13 ms·

Hydrogen brings absolute oodles of troubles with it. For upper stages, where it is much more critical to minimise the wet weight, that trouble is generally wort

by nkoren 14y ago

Hydrogen brings absolute oodles of troubles with it. For upper stages, where it is much more critical to minimise the wet weight, that trouble is generally worthwhile. For lower stages, not so much -- it's generally FAR more cost-effective to just build a heavier stages which will do the same job, but with lower ISP. In the past, 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. (They've said in the past that they're working on a LH2 upper stage already, so if you see them building up a cryogenic capacity, that would explain why). Also, I kinda miss the days when they could get away with calling their next generation rocket the "BFR", and no true geek needed to even ask what that stood for.

ballooney 14y ago
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.
someperson 14y ago
Heh, I didn't realise Musk used to do that. I found a well written and interesting article from that period of time (2005), it's not his first announcement of "BFR" but it's worth a read just to remind ourselves how far SpaceX has come. Also it has some tidbits on his roadmap for the vehicle's development: http://www.thespacereview.com/article/497/1 http://www.thespacereview.com/article/497/1 (Fairly sure he's talking about the super heavy lift, using the Merlin 2 and all) >Musk said he that while he would like to slow down the rate of growth the company has experienced since its inception over three years ago, he envisions that SpaceX could grow as large as 400-500 people, but no larger. “Companies do change once they grow beyond a certain level,” he said. “Once everyone doesn’t know everyone, the company becomes slightly different.” SpaceX now has more than 1800 employees.

by nkoren 14y ago

ballooney 14y ago

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.

someperson 14y ago

Heh, I didn't realise Musk used to do that. I found a well written and interesting article from that period of time (2005), it's not his first announcement of "BFR" but it's worth a read just to remind ourselves how far SpaceX has come. Also it has some tidbits on his roadmap for the vehicle's development: http://www.thespacereview.com/article/497/1 http://www.thespacereview.com/article/497/1 (Fairly sure he's talking about the super heavy lift, using the Merlin 2 and all) >Musk said he that while he would like to slow down the rate of growth the company has experienced since its inception over three years ago, he envisions that SpaceX could grow as large as 400-500 people, but no larger. “Companies do change once they grow beyond a certain level,” he said. “Once everyone doesn’t know everyone, the company becomes slightly different.” SpaceX now has more than 1800 employees.