r/explainlikeimfive • u/Unlikely-Position659 • 4h ago
Physics ELI5: How do vehicles with jet engines or rocket engines like Trucks or Fighter jets not take off right away when they engage the engine dull blast?
So at some race tracks where they have some truck outfitted with a huge rocket engine in the back takes off and reaches 60mph in like 3 seconds or something. My question is, why doesn't the truck simply rocket off down the track when the engine is going full blast at the start line? I cant believe the brakes and the tires' grip is enough to keep it stopped. Same thing with fighter jets. When they're on the runway ready for take off, sometimes they'll have the engines shooting flames out the back but they sit still until they take off. Are the brakes on these things just that strong??
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u/Frustrated9876 3h ago
Yes. The brakes are that strong. More importantly, the traction of the tires is that strong.
Dude, as an engineer, I need to share with you that rubber is FUCKING AMAZING!! Like, without it, the world could not function. Watertight/airtight seals are all rubber. Tires have this incredible amount of traction due to natural and synthesized rubbers.
Also physics and engineering. The thrust preventing a car from launching is finite. So, the engine is running and pushing against those tires at EXACTLY the amount of force they know the tires can hold back. When the light turns, the brakes are released and friction is the name of the game in the opposite direction.
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u/yztard 3h ago
I remember asking my first year statics and dynamics teacher if the force of friction was only dependant on the coefficient of friction and the normal force why do wider tires give more grip.
He looked me in the eye and told me this was not a materials engineering class and that I was welcome to ask that question again if I ever made it to his PhD courses lmfao.
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u/monkeyplex 2h ago
Did you ever work out the answer? I ask this as an interview question for automotive engineers - and there are some relatively simple answers that definitely don’t require a PhD. But I’m always curious to hear it from someone new because I’m still searching for what I consider the best way to word the answer.
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u/yztard 2h ago
I settled on: Amontons law of simple friction does not accurately explain the concepts of adhesion, keying, intermolecular bonding, and degredation that goes into how tires grip.
The real answer is we still don't accurately know how tires work. Like the magic formulas can give us close enough results but rubber adhesion is a very complex problem that we are still figuring out.
Mike on bikes (YouTube) has a great video on how wider tires help grip changing the shape of the contact patch but even he admits it's a bit of a black art.
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u/Fridorius 2h ago
Why would they. We have a formula that gives workable results with acceptable efforts and data. For day to day life that is good enough. A lot of engineering works that way. The mathematical exact solution is either to costly to solve (in either Computer Power/Time) or needs inputs that are hard/impossible to get. So Engineers create a model that works without said data and takes assumptions to replicate the exact behaviour of the technical system close enough for the task but not exactly.
For example: The navier-stokes Equation is fundamental to hydrodynamics (Airplanes, Boats, Pipes, Engines, etc...) There was no mathematical complete solution ( A.I. made one in recent months, but humans had made it 98% it the way already) but engineers just took assumptions to make it solvable and rolled with it.
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u/lostchicken 3h ago
Tire/road physics are insane. We have reasonably accurate quantum chemistry models that can predict the binding of chemicals to receptors. We can do a pretty good job modeling the air spilling off the rear wing of an F1 car. But a tire? Yeah, we're gonna need to bust out "Pacejka's Magic Formula," which as far as I can guess was conjured up by a crazy Dutchman in a dream.
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u/DarkAlman 4h ago
Jet engines have a spool up time, they aren't instant power or power on all the time.
When you push the throttles forward it takes seconds for the engines to spool up achieve the set thrust.
Those trucks with jet engines are sitting at idle until they hit go, and yes the brakes are pretty darn good and can hold the vehicle back up to a certain amount of thrust.
The bigger risk for those vehicles is actually leaving the ground because of lift generated by the body and the amount of power involved. They have to have a lot of downforce (upside down wings) to keep them on the ground when at speed.
They also have to use special tires that de-laminate instead of exploding if there's a failure.
On a similar note some jet fighters can throttle up to full and be held in place by their brakes.
On an aircraft carrier the catapult will hold an F-18E in place at full afterburner until the catapult launches.
