How do rocket engines work out in real spacecraft?

[travis575757]

So in KSP, all you have to do is a press a button and the rocket engine fires. I'm pretty sure this is not how it works in a real orbital vehicle but i can't find much information on it. Specifically what is done before a orbital maneuver is done with the main engine of lets say a Soyuz spacecraft or the Apollo service module. If there is different methods i would like to know. I have found some information on things such as the engines requiring heating and such to pressurize the tank. I want something that is, I guess, step by step how it is done.

[Chaia]

The most basic explanation:

Put some explosives into a tube with only one end open. Burn the explosives and you have solid rocket boosters. Similar this goes for fuel based boosters. Did you ever play with flame torch or something of that kind? That with higher pressure on the exit and you have fuel boosters.

Everything else is just about efficiency. Hotter engines mean more pressure on the expelled gas which results in higher thrust, as the thrust is based on the mass of the gas which gets expelled. Higher pressure = more mass at the same time = higher thrust.

Tanks are pressurized for more capacity. More pre-burned gas in the same volume of tank. Did you ever see how fuel tanks of rocket looks like frozen or something? If these tanks are cooled, you can stuff even more gas in the tanks.

Edit: If you like I can give you more technical explaination with formulas and stuff

The orbital maneuvers are beyond my explaining range, ask some experts here

[OrbitusII]

Very simply, Newton's Third Law. Here's a relevant image from NASA explaining it:

There's a lot more science behind it of course but that's the simplest explanation of how an engine works. I'm not educated on the other things yet, maybe ask me again in a few years.

[Streetwind]

If you fancy an in-depth yet entertaining read that teaches you a lot about what it takes to propel a rocket through space, hit google for a download of the ebook "IGNITION! An Informal History of Liquid Rocket Propellants by John D. Clark". It's out of print, the download is perfectly legal. 233 pages of history about lighting explosive things on fire and hoping for the best

[Alshain]

I kinda wondered how they fire in space too. Considering they need a sparking mechanism on the launchpad.

[astropapi1]

If you fancy an in-depth yet entertaining read that teaches you a lot about what it takes to propel a rocket through space, hit google for a download of the ebook "IGNITION! An Informal History of Liquid Rocket Propellants by John D. Clark". It's out of print, the download is perfectly legal. 233 pages of history about lighting explosive things on fire and hoping for the best

Commenting for later.

[TythosEternal]

So in KSP, all you have to do is a press a button and the rocket engine fires. I'm pretty sure this is not how it works in a real orbital vehicle but i can't find much information on it. Specifically what is done before a orbital maneuver is done with the main engine of lets say a Soyuz spacecraft or the Apollo service module. If there is different methods i would like to know. I have found some information on things such as the engines requiring heating and such to pressurize the tank. I want something that is, I guess, step by step how it is done.

Really, it depends on the engine. A surprisingly small number of rocket engines are capable of arbitrary in-flight restarts. There are a number of extra design considerations you have to make, including feed pressurization, turbopump transients, ignition reusability, and difficulties extending from any dependence on ablative surfaces.

If I understand your question correctly, though, what you're really asking about is the transient problem: how does an engine go from cold start to a fully-opened throttle? (Please correct me if this isn't the case.) This is a tremendously-difficult problem, and one that engineers usually solve empirically on the test stand. The flow behavior that takes place during transient events (I.e., startup, shutdown, and throttling) is turbulent, excitable, frequently discontinuous, and only on the margins of stability.

For example, consider the SSME startup procedure (slide 94 of the PDF linked here: https://www.google.com/url?sa=t&source=web&rct=j&ei=qUsXVM7EBoiONsfsgbAN&url=http://large.stanford.edu/courses/2011/ph240/nguyen1/docs/SSME_PRESENTATION.pdf&cd=4&ved=0CC4QFjAD&usg=AFQjCNE8N8kdYHGKVYKlxhgJFjyLwxi9UQ&sig2=JL7kwmC9VhUk3vixoIDXPg). That's one hell of a convoluted startup curve, with vastly different profiles for each component. For every point of inflection on that contour, there is an empty shell of an exploded SSME (or turbopump) sitting out in the desert that proves that particular step is necessary (OK, not literally, but very close to it). It's fascinating stuff.

The transient problem and related stability issues become more challenging as you scale the size of your engine in an attempt to increase thrust performance. This is an interesting problem that both the Americans and Soviets ran into around the same time when building bigger and heavier rockets during the space race. They ended up taking two very different approaches: NASA's team used some very innovative explosively-driven tap-testing techniques to explore impulse responses for engine under stress. They were then able to isolate and control the most problematic failure modes, which enabled the successful operation of behemoths like the infamous F1. Conversely, the Soviets circumvented the problem by routing a single turbopump systems' products through multiple, smaller combustion chambers and nozzles, an approach they have continued and refined with engines like the RD-180. It's funny how simple choices can lead to significant long-term programmatic discrepancies.

