Question for REAL rocket scientists

How does the fuel and oxidiser in a liquid fueled engine keep from beeing pushed back through the injector with all the extream high preassure in the combustion chamber?

kpklein said:

How does the fuel and oxidiser in a liquid fueled engine keep from beeing pushed back through the injector with all the extream high preassure in the combustion chamber?

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The fuel and oxidizer don't get pushed back up because they are at a higher pressure than the chamber. Remember that the chamber has a giant hole in the back (the nozzle).

The fuel and oxidizer are, on simpler rockets, pressurized by helium, keeping them flowing into the chamber.

On bigger rockets, there are pumps that force them into the chamber at high pressure.

IANARS but ... the Wikipedia article on liquid propellant rockets (https://en.m.wikipedia.org/wiki/Liquid-propellant_rocket) explains that pumps or other devices are used to inject the fuel and oxidizer into the engine at a higher pressure than in the chamber.

Good question, BTW.

-- Roger

Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).

To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.

Thanks. I have been wondering about that for a while.

Mushtang said:

Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).

To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.

Click to expand...

Mushtang,

You are confusing pressure and force. The force on the 1/8" opening is lower but the pressure is still 1000 psi. There smaller the area, the smaller the force for a given pressure. This is in part how hydraulics work. A small force over a small area can be converted to a large force on a greater area at the expense of the distance the fluid/piston travels. The burning fuel also presses evenly (relatively) on the sides of the combustion chamber as well (Pascal's principle). There is a pressure drop along the length of the combustion chamber as the mass flows out the nozzle. The pressure would be equal throughout if there was no mass flow.

Doug

Your all wrong, It's magic. Just ask David Copperfield!:rofl:

dcbertelsen said:

Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).

To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.

Click to expand...

Mushtang,

You are confusing pressure and force. The force on the 1/8" opening is lower but the pressure is still 1000 psi. There smaller the area, the smaller the force for a given pressure. This is in part how hydraulics work. A small force over a small area can be converted to a large force on a greater area at the expense of the distance the fluid/piston travels. The burning fuel also presses evenly (relatively) on the sides of the combustion chamber as well (Pascal's principle). There is a pressure drop along the length of the combustion chamber as the mass flows out the nozzle. The pressure would be equal throughout if there was no mass flow.

Doug

Click to expand...

That's so funny, because last night I was at a gas pump and for some reason my post came to me and I was thinking to myself, "Wait, did I say that the reduced number is also PSI? I hope not, that's not what I meant!! I'll have to correct that if I did. And I bet someone has already caught my error."

Sure enough, you did. Heh. So yes, I agree with you completely. The reduction is not in PSI. A 1000 psi pressure on the surface will produce a force of 1000 lbs force on a one square inch area. On a 1/8" hole there will only be a 12.2 lb force against it.

The numbers were right, the units were jacked up.

Sorry about that.

Mushtang said:

That's so funny, because last night I was at a gas pump and for some reason my post came to me and I was thinking to myself, "Wait, did I say that the reduced number is also PSI? I hope not, that's not what I meant!! I'll have to correct that if I did. And I bet someone has already caught my error."

Sure enough, you did. Heh. So yes, I agree with you completely. The reduction is not in PSI. A 1000 psi pressure on the surface will produce a force of 1000 lbs force on a one square inch area. On a 1/8" hole there will only be a 12.2 lb force against it.

The numbers were right, the units were jacked up.

Sorry about that.

Click to expand...

Your forgiven, just don't make that mistake again!

I read on another website that rockets can't work in space. Since the website used "math" and "science", it must be true.

shrox said:

I read on another website that rockets can't work in space. Since the website used "math" and "science", it must be true.

Click to expand...

Don't go there.

kpklein said:

How does the fuel and oxidiser in a liquid fueled engine keep from beeing pushed back through the injector with all the extream high preassure in the combustion chamber?

Click to expand...

