How Rockets Are Ignited – Things Kerbal Space Program Doesn't Teach



hello it's Scott family here with another episode of things Kerbal space program doesn't teach you know today we're going to talk about lighting the big candle I'm sure we've all followed along with rocket launchers and as the last few seconds tick down we hear the Mission Control or call-out ignition as the seconds tick down to zero ignition of course refers to the main engines lighting and preparing to push the rocket off the pad after everything else checks out of course now after hours of sitting on the pad Alan Shepard was famously hard to say fix your little problem and light this candle coining a phrase that's been adopted and reused by many who have followed in his rocket-powered footsteps but it's not really like lighting a candle I mean I'm sure many of you have seen backyard fireworks that invite you to literally light the blue touch paper and stand well back but I don't think many of you imagine actual rocket scientists with giant matches setting fire to the bottom of a fuse and retreating to a safe distance no in Kerbal space program engine ignition is 100% reliable but the game gives no indication of how the fuel and oxidizer actually ignite simply pumping fuel and oxidizer into a combustion chamber doesn't automatically guarantee that they will combust especially in the case of cryogenic propellants like liquid oxygen and rp1 or liquid hydrogen now rocket scientists have used all sorts of tricks to make sure that when the fuel starts flowing the engine actually starts burning it indeed given the volatility and energy content of fuel bad things have been known to happen when the fuel starts flowing but it doesn't start burning if the fuel flows for too long before actually igniting then the build-up of fuel can lead to something that rocket scientists euphemistically refer to as a hard start as the combustion chamber struggles to cope with the an explosion which is slightly larger than the one it was intended now I'm sure a lot of you armchair rocket scientists are already familiar with hypergolic fuels but for the benefit of those who are unfamiliar Golic fuels are chemicals that when mixed together will spontaneously catch fire with each other combinations like dinitrogen tetroxide and unsymmetrical dimethyl hydrazine solve many of these problems for rocket scientists although it can equally cause a headache for the poor people that have to handle these invariably toxic fuel and oxidizers hypergolic fuels are especially common in engines that are repeatedly lit in space and need to be reliable but even on the ground we have things like the proton booster which uses hypergolic propellants but most other large launchers typically rely on fuels that are a little less toxic the earliest space rockets used a liquid oxygen as an oxidizer which doesn't really hypergolic with any fuels that are generally considered viable for rockets consider that a puddle of kerosene contains a lot of energy but it doesn't spontaneously ignite on contact with the air and that's a good thing you to get it to ignite you have to come in with a flame to start the burning but once it starts burning the flame it's will self-sustained and the same is true in rocket combustion chambers once you start the fire generally it will keep going as long as you could continuously push in fuel and oxidizer into the combustion chamber in some early space launch vehicles ignition inside the combustion chamber would be initiated by a pyrotechnic moving device literally placed inside the combustion chamber it would be secured by wood or plastic and it would have to be placed there manually by some lucky rocket scientist it would crawl up inside the rocket and place it there I hear that the people tasked with this would invariably wear the ignition keys around their neck just to avoid any accidents happening now these Pyro's would be triggered by an electrical signal seconds before the fuel started flowing into the chamber and they would burn long enough to make sure the ignition actually happened at which point the intense combustion and pressures would blow the Pyro's out of the bottom of the rocket engine and it would burn off on its own actually I'm not being entirely truthful when I say early engines because this kind of process is still very much in use today in the Soyuz launcher Soviet engineers use the equivalent of giant high-tech matches that were stuck up inside each of the rocket chambers they would be t-shaped pieces of woods two by fours generally and they would stick up in the middle with a pyro inside the combustion chamber itself now when the signal would be given to launch the electrical signal would trigger the Pyro's and as the Pyro's start to burn there would be a second little piece of wire inside that would have to melt and burn through to show that the pyro was burning the Soyuz has dozens of engines and every single one of these has to get the trigger signal correctly and there was a launch recently where not all of the Pyro's went off and the launch had to be aborted but anyway yeah I guess Russian engineers are still using giant space age matches to launch their Rockets at least in the u.s. later engines took inspiration from hypergolic fuels and started to use a liquid based ignition system the idea is that just before the fuel starts pumping a highly reactive chemical is pumped into the combustion chamber something that will be hypergolic with liquid oxygen the most common substances used for this these days our trifle borane and tri ethyl aluminium a mixture of the two was used to light the f1 rocket on the saturn v and the merlin engines on the SpaceX Rockets the advantage of this system is of course that you can restart the engine if you've got extra ignition fuel which is of course something that is essential to SpaceX as recovery plans trifle boring is known as tab for short and it burns with a brilliant bright green flame which is sometimes visible in some SpaceX videos as the rocket relights incidentally tab is also the same stuff used to ignite the massive engines and afterburners on the sr-71 blackbird the sr-71 mission endurance is actually constrained by the number of times they can relight the afterburners the day they can refuel with jp-7 while they're in flight but they can't refuel the the tab so once they run out of that they can no longer relate the engines and the mission ends anyway if you've got any experience with car you're no doubt wondering whether good old-fashioned spark plug based ignition has any place in rocket science and I'm happy to say that it does at least for hydrogen oxygen engines the j2 engines on the Saturn 5 and the rs.25 engines on the space shuttle both used spark plug based ignition but it's not as simple as putting a spark plug in the ignition chamber what you generally had was a smaller low-pressure combustion chamber where it would be a pre ignition chamber where you had the spark plug it would set fire to the fuel and the oxidizer and that would then blow like a blowtorch into the main combustion chamber where the main fuel and oxygen would actually go into and of course the fury of that would not propagate back into this smaller chamber thereby protecting the somewhat delicate ignition systems at least in the case of the space shuttle main engine by the way a lot of people see videos of the space shuttle launch sequence and the CV sparks going off below the main engines those are actually just there to burn off excess hydrogen and are not actually part of the main engine ignition sequence and speaking of the space shuttle it's also worth mentioning that those giant solid rocket boosters those are lit too many people think they're actually lit from the bottom know they're lit from the top there's a miniature version of the solid rocket off the booster at the top right it's a pyro that fires like a solid rocket booster and it fires down the middle of the engine there's a channel all the way down in the middle of these solid rocket boosters there is carefully shaped to match or to provide the thrust profile they need but of course they're these of course have all sorts of important safety systems to make sure they don't trigger until everything else is in working order with the space shuttle anyway of course this is just one tiny detail from a larger ignition process getting the fuel into the combustion chamber can involve a whole mess of amazingly scary plumbing which more often than not is being driven by miniature versions of the Rockets themselves and of course that is a story for a future episode until then I'm Scott Manley fly safe you

