You may have heard of the Antares rocket that blew up recently (video link at the bottom). You may have also heard that this rocket was fueled by kerosene. Well if you’re like me, and I know I am, you might have thought to yourself, “Why, in this day and age, would you fuel a rocket with kerosene?” Well the answer is rocket science, but it’s still pretty simple.

Most rockets now days use liquid hydrogen for their fuel. But that’s not really a modern invention. Even back in the 1960’s when we went to moon we used liquid hydrogen in a lot of rockets. The upper stages of the big Saturn V that took us to moon used liquid hydrogen. So why use RP-1 now?

In fact back in the 1960’s the first stage of the Saturn V did use kerosene (which rocket scientists call RP-1). If you watch the video of a moon rocket lifting off you’ll see those same orange flames and black sooty smoke coming out of the Saturn V, just before liftoff, that you see from the Antares rocket explosion. The reason they used RP-1 in the first stage is that it’s perfect for low altitude rocket flight.

The reason RP-1 works so well low down is how much energy you get from each gallon of it. For each gallon of RP-1 you burn you get over 4 times the energy you get from burning a gallon of liquid hydrogen. When you’re building a rocket the more gallons of fuel it has to carry the bigger it has to be. The bigger the rocket is the more air it has to push aside as it speeds up, and that uses up valuable energy that should be used getting the rocket up higher.

So why not use RP-1 for the whole rocket? Remember that Saturn V only used RP-1 for the first two and a half minutes or so. Because RP-1 is heavy stuff. 11 gallons of liquid hydrogen weigh less than one gallon of RP-1. Of course once you get high enough there isn’t as much air to push through so you don’t mind if your rocket is big and fat to carry all that low energy, but lightweight, liquid hydrogen. Up high you care more about how much your rocket weighs. After all it’s easier to speed up a lighter rocket. So once you’re up above most of the air your best bet is to switch to liquid hydrogen. Once you’ve gotten up that high it’s all about getting up to the 17,000mph speed you need to achieve orbit.

Of course there is more than one way to skin a cow (and make lovely wallets). The Space Shuttle had this same problem but solved it with solid rocket fuel instead of RP-1. It burned liquid hydrogen in it’s engines and so needed a huge tank to hold it all. To get that big Space Shuttle and that huge tank up above most of the air it needed those two big rockets on either side. They didn’t burn for long (only about 2 minutes) but in that time they shoved the Space Shuttle and it’s big tank up to 150,000ft. Which is above most of the air. That’s above 5 times higher than a jet airliner flies. Once the Shuttle was up there the remaining liquid hydrogen fueled engines on the back were perfect for the job of getting up to speed so it could reach orbit.

If you want more morbid proof of this go watch the Space Shuttle Challenger explosion. You’ll see that, unlike in the Antares explosion, the cloud that’s left over is a nice clean white one. When Challenger's left solid rocket booster failed it burned a hole in the hydrogen tank. All that liquid hydrogen dropped out of the tank and into the rocket exhaust and ignited. When you burn hydrogen and oxygen you get H2O, or water. Water vapor in this case, water vapor takes up a lot more space than liquid hydrogen does. The expansion made enough thrust to break up the vehicle in a bunch of ways. The solid rocket boosters, freed from the rest of the shuttle, flew on out of the cloud. So what you see is pretty much just burned up hydrogen, a.k.a. water. And also one of the saddest days in space history.

TLDR: Kerosene has a lot more energy per gallon than liquid hydrogen.

Antares explosion: https://www.youtube.com/watch?v=NCWunnJXdm0#t=168s

Slow motion Saturn V liftoff: https://www.youtube.com/watch?v=DKtVpvzUF1Y

Challenger explosion: https://www.youtube.com/watch?v=fSTrmJtHLFU#t=43s