Simple... the pressure of the propellant is higher on the liquid line side of the orifice(s) than on the combustion chamber side.

For instance, if the combustion chamber pressure is 750 PSI, the line pump outlet pressure is probably over 1,000 PSI, to account for pressure losses through the line and in the case of some propellants (kerosene or LH2) pressure losses through the regeneratively cooled nozzle and combustion chamber (on regeneratively cooled rocket engines, whether they be of channel-wall or tube-wall construction). The higher pressure on the liquid propellant causes them to squirt through the combustion chamber injector(s) (or orifices on the injector plate face if it's a plate injector engine, or through the pintle nozzle into the combustion chamber on a pintle-injected rocket engine, despite the high pressures and temperatures inside the combustion chamber itself...

The propellant tank design, rocket structures, propellant ducts, valves, and engine components are all designed to work together... and a lot of the design depends on the type of rocket engine. The two basic types are pressure-fed and pump-fed. Most rocket engines are pump fed, especially for large high-thrust primary propulsion booster engines and large upper stage engines. Pump fed engines then break down into several more categories depending on whether they're ablatively cooled (have a solid "liner" in the nozzle which slowly burns away as the engine fires, taking the extreme heat with it) or regeneratively cooled (having either a series of tubes brazed together to construct the nozzle and combustion chamber, or having a double-wall "channel wall" construction, which is easier and cheaper). In addition there are several different types of combustion cycle, from gas generator engines, which are usually the simplest and cheapest, but also usually lower efficiency (though modern design, like on J-2X, is largely making up for that and closing the efficiency gap with other more complex designs like SSME, which uses tap-off cycle staged combustion, but is much more complex and therefore harder to build and more expensive). Other combustion cycles include tap-off cycles (which run the pumps from combustion gases bled from the combustion chambers, raising efficiency) and staged combustion engines which use preburners and lots of other stuff to enhance the combustion. Much like a car engine, the higher the operating pressure in the combustion chamber, generally speaking, the more efficient the engine tends to be. Back in the 60's, car engines running 11:1 or 12:1 compression ratios got better mileage with ordinary carbuerators than many smaller, lighter cars today with computer controlled, fuel injected engines, because of the high compression ratios (unfortunately higher compression ratios require higher octane gasoline, and also the higher combustion temperatures from higher compression ratios increase NOX emissions, which is why in the early 70's the auto-makers started producing low-compression ratio engines, dropping down to around 7.5:1 or so, which drastically reduced power and efficiency... they've since raised compression ratios somewhat with more modern catalytic converters and exhaust gas recirculation and stuff to help lower combustion temperatures and thus lower NOX emissions, while increasing efficiency and power). In the same way, the higher the combustion chamber pressure, the more efficient the rocket engine is (up to a point, anyway, and generally speaking). Staged combustion engines usually operate at VERY high pressures which helps explain their higher efficiency (specific impulse, or ISP in rocket terminology, which is roughly equivalent to "miles per gallon" in car engines).

Now, with pump fed engines, the main concern with the rocket tanks and propellant duct design is to deliver the propellant to the pump inlet impeller face without cavitation. This is important to the sizing of the propellant duct, and also a function of the propellant tank pressurization pressure. Propellant tanks in rockets are pressurized with either nitrogen (for non-cryogenic propellants) or helium (for cryogenic propellants which would liquify or freeze nitrogen) to usually in the range of about 45 PSI or so... this has two effects: 1) this increases the structural rigidity of the tank walls and/or stage structure, in the same way that 35 PSI of air pressure can allow a 1/4 inch thick car tire sidewall to support up to about 1,500 pounds, and 2) this increases the pressure pushing the propellant from the tank through the inlets (which are equipped with anti-vortex fittings to prevent forming "whirlpools" that could allow gas to be sucked into the propellant lines and mixed with the propellant, which would be a very bad thing, like cavitation). This also increases the pressure pushing the propellant down the propellant duct (line) and through the manifolds/valving into the engine's turbopumps, so that there is no cavitation. The turbopump then increases the pressure of the propellant from the tank pressure (minus line/valve pressure losses) to the engine inlet pressure (plus calculate valve/line/regenerative cooling jacket pressure losses). The propellants then flow through the proper valving, intercoolers, regenerative cooling jackets, and ultimately into the engine injector(s) and on into the combustion chamber. The higher pressure created by the pumps allow higher efficiency in the engine, and the pressurization of the propellant tanks allow them to be constructed thinner than they would otherwise have to be to withstand the load of the vehicle structures/payload above them, so long as they can withstand the pressurization of the propellant tanks (the tank pressurization increases the load-bearing capabilities of the otherwise thin tank walls, "stiffening up" the stage, and making it lighter than it would otherwise have to be to withstand the weight and flight loads imposed on it by boosting the upper stage(s) and payload above it).

