A narrated walk through the engine and its five-second start, from a cold, frosted engine to full power. Press play with your sound on.
During bootstrap, boiling hydrogen makes the fuel preburner pressure swing about twice a second. You are the fuel preburner oxygen valve. Admit oxygen at the bottom of each dip, three times, before the 4,600 rpm speed check. Slowed to half speed.
Space bar works too.
A liquid rocket engine can't just be switched on. The RS-25, which flew on the Space Shuttle and now flies on SLS, has to go from a still, frozen machine to pumps spinning over 30,000 rpm in about five seconds, without its flame ever getting too hot, too lean or into the wrong place.
Purge clears air and moisture with nitrogen on the oxygen side and helium on the hydrogen side, since liquid hydrogen would freeze nitrogen solid. Chill down soaks the engine in propellant for over an hour so metal and pumps are cryogenic. Then comes the start: the main fuel valve opens and hydrogen flash-boils in the warm engine, just enough to nudge the turbines. This is a bootstrap start, with no outside spin-up gas. Oxygen is fed to three igniters in tight timing with pressure oscillations. The two preburners and the main chamber "prime" at 1.4, 1.5 and 1.6 seconds, and the computer takes over in closed loop until full power at five seconds.
Press play for the narrated tour, about five minutes. The engine time is slowed so the narration can keep up: the whole start really takes five seconds. Voice, Music and Effects switch the narrator, the score and the engine sounds on or off. Captions shows the narration as text; it starts off on phones and always shows when the voice is off.
Chart plots what the valves, the fuel turbopump and chamber pressure do through the start, plus the fuel pressure oscillation (dashed) that the scripted valve moves have to ride, synced to the timeline. Hover it to read values at any moment, or click and drag on it to scrub.
Tap Ride the waves to play the fuel preburner oxygen valve yourself. Tap a chapter name under the timeline to jump to it.
Turn on Sluggish fuel pump to see what happens when the pump misses its 4,600 rpm check at 1.25 seconds.
X-ray ghosts the hardware so you can see hydrogen (orange), oxygen (blue) and hot gas (amber) moving through it. Tap any part or label for what it does and its live reading.
Space play or pause · N P next or previous scene · V voice · T captions · M music · E effects · ← → scrub (hold Shift for bigger steps) · 1 2 solid or X-ray · K chart · C cinematic camera · R reset view · S sluggish pump · G ride the waves · / hide the interface
Built from Everyday Astronaut's How to Start a Rocket Engine and its companion article, which draw on Robert Biggs' Space Shuttle Main Engine: The First Ten Years. Event times follow that account. Purge and chill down are compressed from hours. The geometry is simplified and the curves between events are illustrative, not flight data.
The RS-25 is one answer to a problem every engine has to solve. Here is how others solve it.
Fuel and oxidizer are premixed in one rubbery solid, so it only needs heat. The SLS boosters light through a chain: redundant detonators fire a booster charge, which lights a small rocket motor that shoots flame down the hollow core. Simple and reliable, but once lit it can’t really be throttled or shut off.
Kerolox only chills the oxygen side, at −183 °C. Falcon 9 starts chilling its Merlins around T−7 minutes. Methalox chills both sides, and vented methane is recaptured or flared. Hydrolox is the coldest, with hydrogen at −253 °C, so the hydrogen side is purged with helium. Hypergolics store at room temperature and need neither chill down nor an igniter, but they’re toxic and lower performance.
Pumps need a turbine, the turbine needs hot gas, and hot gas needs pumped propellant. Ways out: pressure-fed engines skip pumps, but the tanks get too heavy for a first stage. Electric pumps, like Rocket Lab’s Rutherford, run on batteries and don’t scale to engines this size. Gas spin start blasts helium or nitrogen through the turbine. Start cartridges burn a small charge once. Bootstrap, used by the RS-25, lets tank pressure and boiling hydrogen do it, and is the hardest to control.
Pyrotechnic sticks still light Soyuz, once, on the pad. Spark and glow plugs need power. Torch igniters, like the RS-25’s, use a spark to light a tiny engine that lights the big one. Falcon 9 uses TEA-TEB, which ignites on contact with oxygen in a green flash but must be reloaded. Hypergolic propellants light themselves. Raptor’s main chamber is believed to light from its two hot gas streams meeting, with no igniter.
A coasting stage is in free fall, so propellant floats away from the pump inlet. Ullage thrusters nudge the stage so it settles first. Bladder or piston tanks squeeze it toward the outlet. Hot staging, used by Soyuz and Proton, lights the upper stage while the one below is still pushing.
Getting the RS-25 through its first two seconds took 19 test firings, 23 weeks and 8 turbopump replacements. Reaching full power took 18 more tests. SpaceX attacked Raptor’s even tighter coupling by testing about five times a day.
Chilling the engine…