[Seret]

So in KSP, all you have to do is a press a button and the rocket engine fires. I'm pretty sure this is not how it works in a real orbital vehicle

For some types of engines it pretty much is. Some engines are very simple, you open a valve and the two components of the fuel automatically ignite when they mix.

[Dkmdlb]

I kinda wondered how they fire in space too. Considering they need a sparking mechanism on the launchpad.

That's why the so often use hypergolics in engines designed for in-space operation.

A hypergolic rocket propellant combination used in a rocket engine is one where the propellants spontaneously ignite when they come into contact with each other. The two propellant components usually consist of a fuel and an oxidizer. Although hypergolic propellants tend to be difficult to handle because of their extreme toxicity and/or corrosiveness, they can typically be stored as liquids at room temperature and hypergolic engines are easy to ignite reliably and repeatedly.

[B787_300]

SIGH.... time for me to use my education that i am spending soooo much money on to explain...

In Space there is no AIR. Here on earth when you light a fire, the air provides the oxygen to let the burnable material burn. In space or when you need to have something burn really fast (think Gunpowder), you have to have a stored source of oxygen. In gunpowder it comes from Potassium Nitrate (aka Stump Remover, KNO3). The oxygens get broken off in the burning process to burn what ever. For Liquid Engines the most common Oxidizers are LOX, RFNA, WFNA and some other Ammonium compounds. All these mix with varying degrees of success with the fuels commonly LH2, RP1, NH4, and some other very specialized ones... All chemical burning reactions of ORGANIC compunds produce heat, CO2, CO, and H2O. LH2/LOX is a bit simpler creating HEAT and water.

The heat is what is important because it keeps all the reaction products as a gas. Gasses are nice to work with because they are compressible. The nozzle design is of great importance and can really effect the performance of the engine. See DeLaval Nozzles for more. As the gasses are going though the nozzle and thrown out the back of the rocket, there is a resultant force (newtons 3rd law) that pushed the rocket forward. As a side physics note for forces and accelerations of a rocket you CANNOT use F=ma you have to use F=(d/dt)(vm) because both mass and velocity are changing.

Now some one mentioned how rockets are started in space. There are some fuel and oxidizer combinations that are Hypergolic (they ignite upon contact, no external energy is needed). Those are the best to be used in space so you dont have to worry about an igniter system. BUT hypergolics often have the drawback of they give less Thrust and have less Isp (specific impulse). Other upper stages are designed to not use hypergolic propellants, like the F9v1.1 Upper Stage. To ignite the engine there is literally a slug of hypergolic propellant that is fired into the combustion chamber, where it reacts hypergolically with the LOX and starts the reaction (if you want to REALLLY get specific, the TEA/TEB system is not Hypergolic it is Pyrophoric (it ignites when the temp is cold)). On the F91.1 upper there are 3 such slugs meaning the engine can be relight 3 times.

On pads before the ignition of the main engines it is common to see sparklers. Those are not the primary ignition source of the engines. The primary source is either an electric sparker in the engine or a Hypergolic slug as discussed above. The Sparklers are there to burn off any escaping Fuel or oxidizer before it can "pool" on the pad and pose an explosion hazard.

Most of this I have to know because of the degree I am studying for (Aerospace Engineering), the rest I have learned from various sources like watching hours of NASA tv, John D Clark's book IGNITION. Feel free to ask any questions.

[Kryten]

Pyrophoric=ignites in contact with air. Useful for starting engines on the ground, not so much in space.

[B787_300]

Pyrophoric=ignites in contact with air. Useful for starting engines on the ground, not so much in space.

TEA/TEB is pyrophoric which is why they use it, it also just kicks off the RP1/LOX burn

[shynung]

If you fancy an in-depth yet entertaining read that teaches you a lot about what it takes to propel a rocket through space, hit google for a download of the ebook "IGNITION! An Informal History of Liquid Rocket Propellants by John D. Clark". It's out of print, the download is perfectly legal. 233 pages of history about lighting explosive things on fire and hoping for the best

Agreed.

BTW, here it is.

[Elthy]

I dont think you understood the question right. I assume the question is what Astronauts (or people in the controllroom) do to get an engine running, they wont simply press shift...

If im wrong with that its still an interessting question.

[trekkie_]

rockets produce thrust because the pressure pushes off the bell curve. what you basically get is molecules bumping off of it at high pressure and heat, producing a force against the inside of the exhaust area. it's like when you blow up a balloon, the air pushes against the insides, outwards, and causes it to expand. except the rocket is open ended. of course, there is a lot of energy loss because not all the exhaust gets turned into usable thrust.

as for how an engine fires up, the most simple explanation without getting overly complicated is there are these really really strong liquid pumps that pump both the fuel and oxidizer into a chamber where they either ignite on contact or are ignited through some other means, like electrical or a third party catalyst chemical.

and as for the steps needed before maneuvers, well that's highly dependent on the maneuver and the design of the craft. there's always a lot of checks and heating stuff up, etc. But for the most part, astronauts don't really fly things by hand, pre-programming does a lot of that. mostly all an astronaut has to do is make sure the ship is good to go for the maneuver.