In the case of pressure fed engines, which have no turbopumps, the pressure in the tank MUST be higher than the pressure in the combustion chamber, and by enough extra pressure to ensure proper propellant flow rates into the combustion chamber, and good propellant atomization and mixing. This means that whatever the combustion chamber pressure is, one has to calculate or measure the pressure drops and required propellant duct sizing to minimize pressure loss along the flow path, all the way back to the propellant tank. The tanks must then be designed to withstand that level of pressure and MAINTAIN IT throughout the burn of the rocket engine... For this reason, it causes two problems for designing pressure-fed stages-- 1) the propellant tanks must be constructed to be VERY robust to withstand the MUCH higher pressures than those typical of a pump fed engine, and 2) the pressure-fed engine combustion chamber pressure must be designed low enough that a suitable propellant tank can be constructed to withstand the pressure loads required for propellant injection into the combustion chamber, without being too heavy. This lower combustion chamber pressure than a typical pump-fed engine reduces efficiency substantially... which means lower ISP, and therefore more propellant required to do the same amount of work (lift the same payload to orbit, or lower payload capability with the same propellant loadout, depending on how you want to look at it). The other side of the coin is, the propellant tanks have to be made MUCH stronger to withstand the enormous pressure forces trying to blow the tank apart from the inside... stronger generally means thicker, and thicker tank walls means heavier, and more weight means less payload. The strongest, most efficient pressure vessel design (and the simplest to construct) is a sphere. Unfortunately, a sphere is not a particularly efficient shape for a large rocket stage, although the famous Soviet N-1 moon rocket WAS designed with spherical first and second stage tanks for both the LOX and RP-1 propellants, due to their enormous size and the lack of experience in Soviet industry in making such gigantic pressure vessels in other shapes. (Which is why the N-1 looks the way it does, with its conical shape, and also part of the reason its performance capability was about half of the Saturn V's, despite being slightly larger than Saturn V and having half-again the liftoff thrust... the spherical tanks lead to extremely long and heavy (inefficiently designed) intertanks and fore/aft stage structures (forward and aft skirts and thrust structures) which makes the stages much heavier than if they used a cylindrical pressure tank with hemispherical pressure domes on the fore and aft ends as Saturn V did, or common bulkheads). These problems make pressure-fed engines largely restricted to low-thrust requirement, but high-reliability needs such as manuevering engines like the OMS on the shuttle, SPS on the Apollo CSM, the lunar module descent engine and ascent engine (LMDE/LMAE) and other such "thrusters" and such where the complexity of a pump fed design (which can lead to more failure modes, despite the higher efficiency), are outweighed by the simplicity and robustness of a pressure-fed design (which is simpler, despite the lower efficiency). In addition, for such applications using smaller quantities of propellants, spherical tanks are less of a problem, and usually fit much better in smaller stages or service modules, etc. This isn't to say that pressure fed engines MUST be designed around spherical tanks; far from it-- NASA and others have proposed building large, sometimes mega-sized pressure-fed boosters basically since the beginning of the space age-- BUT cylindrical pressure-fed propellant tanks are built more like SRB casings (which must also withstand the combustion chamber pressure since they ARE the combustion chamber!) than they are like typical thin-wall pump-fed rocket liquid propellant tanks. In fact, the largest rocket ever conceived (IIRC), Sea Dragon, would have used gigantic pressure-fed rocket engines and mammoth pressure-fed propellant tanks, launched from the ocean. NASA has also looked at various "minimum cost designs" built around large, high thrust, low efficiency pressure-fed engines and simple but heavy pressure-fed thick-wall tanks for low cost boosters, heck they even had a pressure-fed booster (and then boosterS, ala LRB's) slated for use on shuttle, before they switched to SRB's instead. Unfortunately no large pressure-fed booster engines and stages have ever been constructed ("Large" as in Saturn V class-- N-1 was a pump-fed rocket, in fact with highly efficient staged-combustion engines; the reason they went with spherical tanks was due to engineering/construction issues, not tank pressurization levels).