35 thoughts on “How Rockets Are Ignited – Things Kerbal Space Program Doesn't Teach”

  1. Something I've wondered about is the synchrony of the SRBs. If one ignites and the other doesn't, what happens? Would the space shuttle flip over? Likewise, how do they ensure that they burn out exactly together, as one at full power while the other burns out would be uncontrollable. Do they design them to gradually taper off thrust rather than go out all the sudden so that the difference in thrust is small enough for main engine gymballing to compensate for?

  2. I want your T-shirt soooooo much. I live in Edinburgh is it only available in the US?? Knowing my luck it’ll be custom made? 😢

  3. For the SR71, the first engine start uses TEB after being wound up by large automotive engines. In flight, the engine continues to run, but the TEB is needed to relight the afterburners because the JP7 is very resistant to lighting in the first place. There is a story of one of the start carts catching fire near an SR71. Someone grabbed a pan of jp7 and threw it on the fire putting the fire out.

  4. As I remember, keeping the motors light was a problem as well. I seem to remember during the Apollo development they had do a lot of experiments to stabilize the engines. A way to test was to set off explosives in cones to induce flame outs. Could anyone verify this or add historical details?

  5. In KSP it’s not 100% reliable my game after a few launches it’s just makes the spark noise and does nothing

  6. I've been out in the Mojave Desert and watched people launch rockets to over 12,000 feet. There is always a delay from the time they send electricity to the top of the solid rocket motor and the rocket leaves. There will be a little smoke just before liftoff. How does NASA get an exact time the boosters will light with no pre-ignition smoke?

  7. Scotty, loved this. I said yep a lot. You are an amazing educator duder. When I tell people about launching ICBM's they cant understand that a tertiary rocket motor gets her engaged. I am submarine warfare qualified and I don't know if it was hyperbolic or not. Please do a video on Trident Missiles!!!!

  8. @Scott Manley I just noticed, at 3:33 in this video, that there's a 12" record sleeve behind your right shoulder with the title of "Rocket Fuel". Who made that album? Thanks! 😀

  9. That is so awesome Darth Vader and Stormtrooper t-shirt you have dude love it in the shape of a skull. Killed it

  10. 6:05 any Acquisitions Inc. fans here? I couldn't stop myself from shouting "GREEN FLAME". I've been conditioned, I guess, lol.

  11. The Soviets lit the blue er, sorry the RED touch paper 🤪. Seriously though the Russians are great engineers. The Soyuz rockets are safe, reliable, practical & well engineered. Apart from that they’re the most beautiful rockets to look at too. Respect 👍

  12. Rockets should always be lit by someone important (using a ceremonial match), such as the sitting president. Let's try this!

  13. Hello Rocket Nerds, what is the water that you can see during liftoff spraying in from the side (very well visible during Falcon Heavy Launch https://www.youtube.com/watch?v=sB_nEtZxPog)? btw israel is going to the mün on Thursday 11.4.2019 https://www.youtube.com/watch?v=VYd5vRjsfQE

  14. Big Scott, this is the simplest and best explanation of this topic I’ve ever heard. You Da Man!👍

  15. With >1000 successful ignitions I've found that for reusable systems a 'torch' ignitor burning the same propellants as the main engine. For one shot ignitions, the pyro ignitors are cheap/reliable. Hypergolic are messy,complicated,costly,etc,etc. and yes, I've used them,i.e. TEA, not my cup of.

  16. arstechnica.com/science/2013/04/how-nasa-brought-the-monstrous-f-1-moon-rocket-back-to-life/
    .. part of the NASA SLS team investigating Saturn system

  17. Ummm, Scott……. I think you're wrong on this one. I applied for the job of lighting the Apollo engines with my Bic lighter!

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