The Rocket Turbopump

Quantify the alternative before building the complicated thing.

1. For a chosen tank volume, compute the wall thickness needed to hold 5 bar with a sensible safety factor, and its mass.
2. Repeat for 50 bar, and again for 300 bar.
3. Plot tank mass against pressure.
4. Compare each against the propellant mass the tank would hold.

Tank mass rises roughly in proportion to pressure, and by a few hundred bar the tank weighs more than what is inside it. Since the rocket equation cares about the ratio of full to empty mass, that is fatal — you have added dry mass in exchange for nothing.

Same machine as the turbojet compressor, running on liquid instead of gas.

1. Machine or adapt a centrifugal impeller in a volute housing, driven by a variable-speed motor.
2. Pump water and measure delivered pressure against flow rate at a fixed speed, throttling the outlet in steps.
3. Plot pressure head against flow.
4. Repeat at several speeds and compare.

Head is highest at zero flow and falls as flow increases, and head rises with the SQUARE of rotational speed while flow rises only linearly. That square law is why turbopumps run at tens of thousands of rpm — the pressure you need is bought far more cheaply with speed than with size.

The failure that destroys pumps is not overload — it is the liquid boiling at the inlet.

1. Run the pump and progressively restrict the INLET while listening and watching delivered pressure.
2. Note the point where the sound turns to a gravelly rattle and head collapses.
3. Stop immediately and inspect the impeller leading edges.
4. Now fit an inducer — a small axial screw upstream of the main impeller — and repeat.

Restricting the inlet drops local pressure below the liquid’s vapour pressure, so it boils at the impeller eye; those bubbles collapse violently a moment later in the higher-pressure region and hammer the metal. Cavitation sounds like pumping gravel, destroys head, and pits impellers into lace.

The inducer solves it by raising pressure gently before the main impeller ever sees the flow. It is a low-head pump whose only job is to keep the real pump out of trouble, and every rocket turbopump has one.

Something must spin the pump, and where its energy comes from defines the whole engine.

1. Sketch three arrangements. Gas generator: a small side combustor burns a little propellant, drives the turbine, and dumps its exhaust overboard. Staged combustion: a preburner runs very fuel-rich, drives the turbine, and its exhaust — still full of unburned fuel — goes into the main chamber. Expander: fuel warmed in the regenerative cooling jacket is used to drive the turbine before being injected.
2. For each, trace where the turbine’s working fluid ends up.
3. Mark which arrangements waste propellant.

The gas generator throws away a small percentage of propellant unburned — visible as the dark smoky exhaust alongside the main plume on older engines. Staged combustion wastes none, and pays with far higher turbine pressures and much harder engineering.

The expander cycle is the elegant one and ties straight back to the previous blueprint: the heat picked up while cooling the chamber is used to drive the pump, and then still gets burned. Its limit is that only so much heat is available, which caps engine size — which is why expander engines are upper stages.

One shaft passes from the oxidiser pump to the fuel pump, and a leak between them is not a maintenance issue.

1. Examine the problem: liquid oxygen on one end, kerosene or hydrogen on the other, one rotating shaft between them.
2. Design a seal package — two mechanical face seals with a vented cavity between them.
3. Pipe an inert purge gas into that cavity at a pressure above both sides.
4. Test with air and dyed water, checking that neither fluid reaches the other even when one seal is deliberately degraded.

The intermediate purged cavity is the whole design: any leak past either seal goes into the purge and out to vent, never across. A single seal, however good, has one failure between two propellants that ignite on contact.

Recognise the pattern from earlier in this catalogue — this is the same reasoning as the double seal on the SF6 breaker shaft and the vented interspace in a hydraulic gland. Where a single barrier failing is catastrophic, use two and monitor the space between, so the first failure is detectable rather than fatal.