Hope this helps! OL JR

Mushtang said:

Because the injector openings are so small, they don't see enough pressure to keep a hefty pump from pushing the liquid out. The pressure inside the combustion chamber presses against all surfaces inside the chamber. Since there is an opening on the back end the pressure is unequal, and the pressure from the burning fuel only presses against surfaces on the front and sides of the chamber (this is what pushes the rocket forwards).

To see the difference, a 1000 psi pressure on the surfaces of a combustion chamber will only exert 12.2 psi on a 1/8" diameter opening (Unless my calcs are off). Not that this is the pressure or the openings in rockets, just using these numbers to show how little pressure there is on a small opening compared to an entire square inch of chamber wall.

Click to expand...

Pressure is pressure, regardless of surface area... it doesn't care if it's pressing on a 1/8 inch diameter injector orifice or a 10,000 square inches of combustion chamber wall or nozzle wall. Now the FORCE is different (the TOTAL amount of energy pushing on that surface area is equal to the pressure times the surface area... for instance, 1,000 PSI pushing against 1 square inch creates 1,000 pounds of force on that square inch, but for the surface area of say a 1/8 inch diameter injector hole, the force would be (3.1415 times radius squared (Pi times r squared)= surface area times the pressure (say 1,000 PSI-- you can do the math). But the pressure is the same.

This sort of thing used to trip up farmers in the old days when they used cisterns to supply their house water supply. Cisterns are a low-pressure system typically used with windmills, which deliver the water to a large tank (the cistern) up on a tower (legs) that then feeds water into the house and barn by gravity. Farmers used to believe (mistakenly) that putting a larger drop pipe coming down from the bottom of the cistern to the house would increase water pressure in the house. It does not... the only way to increase water pressure in the house is to have a HIGHER cistern tower... This is why city water towers are SO tall-- the higher the tower, the higher the pressure at the bottom of the water column. A column of water in a standpipe is under (IIRC going strictly from memory) 0.4 PSI per foot of height... SO, for instance, a column of water 10 feet high will be under 4 PSI at the bottom of the pipe, even if it's just open to the air at the top (not being pressurized by a pump or anything else). So, if you had say a 5 foot deep cistern on top of a 10 foot tower, the pressure at the ground level would be 15 times 0.4, or 6 PSI. Of course the pipes come up out of the ground inside the house, so if your sink is 3 feet above the floor, and the house is 2 feet above the ground, that's 5 feet or 2 PSI pressure drop due to gravity pulling down on the water in the pipe going up to the sink. The size of the pipe makes no difference to the PRESSURE, although of course a larger pipe can carry more VOLUME of water, and with a lower pressure drop due to resistance to flow inside the pipe. (Which is why pipes on long runs, say to distant water troughs in a back pasture, are usually a larger size than those for short runs, like to a house near the water well or water main.) For instance, I nearly got cost-sharing from USDA to put water troughs on the back pastures of our farm-- but their rules required the run be measured and their formula applied to calculate the required pipe size for that length run for their requirements. It came out to 400 feet of run, and he checked his pressure-drop charts and found that a 2.5 inch pipe would be required, which would have meant I would have had to hire someone to install the pipe underground, or rent a backhoe and do it myself... which wasn't cheap. I had planned on running 3/4 inch black poly tubing to the trough, which didn't meet their requirements, but WOULD deliver the water to the troughs, albeit at a substantial pressure loss (so the water flows slower, but it still gets there) because I had a tractor-mounted subsoiler that I constructed a ripper tube-laying boot and bolted it to the back of the ripper shank to open a deep slot in the soil, and using an idler pulley at the bottom, the tubing would be fed into the boot and under the pulley, burying it in the ground quite efficiently. As it turns out their requirements were going to cost me more, with them paying half the bill, than I planned to spend in total on the project (my half doing it to THEIR government requirements would cost me more money than just doing it myself my way, and paying ALL the bill myself) so that's what I did-- did it my way, cheaper, and it works fine...

