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I don't know much about Rockets, but all that I have seen, from the Saturn V to SpaceX's Falcon 9 have the engine at the bottom. Doesn't this make the Rocket really unstable, like balancing a pencil on your finger and trying to lift it up? Why do we not pump the exhaust gases up to the top of the rockets before expelling them? Surely that'd give a more stable rocket with less chance of faliure?

Essentially, what reason is there for having the engine at the bottom, rather than at the top like a firework rocket?

geoffc
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7 Answers7

45

..what reason is there for having the engine at the bottom, ..

For the fundamental logical flaw in the desire to move the engines, please see this answer of Russell Borogove - it comes down to The Pendulum Fallacy (the exact same fallacy I made when considering other reasons for 'engines at bottom of stack'). For more on the The Pendulum Fallacy see links at the bottom of this answer and in Russell's answer.

In a nutshell the fallacy is based on the assumption that a 'top engine' layout is more stable than a 'bottom engine' layout. In fact, they are exactly equally as (un)stable and therefore other factors determine the decision behind engine placement.

This list below is some of the other reasons:

  • Heat. Rocket exhaust is very hot, therefore before the rocket is safe from them, the exhaust would have to come out of the engine and past the long fuel/oxidizer tanks (which often contain fuel or oxidizer at 'cryonic temperatures').1
  • Control. Rockets already need sophisticated control even to get where they are intended to go. That ability automatically ensures the rocket 'does not fall over' even under full acceleration.
  • Fuel flow. It is harder to pump fuel up to a rocket engine (against both gravity and the acceleration of the rocket), than to allow the fuel to flow down to the engine. This point has been disputed in comments below, along the lines that it is not a significant difference. We are hoping for more data to determine how much difference it makes.
  • Hagen von Eitzen mentioned in a comment below:- "Many materials and structures are more stable under pressure than under pull; in fact things you push in principle need not even be fixed to one another."
  • Hobbes also points out in a comment:- "It does complicate the engine layout: you'd need a turbopump at the bottom of the stage, and pressurized lines all the way to the top. This would make using a regenerative engine (which dumps the turbopump exhaust into the combustion chamber) much harder."
  1. The 'emergency abort' rockets of particular American (at least) launchers put a small rocket engine above the capsule, but this was only expected to fire for a very short time (and might have used solid propellants that don't need to be pumped anywhere). Also note how the exhausts of the rockets are angled outwards in efforts to keep the capsule cool, but in doing so waste some of the thrust of the rocket engines.

enter image description here

It's a little ironic that this 'real life example' of engines above payload is also shown on the Pendulum Rocket Fallacy page discussed in the answer of Russell Borogove/comment of Philipp. In fact, that small sideways jet near the top of the rocket seems to be a way of changing the direction of the combined rocket/capsule rig. I.E. further evidence that a top engine rocket is not self stabilizing, and requires active guidance.

Community
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Andrew Thompson
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    I suspect fuel flow isn't really an issue: the propellants are pumped at many times atmospheric pressure, adding a few bar due to having to pump uphill won't make much of a difference. It does complicate the engine layout: you'd need a turbopump at the bottom of the stage, and pressurized lines all the way to the top. This would make using a regenerative engine (which dumps the turbopump exhaust into the combustion chamber) much harder. – Hobbes Jun 28 '15 at 17:01
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    Not sure if this applies for accelerations that the human crew can survive, but: Many materials and structures are more stable under pressure than under pull; in fact things you push in principle need not even be fixed to one another – Hagen von Eitzen Jun 28 '15 at 19:50
  • @HagenvonEitzen That's such a good point I decided it had to be part of the answer, so I edited it in. Thanks! – Andrew Thompson Jun 29 '15 at 08:53
  • @Hobbes "(pump up/down) ..won't make much of a difference.." Given rockets operate very near to the limits of the materials and fuels, I could only be convinced by hard data that backs that up. (I'm not pretending I'd necessarily understand the data, but an engineering study that supported that view would be most convincing to me.) OTOH ... – Andrew Thompson Jun 29 '15 at 08:58
  • ... OTOH "It does complicate the engine layout: you'd need a turbopump at the bottom of the stage, and pressurized lines all the way to the top. This would make using a regenerative engine (which dumps the turbopump exhaust into the combustion chamber) much harder." Yep, that is another good point. Mind if I edit it into my answer? (I note you have your own answer.) – Andrew Thompson Jun 29 '15 at 08:58
  • @Andrew, go ahead, use it in your answer. I'll try to find some data on turbopump pressures etc. – Hobbes Jun 29 '15 at 10:04
  • @Hobbes "go ahead, use it in your answer." Thanks! "I'll try to find some data on turbopump pressures etc" And thanks again. I've already shoved that point a little further down the list on the basis that it has been disputed. – Andrew Thompson Jun 29 '15 at 10:33
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    Well, it works in Kerbal Space Program – asawyer Jun 29 '15 at 13:46
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    @asawyer You forgot to provide the mandatory reference. – jpmc26 Jun 29 '15 at 19:12
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    That small side jet exists to force it to be unstable. You don't want it going straight up only to be encountered by the derelict booster behind it. – Joshua Apr 15 '16 at 15:52
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Putting the rocket nozzles nearer the top wouldn't make the rocket any more stable; this is the well-known pendulum rocket fallacy.

