# Why does a jet's combustion energy propel the engine forwards rather than acting equally in all directions to create zero thrust? [duplicate]

I'm a novice. I have high school physics and I've seen many explanations of how a jet engine works. To paraphrase: Fans compress air into a chamber where combustion energy is created, which spews out the back of the engine, propels the plane forwards in a typical Newtonian action/reaction way, and also spins more fans that in turn spin the previously mentioned compressor fans. I know that's not a precise explanation but it just serves to introduce my question.

Why does the combustion chamber not act more like, say, a hand grenade? That is, why doesn't the energy act equally in all directions? What makes it directional? I have never understood this from the dozens of school-level explanations I've seen on the topic. Why does the energy from combustion cause the "low pressure turbines" at the rear of the engine to drive the compressor blades and not the other way around? Or the most likely scenario in my flawed understanding, why doesn't the energy act equally on all the fans in the front and back of the engine to create no net propulsion?

• @Bianfable it will take me a little time to digest the answers there but I think the question is essentially the same as mine. Not sure if I should delete this question now. Jan 26 '21 at 13:42
• @Jay Don't worry about it, it will likely be marked as a duplicate and may help others find the relevant question. Jan 26 '21 at 13:51
• @jay613 The short answer is "combustion doesn't begin until flow direction has already been established" - the longer answer is in that linked question and answer. As an example, when I'd start the engines in my SH-2F helicopter (in the Navy) the compressor would already be turning at 12% rated speed (which means air flowing aft) before we introduced fuel and ingntion into the compressed, flowing air in the combustion section. (T-58-GE-8F turboshaft engine). Jan 26 '21 at 14:28
• @jay613, be careful to not misunderstand some points. Energy does not "act on fans" or anything else. Energy is (in my view) a very difficult concept in newtonian mechanics, and it can be seen as a quantity of the system or fluid (like air). Inside the combustion chamber, chemical energy is given to the air flow. No mechanical force act on it and as KorvinStarmast said, the flow is already established before entering in combustor. Jan 28 '21 at 21:04
• @Acsed. thanks, that gives me more to think about. The idea that the combustion energy is transferred to the flowing air in a somehow directional manner, and not directly to the turbines. The details may be beyond my ability to understand, but it will help me to get past the inadequate school-level explanation. Jan 28 '21 at 21:19

You are making air expand in a contained space but giving it an escape path in a particular direction that is easier to take than making the container blow up. When I was a kid and it was possible for kids to buy firecrackers from the corner store, I used to like to make rockets out of them to propel balsa gliders. You rolled the end of the firecracker between your fingers to make the packing fall out so it was open at one end, and taped it to your glider. When the fuse burned down and the gun powder lit off (just after you threw the glider), the expanding gases went out the open end, making a nice rocket that ran about a second. Close off the open end, like a normal firecracker, so the gases have nowhere to go, and it blew up (which would occasionally destroy my gliders when I didn't roll out enough of the packing - good thing they only cost a dime in 1967).

So air is being made to expand and you give it somewhere to go. Since the front end is open as well, why it doesn't go back out the front is simply because the engine is packing in air from that direction faster and harder than the burning air can push back, relative to its fairly easy path out the tail pipe, so it takes the "easy way out" (jet engines can back flow if things go off kilter - you will sometimes see flames coming out the front during "stall/surge" events).

In taking the easy way out, it's still has to accelerate to high velocity to make it through an opening that is constricted, and there's your Newtonian thrust reaction.

The last part is, you put a windmill, a turbine, in the high velocity flow and use the windmill to drive a compressor to keep the air being packed in at the front. You are extracting some of the energy in the flow going out the turbine section to power air-packing equipment at the other end, to keep the cycle going by itself without any external help. Basically you are redirecting a part of the energy of the burning fuel air mixture to keep the cycle going instead of making thrust.

In a piston engine, you have a similar energy transfer effect, in a radically different format, to self-sustain the cycle. Air burns in a cylinder's combustion chamber and forces the piston down to drive a propeller, but some of that rotation energy is being used to drive the adjacent piston on its compression stroke, redirecting a part of the energy of the burning fuel air mixture to keep the cycle going instead of making torque at the prop.

The jet engine does the same thing in a linear fashion using aerodynamic forces instead of closed spaces inside cylinders, depending on the inertia and velocity of the flow (like a hallway packed with people running in one direction - forget about trying to run the other way) to channel all of the forces where they need to go to keep the cycle going.

• Or match rockets. Take a paper match, a bit of aluminum foil, and a pin. Wrap the foil around the match head, using the pin to leave an escape nozzle along the length of the match. Then hold a lit match under the head. It ignites, the gasses escape out the nozzle, and your match rocket flies off. Jan 26 '21 at 18:09
• Kids I knew used to make match rockets with multiple match heads packed into a pen barrel. They were more dangerous I think. Pack a medicine container with match heads and you have a little flashbang grenade! I put 4 firecracker rockets on another kid's "25 center" balsa glider (the ones with the rubber band prop), 2 on the wingtips and two on the fuselage. We got all the fuses going and launched it. It climbed to about 15 feet, then went shooo shooo shooo BANG!!! Spun down with one wing in balsa shards. Boy was that kid mad... Jan 26 '21 at 18:39

The flow of gasses can be guided mechanically. The turbine blades allow it to pass through, while the fan blades block it.

For example, take a look at the Tesla Valve in which fluid flows easily in onn direction, but is obstructed in the other direction.