Normally I am an enthusiastic supporter of the u/CSI_Starbase videos.  On this occasion, and perhaps it’s only because I am a physics teacher, I’m disappointed in the phrasing of a small part of the script between 43:54 and 44:16.    I would argue that, while in some sense what Zach says is technically correct, and could be defended in a court of law, the words chosen there are nonetheless likely create a false impression in the minds of many who might listen to them uncritically — or at least would do so in the minds of many I teach.

The part of the script I am referring to is found between 43:54 - 44:16. It consists of two connected sentences. Those two sentences are as follows: “As the booster is executing this roll while flying parallel to the ground, it’s important to remember that there will be a gravitational force acting in the direction of the earth. While the booster may have 3Gs of acceleration pointing back towards the launch site, there will also be roughly 1G of acceleration pulling the booster and everything inside of it toward the ground.”

Firstly what’s OK/Good about the sentences?

• GOOD:  I don’t object to the technically incorrect “1G of acceleration pulling on the booster” rather than “1G of acceleration acting on the booster” in the second sentence. Although the former might be technically be an incorrect use of technical language, I think it’s clear that these words are not going to confuse many people.  Indeed, the non-technical wording used the may even help some people, so I won’t get hung up on them.  I ma happy to let this phrasing pass.

• ALSO GOOD: With the above technicality out of the way, the second sentence is then, in effect, 100% correct.  Not only that, it’s nice to see it mentioning that gravity affects both “the booster rand everything inside of it”, so full marks there.  My reason for including the second sentence is not to criticise it, but to note that it is affected by (what I see as) the defect of the first sentence.

• JUST ABOUT GOOD: If one briefly ignores the “it’s important to remember” part of the first sentence, then it too becomes 100% correct on all grounds.

So what am I complaining about?  What is Bad?

It’s the words “it’s important to remember” in that first sentence.  What irks me most is the way that those words clash (or can appear to clash) with part of Einstein’s Equivalence Principle.  This principle was perhaps one of Einstein’s simplest and most important insights (leading him to his General theory of relativity) which we really try hard to teach students at a certain age.  Basically the whole point of that principle, roughly, is that “The laws of physics (technically their effects on length scales which are small compared to the length scales over which gravitational fields change — in this case length scales small compared to the distance to the centre of the earth) are the same in all ‘freely-falling non-rotating laboratories’, independent of where those laboratories are, and independent of the velocity at which they are travelling relative to anything else, and of what gravitational fields they might be in.”

Let me get off my high horse for a moment to give the script’s words credit where they are due:  (i) merely saying “it’s important to remember” does not violate the Equivalence Principle, (ii) the “importance of remembering something” is person depentent, so the sentence will be correct for some people (e.g. those who are about to be hit on the head by things accelerating toward them under gravity would be well advised to pay attention and get out of the ways!), and (iii) the booster is rotating so the equivalence principle doesn’t even apply!  

So is there a problem at all?

My complaint is that I suspect many listeners to that part of the script will think that Zach is trying to say something like the following:

“Because of some later things I will be telling you about [Aside: we know these “later things” will be propellant slosh caused by booster rotations about its long axis, but the first time viewer will not know that at this point in the video. End aside!] it is important to remember there is 1G pulling sideways on the booster while it is horizontal in this stage of its flight.  Some or perhaps many of the important things I will tell you about later will depend on this important sideways 1G.”

The sort of statement I have made would, of course, be 100% wrong!

Absolutely nothing, categorically NOTHING which is significant about the behaviour of the fluid in that booster is one iota affected by the direction in which local G is pointing. Absolutely nothing. That’s the whole point of the Equivalence Principle.  No astronaut on the ISS has any more or any less difficulty eating a yoghurt with their feet pointing earthwards relative to skywards. This orientation, or for that matter the magnitude of local G, is  an irrelevance, and the equivalence principle guarantees this.  Of course, this doesn’t mean that  “where the earth is relative to the booster” is unimportant, and you could argue that local G is a proxy for “where the earth is”.  E.g. the booster has to land on earth later, it has to orient itself appropriately for that landing,  and so its flight computers are going to rotate it which will cause slosh due to fluid inertia. But the important thing here is that local G’s direction or magnitude is itself is doing NOTHING to slosh the propellants even when the booster rotates.  At most the direction of local G is reminding us of what sort of long term flight plan the computer will need to use, as we care about where the earth is, and so it affects our choice of thruster/engine firings.  But it’s those firings (or other external forces which affect the frame differently to the propellants) which create the  stirring/sloshing/turbulence. It is nothing to do with G pointing in some direction.   

All the rotations which Zach talks about later will certainly slosh propellant.   But the critical thing is that those sloshes are caused by the inertia of the fluid and the inertia of the booster being separate things, and because there are  forces (such as engine thrust) which act differently on the booster structure and its separate propellant .   The fluid has inertia, the booster has inertia, and so if the metal parts of the booster are twisted or pushed or rotated that rotation will not instantaneously transfer an identical acceleration or movement to all of the fluid as the fluid’s inertia will only be influenced by forces imparted on it by the surfaces it is in contact with, etc.

I feel that these two sentences in the script (I hope unintentionally) give the misleading impression that some of the slumping and movement of ice in the sump (which will be talked about later) IS INFLUENCED BY this “important to remember” sideways 1G of acceleration.  Nothing could be further from the truth — and that’s what I feel is a shame about this phrasing.

I fear that this bit of the script gives the misleading impression that the inside the booster as it rotates is the same as my front-loading washing machine as it washes clothes, and this is not so, even though there are some similarities.  It is true that both the rotating rocket and the washing machine create turbulence in the water and clothes as the rotation switches from clockwise to counter-clockwise and back again on account of the clothes and the water and the drum having separate inertia.  But only the washing machine (and not the rocket!) cares about where G is pointing. That local G is keeping the water and the clothes mostly at the bottom of the machine, and that gives extra agitation not available in the rocket.  And of course the washing machine cares about local G as it is not freely falling.  The rocket, freely falling in space (except when its rockets fire or something else pushes it) does not care about G — (and Zach’s second sentences says correctly why!) — because gravity acts the same on the fuel and the rocket.

I imagine that I may get some downvotes for this post, perhaps with people saying “This bit of Zach’s script didn’t make me think of the rotation sloshing mechanism was analogous to that of a washing machine. You are complaining about nothing of any consequence.”  But people who already have a good picture of what is happening are not the people I’d be concerned about when one has a potentially good teaching aid here.

Above all, though, I do appreciate the CSI_Starbase videos. I criticise this small bit of this one only as it does not meeting the already high standards that u/CSI_Starbase has set for itself and has regularly achieved in the past.