SpaceX Falcon 9 grounded by 'sledgehammer' winds SpaceX was once again thwarted in its mission to get the SES-9 satellite off the ground from Cape Canaveral yesterday, this time due to "extreme high altitude wind shear", according to Elon Musk. Lest there be any doubt of the possible effect of such wind shear on the Falcon 9 lifter, Musk elaborated: "Hits like a sledgehammer …
Anyone know what is the rationale for supercooled fuel? Obviously you get a higher density, so more energy packed in a given size of tank. But it adds complexity and trouble. Why not increase the diameter of the rocket instead? Is it because they already have a proven mechanical design and are "stretching" it to the limit, or is it because the extra aerodynamic drag caused by a larger diameter rocket would be a significant loss?
This looks like sucking every ounce of performance out of an existing design.
It is an interesting cost equation to work with. Sure - you can push more with a normal Falcon before going to the heavy lifter, but your cost of scrapped launches increases significantly. At this "scrap rate" the extra money you generate from improving the launcher may not be sufficient to pay for the costs incurred when you have to abort and delay a launch.
'It is an interesting cost equation to work with'
Yes it is but you can bet your salary on the fact that it is one which has been fully explored by Space X.
Musk is first and foremost a businessman and as such although he 'gets' technology, as crass as it may seem, it's all about the money.
It's not luck that he became and remains a billionaire, It's shrewd business acumen tied to financial courage and engineering nous.
A larger tank is a heavier tank, so you waste more fuel on lifting the tank. So you don't get that big a gain in propulsion.
It's also hideously expensive to change tank diameter - SpaceX have lots of gear set up for manufacturing, moving and transporting their current size rocket, which would need replacing or adapting.
There may also be issues with larger diameters making transporting the rocket massively more expensive and/or slower. I know the space shuttle booster size was limited by a tunnel that they had to be transported through on their way from the factory to the launch site, as making that tunnel bigger would have been hideously expensive.
A cooler fuel doesn't have any of those problems.
Thanks for answers.
A larger tank is a heavier tank, so you waste more fuel on lifting the tank. So you don't get that big a gain in propulsion.
But (to first order and for small changes in diameter) the volume of a cylinder goes as radius r squared whereas its weight goes in proportion to r. So there's a definite gain. A 5% increase in radius equates to a 5% increase in fuel to tank weight ratio (provided you didn't need to add any strengthening to the walls).
Getting more fuel into the engines with the same plumbing and pumps sounds like the most likely answer, and possibly transportation issues. I was thinking that redesigning the cylindrical rocket body and tanks wouldn't be too expensive. Redesigning the engines, far more so.
And of course Musk and SpaceX would have worked out all the trade-offs a long time ago. I was curious!
@cuddles: the question is not why LOX rather than compressed oxygen. It's why supercooled LOX compared to LOX at its natural temperature, the one at which it starts to boil off. They're also chilling the RP. Liquids are somewhat denser at lower temperatures, but keeping them supercooled is nontrivial.
"@cuddles: the question is not why LOX rather than compressed oxygen. It's why supercooled LOX compared to LOX at its natural temperature, the one at which it starts to boil off. They're also chilling the RP. Liquids are somewhat denser at lower temperatures, but keeping them supercooled is nontrivial."
As others have noted, there's really very little difference. The boiling point of oxygen is -183C, SpaceX is using it at -207C. The cryogenic systems required in either case will be essentially identical (although it's easier to keep a constant temperature at a phase transition, so more feedback and/or flexibility in allowed temperature would be needed). Note that the LOX is not supercooled - that means cooling a liquid to below its freezing temperature, which is -219C for oxygen. The article uses the term "super-chilled", and claims this is more difficult to deal with, but as far as I am aware this is not a meaningful technical term.
"Anyone know what is the rationale for supercooled fuel? Obviously you get a higher density, so more energy packed in a given size of tank. But it adds complexity and trouble. Why not increase the diameter of the rocket instead?"
Using liquid oxygen means you don't need a pressure vessel - liquids aren't really compressible, so everything is kept at standard atmospheric pressure (and this part doesn't make it into space, so you don't need to worry about even dealing with a 1 atmos difference). In order to compress the same amount of oxygen into the same space just by pressurising it without cooling it, you'd need a massively heavy vessel that would basically mean you could never launch a rocket at all.
To give some numbers - LOX is compressed by a factor of 861 relative to stp. High pressure oxygen tanks have a compression factor of around 130, and need tanks that weigh many times more than the actual stored oxygen. So you end up with fuel tanks that not only weigh much more, but also have to be several times larger. Cryogenic systems may be awkward to work with, but the alternative is basically not being able to build rockets at all because they'd be too big and heavy.
But it adds complexity and trouble. Why not increase the diameter of the rocket instead? Why not increase the diameter of the rocket instead?
