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Rockets- Hot Staging vs. Drifting.

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travelling at 3 or 4 Gs of acceleration then that dropping to 0Gs in less than half a second then increasing to 2-3Gs in another half a second could really have some adverse effects on the crews necks

Not really, fighter pilots experience greater those kind of changes in acceleration all the time, and they're sitting up moving their heads around. Astronauts are going to be lying on their backs with their heads nicely restrained. Having said that, jet knucks do get strong necks.

The reason astronauts are lying on their backs is that G is much easier to tolerate if it's acting along that axis, due to the fact the heart-to-brain distance in that orientation is zero. It's G acting vertically that sucks, especially negative G. I've taken about 5ish sustained positive G but felt that 1 negative was worse. Lying on your back you could take a ton of G for a sustained period and you'll be fine, especially if you don't have to do anything like move your arms. Jolt force (ie: rate of change of acceleration) in rockets isn't enough to do any harm either.

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The reason astronauts are lying on their backs is that G is much easier to tolerate if it's acting along that axis, due to the fact the heart-to-brain distance in that orientation is zero. It's G acting vertically that sucks, especially negative G. I've taken about 5ish sustained positive G but felt that 1 negative was worse. Lying on your back you could take a ton of G for a sustained period and you'll be fine, especially if you don't have to do anything like move your arms. Jolt force (ie: rate of change of acceleration) in rockets isn't enough to do any harm either.

I think that was the reasoning behind the F16's reclined pilot seat.

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I think that was the reasoning behind the F16's reclined pilot seat.

Considering the fact that the F16 is designed to make 9G maneuvers, yeah.

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I think that was the reasoning behind the F16's reclined pilot seat.

Yup. Unfortunately it made the pilots crane forward to get good visibility, and that's not healthy when you're hanging the weight of six heads off it. "Viper neck" is a real thing.

Considering the fact that the F16 is designed to make 9G maneuvers, yeah.

That just refers to the maximum that the airframe is built to take (minus safety margin), you wouldn't normally be pulling that much even in ACM. Pull too much G and you have to ground the aircraft, which is why they've got it on the HUD.

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Not really, fighter pilots experience greater those kind of changes in acceleration all the time, and they're sitting up moving their heads around. Astronauts are going to be lying on their backs with their heads nicely restrained. Having said that, jet knucks do get strong necks.

The reason astronauts are lying on their backs is that G is much easier to tolerate if it's acting along that axis, due to the fact the heart-to-brain distance in that orientation is zero. It's G acting vertically that sucks, especially negative G. I've taken about 5ish sustained positive G but felt that 1 negative was worse. Lying on your back you could take a ton of G for a sustained period and you'll be fine, especially if you don't have to do anything like move your arms. Jolt force (ie: rate of change of acceleration) in rockets isn't enough to do any harm either.

The difference is that the fighter pilots and F1 drivers have full control of what they're doing, and they can brace for maneuvers now imagine being a passenger in a rocket and having absolutely no awareness of when the staging will take place or how much time would pass between staging phases. Imagine being in a car which gets hit from behind, the impact itself might not be that hard, but the fact that the victim isn't aware of the impact until it has happened, means that they try to compensate for the change in acceleration. its that what causes the injuries, not the actually g forces endured.

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The difference is that the fighter pilots and F1 drivers have full control of what they're doing, and they can brace for maneuvers now imagine being a passenger in a rocket and having absolutely no awareness of when the staging will take place or how much time would pass between staging phases. Imagine being in a car which gets hit from behind, the impact itself might not be that hard, but the fact that the victim isn't aware of the impact until it has happened, means that they try to compensate for the change in acceleration. its that what causes the injuries, not the actually g forces endured.

The crew in rockets generally always knows when the staging events happen and I think most crew capsule have an MET display. So unless a unplanned change in the ascent profile happens, the crew always knows when staging will take place. Also they have special suits which are just worn at launch and reentry to protect them from any injuries related to high G-forces and fast changes in G-forces.

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The difference is that the fighter pilots and F1 drivers have full control of what they're doing, and they can brace for maneuvers now imagine being a passenger in a rocket and having absolutely no awareness of when the staging will take place or how much time would pass between staging phases. Imagine being in a car which gets hit from behind, the impact itself might not be that hard, but the fact that the victim isn't aware of the impact until it has happened, means that they try to compensate for the change in acceleration. its that what causes the injuries, not the actually g forces endured.

There are multi-seat fighter aircraft, where only one of the pilots knows exactly what to expect G-wise. I've also seen some (hilarious) videos of people riding shotgun in racecars where they clearly are not able to predict where and when the G-forces are coming. Neither scenario is particularly dangerous.

Car accidents are a whole other thing, instantaneous G-loads are often in the 20s and can reach into the 100s.

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The difference is that the fighter pilots and F1 drivers have full control of what they're doing, and they can brace for maneuvers now imagine being a passenger in a rocket and having absolutely no awareness of when the staging will take place or how much time would pass between staging phases. Imagine being in a car which gets hit from behind, the impact itself might not be that hard, but the fact that the victim isn't aware of the impact until it has happened, means that they try to compensate for the change in acceleration. its that what causes the injuries, not the actually g forces endured.