Later and hope this explains it better... OL JR

GDJ said:

Don't go there.

Click to expand...

What he said...

Later! OL JR

PS... something just came to me, thinking about it...

The moon hoaxers claim that it was all filmed in a studio... but the famous "Falcon feather and hammer" experiment on Apollo 15 proves that ain't true... Dave Scott, in replicating a famous experiment described (but untested) by Galileo centuries before, dropped a falcon feather and a hammer at the same time, to prove that Galileo's hypothesis that all objects fall at the same speed in a gravity field regardless of mass, if one subtracts aerodynamic drag effects. Galileo couldn't test his hypothesis due to the lack of a large vacuum chamber, but on the airless surface of the moon, Dave Scott could. He dropped the feather and hammer at the same time, and they hit the moon's surface at the same time, just as Galileo had postulated.

Course I guess the tinfoil hatter's would claim they weighted the feather down or something to overcome the air drag or something stupid like that... oh well... back to your reg-gurrr-ly scheduled program...

One more difficulty you get with pumping fuel/oxidizer into the combustion chamber is that the combustion tends to happen in pulses, it's very difficult to get it to burn at a constant rate. The fuel/ox burns and the gases expand through the nozzle, and afterwards there is a pressure drop in the chamber until the inflowing mixture ignites. This causes the infamous "pogo" effect, that makes a rocket engine behave more like a pulse-jet, with significant resulting vibration. You get the same issue, albeit somewhat less pronounced, with hyrid engines (motors?...) too. That's wny the U/C hybrids like the Ratt Works have a floating injector, to help keep the tank pressure relatively constant. Solid fuel motors don't have as much of an issue, because the fuel/ox is just sitting there waiting to burn... it doesn't need to be introduced into the chamber. Pogo oscillation has been a problem for propulsion engineers since Day One, and nearly killed the Saturn V early on.

cerving said:

One more difficulty you get with pumping fuel/oxidizer into the combustion chamber is that the combustion tends to happen in pulses, it's very difficult to get it to burn at a constant rate. The fuel/ox burns and the gases expand through the nozzle, and afterwards there is a pressure drop in the chamber until the inflowing mixture ignites. This causes the infamous "pogo" effect, that makes a rocket engine behave more like a pulse-jet, with significant resulting vibration. You get the same issue, albeit somewhat less pronounced, with hyrid engines (motors?...) too. That's wny the U/C hybrids like the Ratt Works have a floating injector, to help keep the tank pressure relatively constant. Solid fuel motors don't have as much of an issue, because the fuel/ox is just sitting there waiting to burn... it doesn't need to be introduced into the chamber. Pogo oscillation has been a problem for propulsion engineers since Day One, and nearly killed the Saturn V early on.

Click to expand...