Because it's more complex and trouble to design a new rocket (e.g., change its diameter and even length) than to reconfigure infrastructure on the ground and fiddle with propellant loading schedules. Using super-chilled propellants was a relatively low effort way to get more performance out of the current hardware, for certain aerospace definitions of "low effort."
The super-chilled propellants offer a couple of benefits. First, as you noted, you get more fuel into a fixed volume of tankage. Second, you may get more thrust from the same engines. The propellant pumps demand horsepower mostly based on propellant volume, not mass, which is one of several reasons hydrogen-fueled rocket engines tend to have low-thrust to weight ratios compared to denser fuel engines. Increasing the density of propellants will thus allow a fixed horsepower of pumps to deliver a greater amount of fuel into the combustion chamber per unit time. The engines burn more fuel per unit time, and get greater thrust.
Or, if the propellants warm up too much because you were waiting for a wayward barge to get out of the way, then the engines may deliver less thrust than they were expecting because less dense propellant was going into the engine, and you end up with a pad abort.
But the general point is, propellant chilling - for compatible rockets - is a relatively easy way of getting more fuel and performance into a fixed rocket configuration.
Liquids are somewhat denser at lower temperatures, but keeping them supercooled is nontrivial.
Keeping kerosene chilled is easy in this case - you've got a lot of liquid oxygen handy, and that's colder than is needed for the kerosene. The oxygen boil off can keep it chilled. Also, hydrocarbons can increase in density fairly quickly.
Oxygen's a little more challenging since you need some appropriate refrigerant like nitrogen or hydrogen, and its density change is smaller than kerosene. I was surprised they were supercooling the oxygen.
Anecdotally, the diameter of the SpaceX rockets was limited by the height of the lowest bridge that Elon Musk couldn't pay to have raised or demolished, between the factory and their original test site, minus the height of the low-loader the rockets were shipped on...
One thing to bear in mind is that oxygen boils at -183°C. Chilling oxygen to the reported -207°C is only making it about 24°C colder than "normal" bulk liquid oxygen.
But it's taking the stuff down from (roughly) 90K to 66K, which is quite a big percentage drop, and that bodes well for an increase in density.
To my inexpert brain that extra bit of cooling doesn't seem like a really big bother from the technical point of view.
Another thing to think about: if the oxygen is chilled to well below boiling point, it'll take longer to warm up enough to boil off after the rocket's been fuelled.
But regardless of my witterings: as others have mentioned, this extra-cool fuel/oxidiser arrangement has been worked out by some of the best rocket engineers in the business and if they think it's a good idea, then it's certainly a good idea.
LOX density at -183°C is about 1.15, at -207°C it is about 1.25, so they are getting a little more than 8% more oxidiser into their tank for essentially no weight increase other than the propellant. Add to that no increase in drag from larger components and no manufacturing changes.
The cooler temperatures probably don't require any rocket redesign, other some kind of liquid relief valve to allow for the LOX to expand as it warms up on the platform (fueling lines, most likely). Cold LOX can't take advantage of evaporative cooling to stay at that temperature, so it has a short fuelling-to-launch window.
It all adds up to quite a few extra Joules of energy delivered to the payload (faster/higher/heavier) with no significant cost increase. Unless you scrub lots of launches, that is.
"Cold LOX can't take advantage of evaporative cooling to stay at that temperature,"
It can, but you need to be evaporating liquid nitrogen to do it - and given the stated temperature of the LOX, that's most likely what they're using to superchill it. Elon doesn't like doing engineering using experimental techniques whilst LN2 cooling is well-proven as is the materials technology for handling at this lower temperature.
http://www.engineeringtoolbox.com/boiling-points-fluids-gases-d_155.html says LOX boils at -183, LN2 at -196
The value in cooling the fuels is in the Tsiolkovsky rocket equation. The delta velocity of a rocket is proportional to the natural log of the ratio of total mass to dry mass. Anything at all that adds to the dry mass rips performance from the system mercilessly. Eventually every kilogram of dry mass added is a kilogram of payload not reaching orbit (or at least staging). Just making booster tanks bigger does not yield performance improvements as well as one might wish. Adding more fuel (and hence total mass) without any penalty in dry mass is a bigger win than one might expect. The higher effective fuel flow possible can translate to higher trust - which helps with that additional total mass on liftoff, something that matters a lot when most of the thrust is simply vanishing into balancing gravity. An even marginal change in thrust can have big knock on effects. Overall the ultra cold fuel is as close to a free lunch as one might wish for.
Also worth noting, supercooling the fuel is something done by machinery on the ground which can be more or less as bulky and heavy as you like I doubt there is that much redesign needed for the tanks and engines to handle colder fuel so it's probably more or less extra fuel and more efficient engine cooling for very little additional cost in the flying hardware.
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