In fact, some rockets have used a tractor (engine on top) configuration (Goddard's first liquid-fueled rocket in 1926, for example), but the advantages to the pusher configuration as outlined in the other answers here are dominant.

To be clear, the point of the pendulum fallacy isn't "tractor rockets are unstable"; it's "tractor rockets are not self-stabilizing and are not inherently more stable than pusher rockets given the same aerodynamics and guidance and control systems".

Russell Borogove
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    That's a ridiculous "fallacy." Putting the center of thrust above the center of mass provides a restoring torque to any deviation. That restoring torque may not be adequate to provide stability, but it will help. Other difficulties arise with exhaust, etc. but this "well-known fallacy" is, well, not a fallacy. – Erik Jun 28 '15 at 17:42
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    If the thrust line is misaligned with the center of mass -- as it always is, in practice -- how is that a restoring torque? How does that differ from the low center of thrust case? – Russell Borogove Jun 28 '15 at 17:49
  • Because you hang the payload from the engine assembly. In a conventional rocket (which is a better idea) the engines are rigidly attached. – Erik Jun 28 '15 at 17:51
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    Then an instability leaves you with a spinning engine assembly, from which the payload stably hangs, no? – Russell Borogove Jun 28 '15 at 17:56
  • Just mount the engines with a dihedral angle that reflects the expected magnitude of the possible instabilities. – Erik Jun 28 '15 at 17:59
  • @Erik: ARE the engines in a 'conventional' rocket really rigidly attached? I thought that at least some of the engines on every rocket are mounted in gimbals, for steering. Which you'd need in a tractor configuration anyway, because a rocket isn't launched on a straight path. – jamesqf Jun 28 '15 at 18:03
  • Yes, you would have to add the same gimbals if you wanted to follow a predefined flight path. But if you just wanted a rocket that could ascend without any control systems, the tractor would work. – Erik Jun 28 '15 at 18:04
  • I think the skycrane used on Mars is a good example of this. – Erik Jun 28 '15 at 18:06
  • https://youtu.be/p1WX0CATyn8 – Erik Jun 28 '15 at 18:08
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    Mounting the engines divergently doesn't help unless they are gimbaled or differentially throttled -- the same situation as for base-mounted engines. OP was thinking there was an inherent stability gain in the tractor case, and there simply isn't. – Russell Borogove Jun 28 '15 at 18:23
  • Except there is. Take a look at the Mars video. I don't believe those engines are differentially throttled and they are mounted divergently. The only reason they would need to be gimballed or throttled would be to target a specific landing point. – Erik Jun 28 '15 at 19:38
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    I am not an expert on the skycrane lander, but the engines are deep-throttling; it's certainly actively stabilized, whether by reaction wheels, differential throttle, gimbaling, or some other mechanism. https://en.wikipedia.org/wiki/Mars_Science_Laboratory#Powered_descent – Russell Borogove Jun 28 '15 at 19:47
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    Incidentally, the stabilizing function of dihedral wings on aircraft is a secondary effect emerging from the sideslip induced by the roll; it doesn't apply in the rocket case. https://en.wikipedia.org/wiki/Dihedral_(aeronautics)#How_dihedral_angle_creates_dihedral_effect_and_stabilizes_the_spiral_mode – Russell Borogove Jun 28 '15 at 19:49
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    Let us continue this discussion in chat. – Russell Borogove Jun 28 '15 at 21:00
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    @Erik https://en.wikipedia.org/wiki/Pendulum_rocket_fallacy – Philipp Jun 28 '15 at 22:02
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    @Phillip Yes, that wikipedia article references some old Geocites blog. It also assumes that the connection between the rocket and the engines are rigid. – Erik Jun 28 '15 at 22:05
  • KSP isn't real life, but watch from 1:50 on to see how engines that aren't fixed to the rocket and don't have a control system make top-mounted engines stable. https://youtu.be/Tx4cjP-GRAY – Erik Jun 29 '15 at 14:38
  • Spin-stabilized, and I suspect the turning towards vertical is hinging on an aerodynamic effect -- it would be informative to try the same rocket on the Mun. – Russell Borogove Jun 29 '15 at 15:50
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    The video is from 2012 and I'm unable to reproduce this effect in the current version of KSP, which has had a substantial aerodynamics overhaul. Even with longer, more wiggly mountings for the engines, and the engines deliberately canted to induce roll, if I destabilize it by dropping one engine to 95% thrust for just half a second, it continues to tip until it augers in. – Russell Borogove Jun 29 '15 at 20:29
  • Again, top mounted aids stability -- it doesn't guarantee it. – Erik Jun 29 '15 at 23:02
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    All other things being equal (aerodynamics, center of mass, active stabilization systems), what is the mechanism by which top-mounted thrusters improve stability relative to bottom-mounted? – Russell Borogove Jun 29 '15 at 23:07
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    I think the reason so many people fall for this myth is they don't understand rocket engine thrust is a vector which is not automatically aligned with (and in the opposite direction of) gravity's force vector.