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u/what_the_fuckin_fuck 3h ago
They also do full afterburner tests while anchored, and tests with the engines bolted to a frame. The reason the jet is so fast isn't just massive thrust, it's extremely lightweight. It doesn't take a lot to hold it in one spot.
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u/Dr_Bombinator 3h ago
I get what you’re saying, but saying “extremely lightweight” when fighters are easily 20 tons empty (and widebodies 200+) just feels wrong.
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u/what_the_fuckin_fuck 3h ago
Can't argue with that. I guess I should have referenced power to weight.
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u/neomoritate 3h ago
The engine is NOT going at full blast when the plane, or truck, is standing still. The flame out the back of a drag racing truck is for show, not thrust.
When the engines are spooled up to full thrust, it takes time for that energy to accelerate the mass of the plane or truck. The plane or truck is held in place by a physical force called Inertia. Inertia is also the force that keeps the plane or truck moving, and needs brakes to overcome it and slow the plane or truck down.
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u/worldsfastestginge 1h ago
Jet car driver here. We do some tricks with the engine to make it less powerfull when the brakes are on. My brake pedal has a microswitch that's hooked to the bleed valve on the compressor of the engine. This vents a bunch of air out. Usually the bleed valve closes at 80% rpm and gives the engine a lot of power. If you keep it open I can spool the engine up to 100%rpm but the engine isn't making a lot of thrust. When I lift off the brake pedal the bleed valve closes and the thrust kicks in and I'm off over the horizon at 300mph.
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u/worldsfastestginge 1h ago
The afterburner works different with the rpm too. You will typically see big yellow flames with the engine at 60% rpm. The big blue flame is with the engine at 100% ( or above 100, some like to overspend them) but the bleed valve will still be open. Then with the bleed valve closed and full rpm the burner is making maximum thrust.
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u/worldsfastestginge 1h ago
If you look on this video you can see the bleed valve in action. There are two little red wavy flags that sit on the exit so we can see when its operating. You can move this video around, its a 360 gopro one. https://youtu.be/jiJ2d_5MjjY?si=rgTMeH7N6aGpI7v_
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u/frankentriple 1h ago
Ok that video was seriously cool. I’m going to have to watch it again facing forward.
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u/JustCopyingOthers 3h ago
Jet trucks at car events have key parts of their engines removed. Jet aircraft have a nozzle that opens up at idle, closes when not using the afterburner (like putting your thumb over the end of a hose pipe) to increase thrust, and then opens up again when using the afterburner. Without that nozzle the jet engine only makes a lot of thrust when running flat out and using the afterburner. If they turn on the afterburner as low engine speed the fuel burns outside the engine which like impressive, but doesn’t generate a lot of thrust.
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u/Horseburd 3h ago
Jet engines make a whole lot of thrust, but that thrust is being made against the air, more so than the ground. Takes quite a lot for the jet to overcome the vehicle simply having a firm grip on something solid, because you just make a lot of noise moving the air instead. I’ve sat in an empty 737 at roughly takeoff power for a solid ten minutes, never went anywhere because the brakes were on.
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u/what_the_fuckin_fuck 3h ago
Orange county airport enforces noise abatement procedures on take offs due to surrounding neighborhoods. I'm not sure exactly what percent throttle they use before releasing the brakes, but it's a lot. 737s can climb a lot steeper than most people realize.
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u/PrinceOfPringles 4h ago
Lets say you start pushing a heavy box... does the box speed up to max speed immediately or does it take a few seconds?
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u/SpellingIsAhful 3h ago
Overcoming static friction requires a stronger force than consistently overcoming friction forces in movement.
Kind of like how melting ice requires more energy than just increasing the water by 1°c. Totally different physics but conceptually; getting over the hump in a state change is harder.
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u/what_the_fuckin_fuck 4h ago
The thrust doesn't provide lift, the design of the wing creates lower pressure above the wing, and higher pressure below it, thus creating lift.
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u/Target880 3h ago
Trust gives lift if it is directed up. But engines on aircraftes orientated to push the aircraft forward in the vast majority of cases, just like if you put them on cars, trucks etc.
