# Higher nozzle exit velocity generally means higher thrust right?

Higher nozzle exit velocity generally means higher thrust right? If that is the case, then a higher heat capacity could potentially lead to a higher exit velocity, thus higher thrust?

• Have you looked at the thrust equation? Feb 13, 2023 at 0:09
• Was just looking for confirmation, i sometimes lose confidence and need some affirmation if my knowledge is correct or if I am missing something. Thank you. Feb 13, 2023 at 0:15

Yes, higher nozzle exit velocity means higher thrust

No, heat capacity (presumably you mean of propellants) is unrelated to rocket performance.

The definition of “heat capacity” is the number of heat units needed to raise the temperature of a substance by one degree. (Oxford Dictionary). It is not really related to rocket propulsion.

I suspect you are referring to Heat of Reaction (AKA Enthalpy of Reaction). This is the energy released by a chemical reaction such as combustion of rocket fuel.

In general, higher Heat of Reaction produces higher temperature and more thrust, but this is not a useful generalization since different fuels also produce different combustion products and therefore different thrust at a given temperature.

Thrust is proportional to velocity of exhaust gas. At a given temperature, light molecules have higher velocity than heavier molecules. So (once again at a given temperature) steam (from the exhaust of burning oxygen and hydrogen) will produce more thrust than steam plus CO2 (from the exhaust of burning oxygen and kerosene).

Heat of reaction of H2 and O2 is 130 MJ/kg of H2.

Heat of reaction of petroleum fuels is 45 MJ/Kg of fuel.

https://world-nuclear.org/information-library/facts-and-figures/heat-values-of-various-fuels.aspx

Hydrogen/oxygen has higher specific impulse than kerosene/oxygen because of its higher Heat of Reaction and the low molecular weight of the exhaust… not because of the Heat Capacity of the propellants or their exhaust.

• Heat capacity is pretty important for expander cycle engines, and all nozzles that use regenerative cooling. Feb 13, 2023 at 4:48
• @OrganicMarble ... would it be Heat Capacity or Heat of Vaporization that is important for expander cycle engines? I don't think either is directly related to thrust, but I'm not an engineer. Feb 13, 2023 at 5:29
• Check out the answers here space.stackexchange.com/q/41165/6944 "The secondary limiter on expander cycles is the amount of enthalpy you can pick up from cooling your combustion devices."..."Another problem is the specific heat of different propellants make some better than others for different cycles." Feb 13, 2023 at 5:34
• thank you very much for the comprehensive answer! Now I understand the background knowledge for this :) Feb 13, 2023 at 8:15