Rocket engines- why do they have little exhausts on them?

[kerbiloid]

(That strange moment when you had first time realized that the booster fuel tanks are not just barrels, but witch cauldrons full of boiling poison.)

Edited December 23, 2019 by kerbiloid

[sevenperforce]

On 12/22/2019 at 9:50 AM, farmerben said:

Hypergolic-monopropellants offer some interesting oppotunities.    N₂H₄ transitions into N₂ and 2H₂  irreversibly, and with substantially less heat that combustion.  So it could be fed back into the tank as a pressurizer.  N₂O₄ breaks apart in a reversible manner, so it is a different beast entirely.  

I don't have certainty on this, but it seems to me that:  Hydrazine and dinitrogen tetroxide could support unique types of fuel pressure systems that do not work with more stable fuels.  And there is a possibility reducing weight and number of parts by doing so.  

Hydrazine undergoes thermal decomposition at 3000 K, so you can dump in oxidizer willy-nilly to pressurize, or loop a small amount into a catalytic decomposition chamber with the same effect.

This does not work so well with HTP, which undergoes thermal decomposition at 150 C.

[farmerben]

A hydrazine pressure system could forego turbopumps and helium inflation systems in exchange for a much simpler system.  

We can take heat and convert our fuel into hydrogen and nitrogen without opening any loops.  Running the main engine hypergolicly at full power gives plenty of heat.  Turbomachinery not required.  

[kerbiloid]

ISP of monoprop decomposition is much lower than of the fuel burning.
So, either you have a small auxilliary monoprop engine, usually pressure-fed, or a bi-propellant system with oxidizer where you don't need to decompose the fuel as monoprop (just add oxidizer and get a full-featured hypergolic reaction).