I've run into problems a number of times trying to explain something which I think is really easy and obvious, and yet widely misunderstood. It seems to run into intense resistance from people who think they already know why I'm wrong before I start. I apologise for the tone, like a cannibal missionary talking to mild Anglicans, but it's getting to me badly. Am I really confused, or really bad at explaining, or both, or am I merely struggling with other people's aggressive human stupidity as well as my own?
Really, causally, how do chemical rockets work?
Rockets are propelled by the gas coming out at the back, aren't they? Nope. They aren't.
If this troubles you not at all, please, go away and do something more useful. If you already think I have no idea what I'm talking about, please read on.
Anyone who pays enough attention to tell me where, not just that, I'm wrong will be doing me a service. Previous claims that I am wrong (a Flat Earther, anti-science, unteachably stupid) have not convinced me of anything except that somebody doesn't get it.
We all know (I hope) that rockets are indeed quite able to propel themselves through a vacuum. They don't need any external anything to push against. Newton's Third Law, and all that. I wouldn't bother to mention this, except that those who don't already realise it seem strangely hard to teach otherwise, and those who do know it already seem to believe that I don't.
What propels a rocket? You'd think this question wouldn't drive anyone particularly insane, but people who don't realise that they don't know are driven to paroxysms of angry denial. Whatever they come up with just must be an explanation!
The problem exists at all because the whole idea that there is a difference between a covering law and a causal explanation seems utterly unintelligible to some. I'm not talking about sophisticated avoidance of causal explanations, perhaps on the grounds that they aren't as useful or as universal as was once thought. I'm talking about people who are perfectly happy with causal explanations, and think that they have one, and that I am misunderstanding it.
Repeat until clear: I know how to solve those elementary mechanics problems from 'A'-level papers too. I am not proposing alternative physics. But these mechanical exercises are naturally causal; efficient causes acquired their prestige from being invoked for exactly such problems. It really should be possible to give a fully causal explanation, and something odd is going on if one proves elusive.
No amount of telling me about the conservation of linear momentum will help. I know linear momentum is conserved. That doesn't offer a causal explanation of anything. You can't get causal explanations out of raw conservation laws. Telling me that there is a change in the momentum associated with the exhaust gases, so there must be a change in the momentum of the rocket, does not tell me why either happens, nor where, nor when, nor to what extent. There should be forces making momenta change. That's what causal explanations in mechanics are like.
I could expand at tedious length on lots of forces incidentally brought into play. I'll content myself with the big obvious relevant ones which affect longitudinal acceleration.
No amount of gas travelling backwards relative to the rocket will somehow push the rocket forwards. It can't. Its only effect is to apply a bit of drag to the rocket, and that drag is backwards. Where's the bloody force doing the accelerating? And don't tell me "there must be one", and I am a post-dated superstitious Mediaeval peasant to say otherwise. I agree that there must be one, but I see no reason to agree that this "must" be it. On the contrary, it's immediately obvious that it isn't and it can't be.
Exploding rocket-fuel doesn't push specially in one direction rather than another. Indeed, conservation of momentum lets you know that it doesn't. The aggregate momentum of the exploding fuel is the same as the aggregate momentum of the trickle of propellant mixture into the combustion chamber; tiny, and very likely backwards, anyway.
The fuel that directly flies out of the back of the rocket doesn't propel the rocket. It can't. The fuel that travels back through the rocket bouncing off the sides and the lip of the exhaust nozzle is actually impeding the rocket. The fuel that explodes forwards bounces off the closed end of the reaction chamber, and joins the rest of the gas flowing backwards, adding its little bit of drag.
Where is the rocket propelled? It ought to be utterly obvious. The rocket is propelled by the not-yet-exhaust rushing forward. More precisely, it's the bouncing off the closed end of the chamber. It's pretty much an elastic collision, and the momentum of the gas relative to the rocket pretty much reverses. That's where the momentum of the gas changes in a useful direction; and the same impact which makes the gas change direction pushes the rocket forwards. Action equal and opposite to reaction, and this time a large action with a large reaction, in the right direction.
It's exactly what a five-year-old might think propelling a rocket by a controlled explosion would be like; the explosion pushes forwards on the rocket. Because that's what happens, and the five-year-old is right.
A strange determination to know better comes over people, and they decide that it can't be that simple, and whatever complicated thing is true instead, they must already know it, even though they can't say what it is. They just pull out their argument from momentum and tell themselves that I obviously can't be following it.