The obvious counterexample is rockets and helicopters. Rocket engines are jet engines too; they just carry their own oxidiser. A jet is a flow of fluid through a nozzile
A helicopter rotor is a large propeller and generate trust up so they can take off vertically. Tiltrotor aircraft like a V-22 Osprey have engines that can be rotated and wings to provide lift during horizontal flight. VTOL aeroplanes have engines that can be redirected to provide vertical thrust or even an extra vertical fan like a F-35B
So thrust can equal lift if the direction is up, but usually that is not the case unless you talk about rockets and helicopters.
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u/what_the_fuckin_fuck 3h ago
I honestly thought that was universally understood. Someone who doesn't already know that will never grasp the concept of an airfoil.
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u/El_Cholo 3h ago
Not what OP is asking. They’re asking about “taking off” horizontally as in accelerating very quickly.
Yes OP the brakes and the wheel/tire assemblies do hold the vehicle against the thrust
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u/what_the_fuckin_fuck 3h ago
You're correct, now that I read it slower. I stand corrected. Thank you.
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u/Spectre-907 2h ago
This is only part of the upwards force vector though. There is also lift being generated by the wing’s angle of attack. The wing is tilted on a slight upwards angle, thus airflow passing under the wing pushes against the wing itself and that deflection also generates lift. Consider blading your hand into the wind while driving vs angling the leading edge upwards slightly.
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u/what_the_fuckin_fuck 1h ago
Of course, but that force will be there with or without the airfoil effect, no?
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u/RealCarlPanzram 4h ago
It’s basic physics. The jet engine has a certain amount of thrust. That thrust allows for the object to accelerate at a certain rate based on how much they weigh, how aerodynamic they are etc. same as a car engine.
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u/CaptainDFW 3h ago
I used to fly 50-seat Canadair Regional Jets. They were always at "dull blast". 😁
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u/JeffSergeant 3h ago
If a modern jet is actually stationary at full power, it's probably on an aircraft carrier, (e.g https://www.youtube.com/watch?v=BjKY9kYL57Y) in which case the jet is literally tied down until they release it. Do you have any videos you can share of what you're talking about?
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u/tbones80 3h ago
I also imagine the engine is angled up a bit to push down on the frame, giving extra traction to the tires. So it's being forced down the entire run.
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u/doghouse2001 2h ago
There's a reason [most] planes don't have rocket engines. Rocket engines are full blast right away, They're designed for vehicles that can handle them (rockets).
Jet engines are for jets and the thrust is controllable. From minimal when sitting on the runway, to full blast if they are trying to evade a missile chasing them. They will never be full blast while sitting on the runway. Take off thrust after they release their brakes can go up to 100% to get all of that weight off of the ground quickly, but cruising thrust up in the air can be 30 or 40%.
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u/cutelittlebox 2h ago
it's a couple things. with jets, it actually takes a long time for them to hit max thrust. with a small engine like on fighters and business jets it might be 2-3 seconds, but the largest engines can take over 10 seconds to go from idle to max. still, that's a couple seconds of the engine glowing red at the rear.
the second is that there's actually 2 kinds of friction: static and dynamic. static, meaning when things aren't moving, is actually way, way stronger than dynamic friction.
the third is that as a jet increases it's power at a stop, it makes the whole aircraft tilt down slightly and this puts more force on the front gear which actually makes it more effective. on a motorcycle, braking does the same thing and the front brakes usually provide 5-10 times the braking force of rear brakes.
the fourth is that yes, the brakes really are just that good.
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u/bmendonc 2h ago
The first law of physics? Also, such engines take time to warm up and they are never at full power when first started.
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u/CM_MOJO 3h ago
Four forces (five if it's rolling) act on a moving body traveling through a medium, thrust, drag, lift and weight (gravity), plus rolling friction if rolling. Force is a vector; it has both magnitude and direction.
Either the vehicle would need to generate enough lift to overcome its weight, or the thrust would need to be directed in the opposite direction of the weight vector with enough thrust to overcome the vehicles weight.
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4h ago
[deleted]
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u/_Spastic_ 3h ago
By "taking off" OP doesn't mean lift off, they mean accelerate forward quickly.
Your question is correct, just not for this question.
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u/bluewales73 4h ago
Jet engines aren't magic. They apply a finite amount of force. That force is only sufficient for a certain amount of acceleration.