Editing Rocket engine (section)

==Mechanical issues==
Rocket combustion chambers are normally operated at fairly high pressure, typically 10–200{{nbsp}}bar (1–20{{nbsp}}MPa, 150–3,000{{nbsp}}psi). When operated within significant atmospheric pressure, higher combustion chamber pressures give better performance by permitting a larger and more efficient nozzle to be fitted without it being grossly overexpanded.

However, these high pressures cause the outermost part of the chamber to be under very large [[hoop stress]]es – rocket engines are [[pressure vessel]]s.

Worse, due to the high temperatures created in rocket engines the materials used tend to have a significantly lowered working tensile strength.

In addition, significant temperature gradients are set up in the walls of the chamber and nozzle, these cause differential expansion of the inner liner that create [[internal stresses]].

=== Hard starts ===
A '''hard start''' refers to an over-pressure condition during start of a rocket engine at ignition. In the worst cases, this takes the form of an unconfined explosion, resulting in the damage or destruction of the engine.

Rocket fuels, [[hypergolic]] or otherwise, must be introduced into the combustion chamber at the correct rate in order to have a controlled rate of production of hot gas.<ref>{{Cite web |title=Introducing Propellant into a Combustion Chamber |url=https://www.idc-online.com/technical_references/pdfs/mechanical_engineering/Introducing_Propellant_into_a_Combustion_Chamber.pdf |access-date=February 16, 2024 |website=IDC Online}}</ref> A "hard start" indicates that the quantity of combustible propellant that entered the combustion chamber prior to ignition was too large. The result is an excessive spike of pressure, possibly leading to structural failure or explosion.

Avoiding hard starts involves careful timing of the ignition relative to valve timing or varying the mixture ratio so as to limit the maximum pressure that can occur or simply ensuring an adequate ignition source is present well prior to propellant entering the chamber.

Explosions from hard starts usually cannot happen with purely gaseous propellants, since the amount of the gas present in the chamber is limited by the injector area relative to the throat area, and for practical designs, propellant mass escapes too quickly to be an issue.

A famous example of a hard start was the explosion of [[Wernher von Braun]]'s "1W" engine during a demonstration to General [[Walter Dornberger]] on December 21, 1932. Delayed ignition allowed the chamber to fill with alcohol and liquid oxygen, which exploded violently. Shrapnel was embedded in the walls, but nobody was hit.