# rocket engine design

Do we need to preheat both the fuel e.g. LH2, and the oxidizer e.g. LOX, by running conduits around the nozzle? I presume both being cryogenic, preheating would be a must otherwise, engines will have low efficiency

Actually there is no need to preheat the oxidizer nor the fuel. Thermodynamically it even lower the efficiency.

If there is liquid running conduits around the nozzle it is there to cool the nozzle!

You can improve the efficency by getting the exhaust gases hotter, but to hot would melt the nozzle. So you are "investing" a lost by heating your fuel/oxidizer for a higher temperature at the output. That is "giving you more efficency" than you have "invested" in the first place.

You are basially playing with Carnot's theorem.

EDIT:

So you use a part of your cold fuel to cool the nozzle.

an example: Wikipedia Rocketdyne F-1

some of the fuel first traveled in 178 tubes down the length of the thrust chamber — which formed approximately the upper half of the exhaust nozzle — and back in order to cool the nozzle.

• Thanks for the reply. One more related confusion. Since you have said "preheating" might lower the the efficiency thermodynamicaly, do we not waste some energy in bringing the propellants, (which I presume are sub zero temp.), to their flash point before they start burning? Also, why do we have only one of the propellants running around the conduits of nozzle? I have visited the link you suggested, but could not find satisfactory clarification. Commented Apr 23, 2020 at 13:15
• Good Point, I made a serious error using the Carnot's theorem. You still want the exhaust gases to be as hot a possible, but you do not need the "input" to be cold, but the "environment" (quite lucky, we are in space). In fact you are right, warming fuel/oxidizer to flash point during burning lowers efficency. Nethertheless the main purpose is cooloing the nozzle so it does not melt. Commented Apr 23, 2020 at 13:33
• Why only the fuel not the oxidizer? As an engineer I would assume it is much easier to handle fuel than oxidizer. Liquid Oxygen is very corrosive and getting it to hot would result in blowing up your nozzle much quickier. Basically have a look at the second and third paragraph under "Designe" Commented Apr 23, 2020 at 13:37
• Thanks a lot. While I am aware that I should post this as a new question, since you have bothered to reply to my comment, I felt perhaps you might let me know on this as well: Why did the NASA guys / Designers of Saturn V, never used "Electric Motors" for driving the "Turbopumps"? If I look at it, the weight addition would have been less as compared to Turbines + Gas generators and related tubing etc. Unless the "battery" required to operate the motors was not felt to be dependable/trustworthy, and its weight was more than affordable. Request pls respond. Thanks. Commented Apr 23, 2020 at 16:41
• Sorry, we never get so deep in our space engine lectures at university. I just could guess (reailibility, Mass, power) but i do not know it. Commented Apr 23, 2020 at 17:13