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What are the main key lessons has rocket science helped you to discover?

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The oddest discovery – for me – is the fact that the rocket exhaust…that enormous jet of high power hot gasses is actually at very ordinary pressures.

You kind of imagine that with all of that raw power, the exhaust flow would be at some ungodly pressure – but actually, it’s so low in pressure that under some circumstances there can actually be a problem with the outside air pushing INTO the exhaust bell!!

It’s a phenomenon known as “flow separation”.

So if you look carefully around the edges of the bells – you see there is this
irregular white stuff that looks like it’s flowing around the edges (in video, it’s kinda dancing around there)…what you’re seeing is where the rocket exhaust isn’t quite expanding enough to completely fill the bell – and the outside air is pushing inwards. So despite all of that phenomenal power and thrust – regular old air pressure can push back against it enough to push upwards INTO the bell.

Blew me away when I heard that.

If you already understand this stuff – you can just shake your head and be amazed that I had to figure this out.

If you too are somewhat amazed – then keep reading and I’ll explain it.

WHAT THE BELL IS FOR:

The bell of a rocket has the job of keeping the flow of gas out from the reaction chamber as parallel as possible.

Kinda like a laser beam that neither grows nor shrinks as it shoots out into the environment.

This makes sense because if any of the exhaust gasses go out to the sides – widening the exhaust plume – then some of its energy is consumed in pushing it laterally – and because of the law of Conservation of Momentum – that’s motion that’s not resulting in the rocket going upwards…it’s inefficient.

So a 100% PERFECT nozzle would produce a perfectly cylindrical flow of gas out of the back of the rocket.

But the problem with doing that in Earth’s atmosphere is air pressure. The pressure of the atmosphere is pushing inwards on the gas causing the flow to be compressed inwards – which results in the gas to be not moving out as a perfectly parallel cylinder.

And out in space, the pressure of the exhaust gas itself is higher than the vacuum of space so it balloons outwards – with the same overall loss of efficiency.

So the rocket bell is designed to allow the high pressure exhaust gasses coming from the reaction chamber to spread out within the bell just enough that when it leaves the end – it’s at EXACTLY air pressure – and there is no inefficient inward squeeze or outward push.

Rocket scientists call an inward squeeze “Under-expansion” and an outward push as “Over-expansion” – and neither is desirable.

Hence you design the bell to allow the exhaust gasses to expand just enough to perfectly match the air pressure.

HOWEVER:

Air pressure changes with altitude – at launch, it’s 1 atmosphere – and by the time you get to orbit it’s zero atmospheres.

So, annoyingly – the ideal size of bell changes with altitude. At sea level, it’s rather small to keep the exhaust at sufficiently high pressure to match air pressure.

But in orbit – you really want an infinitely large bell to expand the gasses out to zero pressure. (Obviously you can’t do that – but you want to make it as large as you reasonably can.)

So if you set up the bell housing to be perfect at sea level – then it’ll be awful once you get to (say) 20,000′ of altitude where the air is starting to get thin.

But if you design it to be great at 100,000′ – then it’ll be awful at sea level.

THE ART OF THE COMPROMISE:

So what the rocket scientists do is to pick some intermediate altitude and set up the bell to be the right size at that altitude – it’ll be a little less than perfect at sea level and a little less than perfect above that altitude but that’s life.

THE COOL PART:

Once you know all of this – you can look at rocket exhausts and tell how efficiently they are running.

When there is UNDER-EXPANSION – the air pushes in on the exhaust too hard and squashes it down to a little point – which then kinda crosses over and expands again.

You get a row of pretty diamond shapes trailing behind the rocket…you see them a lot in ground-testing:

Why so much in ground testing? Well, because you’re doing the test at sea level – which is (inevitably) lower altitude than the “compromise” altitude that they should have designed the nozzle for.

So – you may now declare with confidence: “Oh – they under-expanded that one!”

These two images are from a recent Falcon-9 launch – and at sea level (on the left) you can see how the exhaust gasses are being squeezed to a point below the rocket (“under-expanded”) and on the image on the right (which was at an altitude of about 22km – the exhaust is ballooning out to maybe ten times the width of the rocket. (“over-expanded”). The color of the exhaust is also rather telling…on the left it’s white hot because the hot gasses are being squeezed – but on the right, they’re expanding outwards and cooling rapidly to a much cooler yellowish color.

In extreme conditions, the pressure of the exhaust gasses isn’t enough to allow it to even reach the end of the rocket bell without being compressed inwards – and then the outside air can push backwards into the nozzle.
This is the cause of that “Flow Separation” – and it happens when the air pressure is too high and the nozzle diameter is too large:

Then, after the nine main engines cut off – and the 1st stage has separated, we see this interesting shot:

This was taken from the second stage – looking downwards at the moment of separation. The white engine nozzle belongs to the single engine of the second stage – but you can see that it’s HUGE! It’s almost the same diameter as the entire first stage (which has NINE rockets packed in beneath it).

So why such a large bell? It’s because we’re now in a vacuum – we want massive expansion of the exhaust gasses in order to minimize over-expansion – and an infinitely large bell would be great…if a little impractical.

So the SpaceX designers wisely created the largest rocket bell they could fit inside the first stage fairing!

COMPROMISE IN ACTION!

So going back to that first photo – the REASON that air is sneaking in around the bell housing is because the exhaust is being pushed inwards by the atmosphere which is actually at higher pressure than the rocket exhaust…and that’s because they designed these engines to hit peak efficiency – with the perfect expansion ratio – at somewhat higher altitude than from which they’re launching.

CONCLUSION:

I love to geek out on this stuff – and when you can watch a launch with actual understanding of what’s going on under the hood (or the “bell” in this case) – it adds so much to the enjoyment.

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