    I believe the closest analogy is balancing a rod from the bottom in terms of the diverging stability of the opposing forces.

     –  Apr 16 '18 at 21:14
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Besides the heat factor brought up by Andrew Thompson, I think another big issue is decoupling spent stages. If the stage is below you during the decouple, there is no way it could physically come back up and hit you again (unless you decouple while the thruster is still powered). If the stage is above you, then you need some kind of trick to avoid the rest of the rocket ramming into the now-empty decoupled stage.

Another related point is the decouple itself: By its nature, the decoupler applies downward force on the element below and upward force on the element above. If the element below is the discarded stage, great! Now the junk is falling faster and the rocket got a slight boost. But if it's the other way around then the junk gets a boost and the rest of the rocket slows down, wasting Delta-V.

Superbest
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  • Playing devil's advocate, decouple while the first stage is still firing. At the moment of decoupling, its motors are now powering a considerably smaller mass, so the almost-spent stage will accelerate away from the rest of the rocket at high speed. And I don't think decoupling will exert a significant force on either component. Don't they just blow explosive bolts holding the stages together? – David Richerby Jun 30 '15 at 09:15
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    If your engines are on top, you could drop tanks and keep your engines. – Joshua Jun 30 '15 at 15:28
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    @Joshua You could, but often staging is set up so that subsequent stages use different engines also (often with less thrust but more Isp) so you may end up having to get rid of the whole stack anyway. But being able to decouple empty tanks while keeping other full tanks and the engine is quite useful, and perhaps a better argument for OP's suggestion than stability. – Superbest Jun 30 '15 at 20:07
  • @Joshua - I like that concept a lot. But Superbest is right: ditching the stages does allow you to ditch the combustion chamber/expansion nozzle/fuel-oxidiser combination for one that suits the altitude better. It would be great to save the weight of the engines, but from a static launch you need much more acceleration at low altitude, so carrying all of your engines into orbit is a waste. Maybe a radial tractor configuration: 6 engines, opposite pairs have matched characteristics, ditch pairs as you ascend. – WillC Dec 12 '16 at 06:58
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The thought of placing the engine on top of the rocket never crossed my mind. One of the reasons against it I see is as follows. In case of the traditional layout (engine at the bottom), the thrust vector of ALL engines is exactly in the direction of the intended movement of the rocket. While in case of the engine-at-the-top layout, we cannot direct the exhaust along the line of the movement, simply because underneath the engine there's the rest of the rocket. That means we have to use several smaller engines, each throwing its exhaust at an angle to the rocket body (like in the picture in the accepted answer). And the thrust vectors of all engines should sum up to create the main thrust vector in the direction of intended movement. Now, the more the angle the less power of each engine will be applied to move the rocket. When the angle is 90 degrees, there will be no thrust, with engines completely compensating each other's thrust. In other words, placing engine at the top makes the whole setup less effective energy-wise, i.e. more fuel will be required for the same performance.

Vitya Smirnoff
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When you exhaust hot gases at the top of the rocket, you have to shield the rocket body from those gases. You don't want the body to heat up and boil the propellant and oxygen.

Those shields would have to be thick and heavy, reducing the rocket's performance.

Also, thousands of rocket launches (and millions of flight hours by airplanes with jet engine exhausts at the tail) have shown that stability isn't an issue worth worrying about.

Hobbes
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They are not always at the base of the rocket. Here are two, albeit slightly quirky potential counterexamples, although in both cases it seems putting the nozzle near the top in a "tractor" configuration does not improve stability.

Goddard Rocket, from this answer

Goddard Rocket

Stabilo rocket, from this question

Stabilo rocket

uhoh
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  • counter examples designed by people who thought putting the engines at the top would increase stability. –  Dec 18 '20 at 18:28
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The other answers have covered why you don't put the rocket motor at the top but not covered this point of the question.

Why do we not pump the exhaust gases up to the top of the rockets before expelling them?

If you're pumping the exhaust gasses to the top of the rocket, the thrust of your rocket is exactly the power of the pump, minus losses due to inefficiency. What's powering the pump? Wouldn't it be better to use that power source to drive the rocket directly, instead of using it to move around the waste products of some messy chemical reaction?

So lets assume you just mean diverting the exhaust from a burner at the bottom of the rocket to nozzles at the nose. The power of a rocket comes from carefully shaping the combustion chamber and exhaust nozzle. Only a part of the thrust comes from the pressure of the exhaust gasses causing them to squirt out of the combustion chamber. Most of the thrust comes from the shaping of the nozzle, which lowers the pressure of the exhaust gasses and accelerates them to very high speeds. Fitting a pipe from the bottom of the rocket to the top (which would have to contain at least one 180° turn) would completely ruin the geometry of the nozzle, almost certainly to the point of making it not work at all.

You'd also need to contain the extremely hot exhaust gasses, which would be trying very hard to melt your pipe, losing energy all the way.

David Richerby
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