Reply to post: Re: Why super-cooled fuel?

Re: Why super-cooled fuel?

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.