Edited September 16, 2014 by trekkie_

[shynung]

I dont think you understood the question right. I assume the question is what Astronauts (or people in the controllroom) do to get an engine running, they wont simply press shift...

If im wrong with that its still an interessting question.

I'm guessing there's a separate control for engine throttle and starting, each controlling the injector inlet pressure and injector valve + ignition system, respectively. Some engines can only be turned on/off, so they might have a control switch. Others, like the Apollo LM descent engine, can be throttled very far down, and likely to have a throttle stick similar to an aircraft.

Of course, that's if the spacecraft in question is entirely manually-operated, a la KSP. Real spacecrafts have more advanced computer-controlled systems, where the astronauts or mission control could simply type in delta-v and t-minus information for the next maneuver burn, and the computer then calculates the time to start and end the burns, along with throttle level, and either automatically do the burn themselves, or guide the astronaut that holds the controls to make the burn according to its results.

[Nuke]

i rather like pressure fed systems like hydrazine engines. they are vastly simple compered to something like the ssme. you only have a valve to the combustion chamber and likely a second for an inert pressurant. then you have a simple bladder in tank design. you inject pressurant into the tank to compress the bladder, and then open the valve that sends the contents of the bladder to the engine. comes in contact with that platinum catalyst and boom, ignition! starting the engine likely entails flipping a switch or two (though this kind of engine is more likely to be seen in a space probe than a manned vessel).

[magnemoe]

I dont think you understood the question right. I assume the question is what Astronauts (or people in the controllroom) do to get an engine running, they wont simply press shift...

If im wrong with that its still an interessting question.

Mostly they program the burn somehow in a less intuitive way than in KSP, however I guess the result work a lot like a mechjeb burn, rocket keep direction, wait until the set time and burn the required fuel then shut down again. This is for an burn in orbit, either course correction or an large burn to moon or other places.

Liftoff is automated, this is done because the computer runs main engines up to max power, check that everything look fine and then release and fire SRB.

[Nibb31]

I dont think you understood the question right. I assume the question is what Astronauts (or people in the controllroom) do to get an engine running, they wont simply press shift...

If im wrong with that its still an interessting question.

This is also how I understood the OP question.

During Apollo, there were all sorts of procedures that the astronauts had to follow before a burn. Different engines with different propellants were used for different burns, so each engine had different procedures. They had to stir tanks, enable various safeguards, disable others, check consumable levels and pressures.... For the whole detail, you should have a look at the Apollo Operations Handbook which describes all the procedures. You'll understand why Apollo astronauts required several years of training to get the hang of all the systems.

The Apollo LM engine was probably the simplest engine because it used hypergolics, meaning that the fuel and oxydizer ignite spontaneously on contact. They used this because it's the most reliable type of engine and uses the less amount of moving parts. And yes, it did have a big yellow START button. Of course, there were safeguards: you had to enable several circuit breakers and activate some safety valves before you could start the engine.

However, most burns on Apollo were controlled by the Apollo PGNCS computer. They would input the parameters of the burn (basically the XYZ orientation of the spacecraft, start time, and duration), and the computer would count down, point the spacecraft into the correct direction, and start and stop the burn accordingly. There was no throttle (there rarely is a throttle on rocket engines), so the duration of the burn controlled the dV that was imparted.

Nowadays, modern spacecraft work pretty much in the same way. Burns are controlled by a computer. The role of the astronauts is to monitor the systems and input the burn parameters into the computer. Flying a spacecraft is much closer to using MechJeb than flying an aircraft with a stick and throttle.

Edited September 16, 2014 by Nibb31

[Seret]

I'd imagine modern spacecraft are considerably simpler if comparable advances in aircraft are anything to go by. I used to work on two main types, starting the engine on the 1960's vintage one was a complicated process involving several people and specialised GSE. The 1990's vintage one had a startup procedure of select "Master engine on", press "Engine start" and watch the RPM spool up.

[mrfox]

Or... in the case of apollo 13, they had to do a mid-course correction burn using their lander engine (the one that hasnt been damaged by the previous explosion), with no nav computer because they needed to conserve battery power for re-entry - just a stopwatch, a mark on the window, a big start/stop button and lots of jockeying on the joystick - just like KSP!

[mrfox]

http://www.capcomespace.net/dossiers/espace_US/apollo/apollo13/transcript.htm#mcc5

Edited September 16, 2014 by mrfox

[magnemoe]

Or... in the case of apollo 13, they had to do a mid-course correction burn using their lander engine (the one that hasnt been damaged by the previous explosion), with no nav computer because they needed to conserve battery power for re-entry - just a stopwatch, a mark on the window, a big start/stop button and lots of jockeying on the joystick - just like KSP!

yes, and that is another thing, you need to get potion data from ground control. This is true for any craft, at least not in orbit around another planet where you probably mostly use it for potion.

[Laie]

I assume the question is what Astronauts (or people in the controllroom) do to get an engine running, they wont simply press shift...

Well, not exactly SHIFT, but I suspect there is a button. I'm quite sure that they don't need to get out and turn a crank.