How are rockets tested (hot fire)?

Question

Given that the majority of the operation of a chemical rocket is at different altitudes / pressures than at ground level, how are hot fire ground tests conducted to gather useful information about efficiency, thrust, etc?

I imagine there is some process of 'truing' the thrust, as well - making sure the vector points in a particular direction, like a bike wheel. How is this confirmed? Additionally...how do you gather information from a stream of hot exhaust that is several thousand degrees?

Maybe my assumptions are way off-base. Just curious what some of the actual engineering processes are.

Answer 1

I'm gonna say it straight off, I'm no expert. But the first part of your question seems relatively straightforward to answer. The easiest is by building test facilities at different altitudes, get the performance data you're after at each altitude you can, and then extrapolate for whichever atmospheric pressure you later require.

E.g. ISRO has a high-altitude test facility at Mahendragiri, India (1,654 m / 5,427 ft), JAXA can simulate atmospheric conditions of an altitude of approximately 30 km at their High Altitude for Rocket Engine Test Facility, SpaceX is leasing a launch pad in Las Cruces, New Mexico (4,000 ft / 1,219 m), DLR has an Altitude Simulation Test facility in Lampoldshausen, and so on.

The thrust itself is usually measured by load cells, which have multiple strain gauges oriented at different angles (usually four of them, two on each side oriented perpendicular to each other) and convert deformations to them (strain / load) into electric signal. Placing multiple load cells to measure force loads on the rigid rocket engine frame it is mounted to during test-fire should be sufficient to measure the thrust vector, since you can measure forces each side of the frame is a subject to.

The exhaust plume itself is usually measured with multiple infrared cameras that record its heat signature from various angles in real-time. All this data would be combined with data from a variety of sensors built into the rocket engine itself, such as e.g. measuring propellants pressure in its injectors (injection rate), and so on.

Answer 2

In a vacuum chamber in Ohio.

NASA's Spacecraft Propulsion Research Facility (B-2) is the world's only facility capable of testing full-scale upper-stage launch vehicles and rocket engines under simulated high-altitude conditions.

There is also the A3 test stand at NASA Stennis Space Center which is capable of flight duration (up to 550 seconds) hot fire test of engines or (with modifications) of stages at a 100,000 foot simulated altitude.

Answer 3

The effect of external pressure on a rocket engine is backpressure at the exit plane of the nozzle. The combustion chamber is well insulated from that by the sonic flow at the throat o the nozzle. High altitude engines have large nozzles that expand the gas to a pressure below ground atmosphere. It is quite a good approximation to put a smaller nozzle on engines that will operate at high altitudes and test them in atmospheric pressure. The effect on performance can then be modeled.