Correct me if I'm wrong, but astronauts who ride rockets typically knows how is it going to be done. Most likely, they would have known when to expect staging events, how and when to do maneuver burns, and what to do in a typical something-went-wrong incident far before they stepped into the rocket. Even if an unfamiliar situation arises, ground control is always in contact, and is usually capable to transmit information regarding appropriate actions to be taken.

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As people have said, astronauts know when to expect major events during ascent like engines firing. Also, it really doesn't matter. When you're lying on your back you can easily take the acceleration both in terms of jolt and absolute g, whether you anticipate it or not.

I've ridden back seat in a combat aircraft. I think having control of the stick would probably help a bit, but your amount of g tolerance, the direction, rate and magnitude of g would count for more. I went green even though I had the g suit and was straining like a mother hubbard, there's only so much you can do even if you know what's coming!

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of course your right, the astronauts will know more or less when the staging phases are going to take place to within a few seconds, but a very delicate and lightweight piece of engineering such as a satellite which has been designed to be manufactured on earth and operate in microgravity may not be able to counteract such forces, besides for the quantity of fuel it would take to control the G forces to make the crew and cargo have a more comfortable journey would be negligible compared to the total fuel required to reach orbit in the first place. Also preburning during staging would help to force the earlier stages into a descent, and this could either force the hardware on a predictable course which will guarantee that it breaks up on reentry, or it can target a safe (ocean) area for hardware which might not break up.

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a very delicate and lightweight piece of engineering such as a satellite which has been designed to be manufactured on earth and operate in microgravity may not be able to counteract such forces

Read this post from earlier in this thread.

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Oh dear. I wonder why such a thing have to exist. If this thing is on the second stage, it will disintegrate the first stage booster when it hot-stages.

P.S. Looks like the censoring software gets a little overboard.

Actually that happened on the pad in Russia with a fully fueled ClF3 first stage exploding under a ClF3 second stage that was accidentally ignited during launch prep.

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Actually that happened on the pad in Russia with a fully fueled ClF3 first stage exploding under a ClF3 second stage that was accidentally ignited during launch prep.

Source, please? You got me curious.

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Source, please?

The Nedelin catastrophe.

"A short circuit in the replaced main sequencer caused the second stage engines, while being tested before launch, to fire inadvertently. This detonated the first-stage fuel tanks directly below, destroying the missile in an enormous explosion."

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its that what causes the injuries, not the actually g forces endured.

i'm just gonna go really off topic here, but i've been dying to know for years: why do people replace the second "that" with "what" here? i have never seen this written anywhere except in colloquial internet writing.

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i'm just gonna go really off topic here, but i've been dying to know for years: why do people replace the second "that" with "what" here? i have never seen this written anywhere except in colloquial internet writing.

I apologize, its a colloquial term used in parts of the north east England, proper English would of course be "it's that that causes" or "it's that which causes" but occasionally regional accents seem to seep into people's writings :)

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why do people replace the second "that" with "what"

Because it's funny.

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i'm just gonna go really off topic here, but i've been dying to know for years: why do people replace the second "that" with "what" here? i have never seen this written anywhere except in colloquial internet writing.

I think it's like an answer for "What's that causes..."

"What's that causes the injuries?"

"That's what causes the injuries."

Edited by Brenok

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I think it's like an answer for "What's that causes..."

"What's that causes the injuries?"

"That's what causes the injuries."

This whole comment makes very little sense

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I think it's like an answer for "What's that causes..."

Except nobody says 'what's that causes'.

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Oy! Stick to the topic, please.

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Actually that happened on the pad in Russia with a fully fueled ClF3 first stage exploding under a ClF3 second stage that was accidentally ignited during launch prep.

According to the source that you cited, "hypergolic UDMH [was used] as the fuel and a saturated solution of dinitrogen tetroxide in nitric acid [was used] as the oxidizer." None of these chemicals actually have fluorine in them, let alone ClF3. (Unsymmetrical dimethyl hydrazine, or UDMH, is H2NN(CNH3)2, containing only hydrogen, carbon and nitrogen.)

Sorry if I'm being harsh, I just want to clarify.

Anyways.

One benefit of hot-staging in model rocketry (I don't know if it applies here) is that all of the electronics and fancy gadgets used for staging can be jettisoned after the top stage is lit. (The electronics are mounted on the top part of the first stage and thus are separated when the stages are decoupled.) This probably doesn't hold as much significance once the rockets are scaled up, though.

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I had no clue about hot staging, and had only read about The Nedelin catastrophe, never seen video.

Cool thread!

/I did know about O2F2, though by the name FOOF. :)

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I had no clue about hot staging, and had only read about The Nedelin catastrophe, never seen video.

Cool thread!

/I did know about O2F2, though by the name FOOF. :)

Lol, I like FOOF better. Implies a nice genteel understated sort of reaction.

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