RS-25 conducts sixth SLS Static Fire test

by Philip Sloss

NASA has conduct another static fire test firing of RS-25 engine number 0525 on Thursday at the Stennis Space Center in Mississippi, continuing development and adaptation of the engine for use with the Space Launch System (SLS). The test was number six in the opening series of seven static fires tasked with gathering development and certification data.

RS-25 Test Six:

Engine number 0525, one of two development engines retained from the Space Shuttle Program (SSP) was fired from the A-1 Test Stand at Stennis in what was the penultimate test of the current series of objectives.

The test series is evaluating the performance of a new engine controller – a unit that controls engine valve settings to produce efficient combustion, and communicates with the vehicle to accept throttle settings and send sensor data.

The controllers on the RS-25Ds used during the Shuttle Program proved to be highly reliable. However, the new controller is utilizing updated hardware and software configured to operate with the new SLS avionics architecture.

“The objectives for this seven-test series have been – first of all – to get development and certification data for our new (engine) controller and software – (to) see how the new controller and software runs with the engine,” noted Steve Wofford, manager of the SLS Liquid Engines Office at Marshall Space Flight Center, in an interview with before the test.

2015-07-18-021742“(We want to) get calibration data on that new controller to see if it controls the valves to the level of precision and interfaces with the sensors in the closed-loop control system the way we want it to – (to) put it through its paces.”

The Stennis team are also certifying the engine to the new SLS propellant inlet conditions “for both start and run”, and to anchor the analytical models in terms of loads and engine performance and thermal environment for the new SLS vehicle.

“We have analytical models and we have test data and this will help sync up those empirical and analytical models,” added Mr. Wofford.

2015-08-13-014135The test objectives even include changes to the insulation used on the engine nozzle, as they prepare for life as a team of four on the aft of the SLS, compared to the three engines used on the Shuttle orbiters.

“We have some new ablative insulation that we’re putting on the nozzle to protect it from the different heating environment on SLS, so we’re getting durability data on how well it sticks and make sure it’s got the proper adhesion and will do its job in the environment,” Mr. Wofford noted.

“Those overall objectives get decomposed into individual test objectives for test-to-test purposes.”

One of the objectives for Thursday’s test is to see how test engine 0525 operates when subjected to a more extreme start condition than NASA would expect to see in an actual flight, exploring the engine “start box.”

“A start box is basically a plot of what the engine can handle in terms of temperature versus pressure for the inlet conditions,” Mr. Wofford added. “So the propellant inlet conditions have to be inside that box before you can start the engine.”

Mr. Wofford said that NASA tests the engines on the ground to a larger box than they would expect to see during flight, providing operational margin.

“That margin is a big part of how we manage risk on the engines. If we throw bigger challenges to it in the ground (testing) than we ever expect to see in flight, then we know we can handle what we expect to see in flight.”

2015-08-13-014255For test six, engineers intentionally introduced propellant inlet conditions to hit one of the corners of the start box – the “hot-fast start” on the LOX (liquid oxygen) side.

“That means the high-pressure oxidizer pump spools up faster than it normally would if you were in the center of the box,” Mr. Wofford explained. “We’re going to see what the engine does under those circumstances. The models show that it will be just fine, but we test it to make sure and to anchor those models.”

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Other objectives for the test included testing a liquid hydrogen chill-cycle that is shorter for SLS than it was for Shuttle and for throttling the engine up to 109 percent shortly after it reaches main-stage at the start of the test. The plan is for the engine to fire for 535 seconds.

2015-08-13-014335Ignition occurred on schedule at 4 pm local time (Central Daylight Time) or 2100 GMT on Thursday, but the test team were on station beginning many hours before that.

“We’re starting at 6 am,” Ronald Rigney, RS-25 project manager at Stennis explained before the test. “We’re actually starting some of the count early because (engine start) is set at a specific time, but we’re also starting at that time because of some of the chill requirements we have for this test.

“We’ll be starting with sensor setups at 6 am and then we’ll go through a standard set of procedures that we would for any test to make sure that our calibrations for our sensors are all set properly.”

2015-08-13-014435In providing an overview of some of the pre-test timeline, the complexity of the path to ignition was explained by Mr. Rigney.

“We run (the engine test) off of facility liquid oxygen run tanks and fuel (liquid hydrogen) tanks, but we also have interconnected piping that allows us to back-feed those systems during a hot-fire from barge tanks.

“Early in the morning we top our run tanks off (to) get ourselves to a start level. (We will) prepare the engine for receiving its proper purges. Those will happen before noon and then we’ll go into LOX chilldown. (Liquid hydrogen) fuel (chilldown) will happen around one o’clock.

2015-08-13-014629“Then we’ll be going through several different chill-type flow-rate conditions to demonstrate different environments for the folks with the vehicle to use for data to plan for their eventual chill procedures they will use. We’ll be going through that from about one o’clock all the way up to four (o’clock).”

Mr. Rigney noted that the test team doesn’t normally target a specific time of ignition, but some of the objectives for this test require more specific timing.

“We don’t normally operate on timed countdowns,” he explained, “we have a lot of freedom of time normally, so we’re working off of events primarily.

“For this test though, our chilldown procedure needs to hit as short of a duration as we can – close to the 90-minute mark – which is causing us to be more time-oriented. (That) happens to line up with somewhere around 4 o’clock.”

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Should all go to plan with the post firing review, the final test in this series will take place on August 27.

(Images: Via NASA and L2 content from L2’s SLS specific L2 section. SLS Render by L2 artist Nathan Koga – lead photo by Philip Sloss)

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