Rockets- Hot Staging vs. Drifting.

[Motokid600]

Im hard pressed to find any substantial information on hot staging rockets. The concept intrigues me. And it seems that the Russians seem to take a liking to the process. For those of you who don't know the process of hot staging involves firing the upper stage engines before the lower stage cuts out and separates. This raises a good amount of questions for me. First of all.. why do it? Most rockets stage, then drift for a good amount of time as the two stages fall away from eachother. But as on the Russian Soyuz the 3rd stage fires iirc a good 30 seconds before the main stage separates.

How does this not damage the rocket? I imagine the exhaust hitting the lower stage tank and rebounding back up into the engines causing huge damage. So why do this? Also.. drifting raises a few questions too. What determines how long the drift is before the next stage fires? The Saturn V for instance drifted a good amount of time before the J-2s ignited. Again im guessing so the exhaust is clear of obstruction?

And again iirc the Russian N1 was to be ALL hot staged... saying if it ever got to that point. So what are some good examples of this? Do any present day rockets other then the Soyuz hot stage?

And a bit of an off topic question.. why is it I cant do this in KSP? Drifting I mean... Even with FAR stage separation in ksp is never as clean as it is in real life. The stages tumble, the rocket dips... its just not clean. Why?

[R0cketC0der]

It damages the first stage of course, but you obviously don't care about that. I'd worry about the first stage exploding from unspent propellant igniting, but this doesn't seem to happen.

That doesn't happen because the fuel and oxidizer need to be mixed to ignite. So unless you heat up the first stage enough that the structural section between the oxidizer and fuel tanks are melting this doesn't happen.

[Francesco]

I But as on the Russian Soyuz the 3rd stage fires iirc a good 30 seconds before the main stage separates.

Nah, it's not that much, it's just a couple of seconds.

See it in this amazing video:

[goldenpeach]

Why do they fire the 3rd stage before the second stage separate?

Is it to have the engine at full throttle when the main stage separate?

[sgt_flyer]

The answer lies in the age of the soyuz concept. At this time, they were not sure they would have been able to ignite engines in a drifting rocket (because the fuel would also be in free fall). (Like Atlas LV-3B) so they ignite the upper stage (in soyuz case) while the lower stage still gives a positive acceleration. (Also, this means they don't need SRB / or pressure fed ullage engines)

Also, the top of the soyuz core stage fuel tank is protected with a conical shape, to further help to deflect the upperstage exhaust during hotfire.

For nasa's rockets, they finally settled in using ullage systems (like the saturn V insterstage ring, which had several ullage engines on it)

For ullage, they finally told that a positive acceleration of a few tenth of a g would be enough to settle the fuel in the tanks. (Although they would lose some delta-v during the drifting)

The turbopumps / gas generators are started as soon as they can pump fuel, then the engine ignited.

The S-IVB upperstage had some pressure fed auxilliary tanks (filled at launch, then refilled from the main tanks while the engine was running) for it's ullage engines, to be able to restart it for the transfer burn.

[R0cketC0der]

I think the reason they do it is because the fuel needs to be settled down at the bottom of the tank when the engine ignites. When the rocket doesn't accelerate, the fuel spreads over the tank and ignition might fail. This is why engines that are designed to ignitein 0-g are either pressure-fed, meaning that the tank has enough pressure so there is always enough fuel at the bottom of the tank, or they use ullage motors, which are smaller engines that usually use solid fuel and fire for a short period of time before the actual engine ignites so the fuel can settle down.

Edit: got ninja'd by a way better reply

[KASASpace]

That doesn't happen because the fuel and oxidizer need to be mixed to ignite. So unless you heat up the first stage enough that the structural section between the oxidizer and fuel tanks are melting this doesn't happen.

Well, do they use LOX? I'm sure they handle it correctly, its just that LOX can burn on itself.......

[Red Iron Crown]

Well, do they use LOX? I'm sure they handle it correctly, its just that LOX can burn on itself.......

Please share the exothermic reaction O₂ has with itself.

[Guest]

Do any present day rockets other then the Soyuz hot stage?

Yes, mostly Russian ones. The rule of thumb is to look for Warren Truss interstages instead of solid ones. Proton does hot staging, so does the Zenit. Long March also does, and Titan IV did when it was still flying. The only US rockets to use hot staging were the Titans, in their case they had vents cut out, not truss interstages, but the idea is the same.

