 4. Although hydrogen doesn't suffer from coking on the combustion surfaces the way that hydrocarbon engines do, it embrittelises (is that a word?) everything it touches, making between-flight inspections a much more crucial and delicate process. There are hydrocarbon engines which have been fired thousands of times between rebuilds, for hundreds of hours of firing time. Show me a hydrogen engine is even 10% as durable. Of course when you're in orbit, neither drag nor gravity loses matter, and the nature of the Rocket Equation dictates that every ounce of gross weight really matters. So LH2 makes undoubted sense there. But the exact inverse is true when you're launching off the ground. Unless some fundamental breakthroughs have occurred which I'm not aware of (always possible), I will remain very surprised if SpaceX is pursuing cryogenic first stages.

ballooney 14y ago
All valid and I'm enjoying the discussion but I would address the points as: 1) Sure at the moment, and it will be difficult to match economies of scale with the oil industry, but the amount of research money going into hydrogen production at scale is enormous. It's only getting cheaper, and while 'The Hydrogen Economy' has been promised since the 70s and hasn't yet arrived, it's definitely coming. I'm sort of thinking about this debate in a 10+ years out sense rather than right now, I admit. 2&3. Sure, but that only becomes a smaller problem as rockets get bigger (i.e. very heavy launchers such as this one being proposed) and the cube/square law works in your favour (i.e. tank volume increases faster than frontal area to which the drag is proportional). T/W is often limited anyway by payload constraints rather than rocket engineering constraints, especially in manned systems. Doesn't matter if the vehicle can do 5G off the tower, the people inside it can't. 4) 'Embrittels' I would guess? Anyway, it occurs in most metals but certainly not all metals and very certainly not "everything it touches". This is the clue for the way out of this problem. I obviously couldn't show you a hydrogen engine with the same track record as no operational hydrogen oxygen engines have been built with a clean sheet using what we know today. That said, a few are being designed precisely as we speak for very long life operation in reusable vehicles. I don't think the chickens have come in to roost yet.
- nkoren 14y ago
  I'm enjoying the conversation too! Your point about the scaling properties making LH2 more favourable for larger vehicles is something I hadn't properly appreciated in those terms, but is certainly correct. As far as T/W goes, I think that you do need to be able to pull 3G off the tower, and moreover do it with one propulsion system (or cluster of identical systems). Again from a systems-engineering point of view, the point where a launch vehicle design jumps the shark is where you're strapping together completely disparate types of rockets to get the desired effect. I don't think that an LH2 engine has ever actually done this, although the the X-33 designs perhaps could have worked. (I always thought that the Venturestar was emblematic of everything that can go wrong with an LH2-driven design, since it basically turned into a (non-)flying kitchen sink exposition. But the DC-Y was elegant enough; I would have loved to see somebody make an honest attempt at that.) So basically, it sounds like there are three things required to make a cryogenic first stage/RLV viable: 1. A market for large payloads -- large enough to allow the scaling laws to work out in LH2's favour -- with sufficient frequency to pay off the development costs. As such, this doesn't exist yet, but it's certainly conceivable that someday it could. 2. New types of LH2 engine designs, which are much more durable than SSMEs (which I guess would be the current benchmark of LOX/LH2 durability? Or would that be RL-10s?) Presumably with some substantial revolutions in materials sciences. This may be well underway already (as you allude), but is not public info yet. 3. New, cheaper ways of generating hydrogen. As long as the lowest-cost method is steam reformation, then hydrocarbons will, by definition, be cheaper. If that's the case, then you'd end up in a curious situation where LH2 might only be economical in the middle of the spectrum. In an ELV which is primarily concerned with capital costs, hydrocarbons clearly are more more cost-effective, since they're much easier to develop and build. On the other hand, in an RLV which achieves true airline-like operations -- where the cost of fuel begins to become a meaningful concern, which it currently is not -- then hydrocarbons win again. That might only leave a thin band in the middle where LH2-based designs are the most cost-effective. On the other hand, a new low-cost hydrogen-generation technique (eg., bacteria that exhale it) could change the game. So yes, I guess I'd echo your sentiment that this is something that could be relevant in a decade, if all three of those items develop in the right way. Definitely promising enough to merit some R&D, but not yet assured or inevitable.

Roboprog 14y ago

Where does liquid methane CH4 fall in the spectrum between H2 and Kerosene?