Solid motors are subject to something similar to pogo, called "motor buzz". It's caused by much the same thing-- pressure pulses in the motor causing combustion to speed up or slow down... As a solid motor burns, the expanding propellant gases can create a reverberating pressure wave that travels up and down the motor core as it's burning, many times a second. This "buzz" is sometimes audible (somewhat like the whistle of a "skyrocket", but from different causes). Motor buzz can be as serious an issue for solid motors as pogo is for liquid engines. Ares I was projected to have severe motor buzz problems due to the additional length of the five-segment SRM versus the four-segment shuttle booster, and also the fact that the shuttle boosters, mounted in pairs on either side of the ET and pushing against a thrust beam under the LOX tank at the front end of the external tank, were free to vibrate up and down by about six inches in flight, using the thrust cross-beam as a kind of "shock absorber" to damp the vibrations instead of transferring them entirely to the stack. With the upper stage mounted on top of the single SRM on Ares I, there was no cross-beam, and therefore no "shock absorber" for the motor buzz. Ares I was being designed to use four huge "C" shaped mounts in the interstage to isolate the upper stage from the first stage motor buzz, allowing the "C" shaped mounts to expand and contract under the loads imposed by the motor buzz. Interestingly enough, ALL solid motors experience SOME level of motor buzz, from 1/4A motors up to Shuttle SRB's-- it's sort of like a tuned pipe on a model airplane engine, or headers on a racecar, where the timing of the pulses is taken into account in the construction of it so that it creates a partial vacuum at the exhaust port to help scavenge the cylinder of spent gases and increase power. Of course with motor buzz, the idea is to design the motor to minimize the effects of it to the greatest extent possible.

While combustion instability in liquid rocket engines can be a problem, it's a fairly straightforward engineering problem to solve nowdays. Pogo isn't DIRECTLY caused by combustion instability, BUT it IS a contributing factor... Pogo is usually a coupling of combustion instability and the natural frequency of the propellant ducts of the rocket... when the two frequencies come into phase, the rocket motor produces slightly more thrust, coupling with a pressure wave travelling down the propellant duct and through the valvework and turbopumps of the engine, creating a pressure spike in the propellant, which of course injects more propellant into the combustion chamber, creating another spike. This is followed by the "wave trough" where the spike tapers off in tune with the reverberation of the pressure wave up the propellant duct, causing lower pressure at the injectors, reducing propellant flow and thus engine thrust. Then the waves reverberate again and the pressure spikes again, causing another thrust "spike... this cycle can happen very quickly and if the rocket design causes these two frequencies to be "in tune" it can rapidly turn into a feedback loop increasing in intensity.

Usually the solution to pogo is to add accumulators (pressurized gas bladders) to the propellant ducts upstream of the turbopump inlets, to "absorb" these pressure spikes in the propellant duct, or else to change the duct design to decouple the frequencies of the combustion instability from the natural frequency of the propellant duct reverberations.

That's how they solved it on Saturn V-- adding accumulators to the first stage propellant lines. They didn't do it to start with because accumulators are another component that adds weight, and thus takes away from payload. The Gemini-Titans also suffered from some pretty bad pogo effects, which were tolerable in a nuclear missile (since warheads are inanimate and designed to withstand over about 40g's of deceleration at reentry anyway) but were quite bothersome in a crewed launch vehicle. Can't recall offhand if they added accumulators to the Gemini Titans or not...

Later! OL JR

luke strawwalker said:

Pressure is pressure, regardless of surface area... it doesn't care if it's pressing on a 1/8 inch diameter injector orifice or a 10,000 square inches of combustion chamber wall or nozzle wall. Now the FORCE is different

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Thanks for correcting me, after someone else had already pointed it out, and then I came back and explained that I understood the mistake (typo) I'd made.

Mushtang said:

Thanks for correcting me, after someone else had already pointed it out, and then I came back and explained that I understood the mistake (typo) I'd made.

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Sorry, replied as I read the thread...

I don't read the whole thread and all the replies and then go back and answer stuff-- it screws up my train of thought... LOL

So, sometimes it makes me look redundant (or worse)... no offense meant...

Have a good one! OL JR