Hot staging is a lot simpler than standard method, but you do waste some fuel that way. Basically, you can get rid of all the separation pyrotechnics, retrorockets and ullage motors. Instead, the thrust from the upper stage just pushes the lower one off. However, if a coasting period is required (as is often the case when not launching to LEO), this kind of staging simply doesn't work. Also, the savings decrease if your upper stage needs be restarted in orbit, since then, you pack ullage systems anyway, and ones that would be restartable as well (they're often combined with the upper stage's RCS). Not to mention you're putting additional stress on the lower stage tank, which means it's also unfit for reusable launchers. A "big dumb booster" would be better off with hot staging, but two stage, restartable designs (most modern rockets are those) are better off with standard staging.

[sgt_flyer]

Well, there is still pyrotechnic bolts for the separation between the stages. But there's no need for retrorockets

And if you go beyond LEO, you generally have a either a restartable transfer stage or at least a ullage system on the transfer stage (which might even be done through the stage's pressure fed RCS system)

(Fregat for soyuz, Blok D / Briz M for proton, etc)

Some of Ariane V upperstages (EPS , and the specific upperstage made for the ATV) were also restartable a few times. Current ESC-A is not, but the planned vinci upperstage is.

The rest of the Ariane V rocket don't need any ullage - as the main engine and the SRB's are fired from launch.

[KASASpace]

Please share the exothermic reaction O₂ has with itself.

And third, LOX in its own right is a dangerously volatile chemical because, of course, as the supreme oxidizer of all oxidizers, it can burn on itself.

-How to Build Your Own Spaceship, Piers Bizony

[softweir]

I think he said a wrong thing there. The danger with O2 is that it can react with almost any metal that might be used in spaceship construction: this is what happened to Apollo 13 - a short circuit in a motor used to stir the liquid in a LOX tank heated the motor enough to cause it and the O2 to burn together with an almost instantaneous explosion. But on its own, O2 can't burn with itself, despite that quote.

Incidentally, even his hyperbole that O2 is the oxidiser of oxidisers is wrong - that honour goes to fluorine which is reactive enough it can oxidise helium!

[KASASpace]

I think he said a wrong thing there. The danger with O2 is that it can react with almost any metal that might be used in spaceship construction: this is what happened to Apollo 13 - a short circuit in a motor used to stir the liquid in a LOX tank heated the motor enough to cause it and the O2 to burn together with an almost instantaneous explosion. But on its own, O2 can't burn with itself, despite that quote.

Incidentally, even his hyperbole that O2 is the oxidiser of oxidisers is wrong - that honour goes to fluorine which is reactive enough it can oxidise helium!

Hmm, well, if even Helium can be oxidized, and it's a Noble Gas, then what is their to prevent LOX oxidizing itself?

Yeah, Fluorine is the best, it's just really expensive and even MORE DANGEROUS.

[Z-Man]

Hmm, well, if even Helium can be oxidized, and it's a Noble Gas, then what is their to prevent LOX oxidizing itself?

The same thing that prevents mud from getting dirty That it already is as oxidized as it can get without adding another substance, in both senses of the word.

Definition 1: Having reacted with Oxygen.

Definition 2: Gave away one or several electrons at least partially to another atom. Since all the atoms are the same, there is no incentive for the electrons to move anywhere between them.

It could form Ozone, but that is a stronger oxidizer than O2 and less tightly bound, and thus a) reduced relative to O2 and the reaction does not happen on its own.

[KASASpace]

The same thing that prevents mud from getting dirty That it already is as oxidized as it can get without adding another substance, in both senses of the word.

Definition 1: Having reacted with Oxygen.

Definition 2: Gave away one or several electrons at least partially to another atom. Since all the atoms are the same, there is no incentive for the electrons to move anywhere between them.

It could form Ozone, but that is a stronger oxidizer than O2 and less tightly bound, and thus a) reduced relative to O2 and the reaction does not happen on its own.

I never said the reaction could happen on its own. It requires energy. Even the heat from the hot exhaust could cause this, but LOX requires good insulation anyways.

[Seret]

LOX requires fuel for combustion just like anything else. However, it is potentially dangerous when brought into contact with just about anything that isn't scrupulously clean, so in a way you're both right. In an engineering environment the risk of your LOX coming into contact with dirt, lubricants and fuels is ubiquitous enough that LOX needs to be handled carefully.

Strictly speaking, no it can't burn on itself. But it can burn with enough things and with such intensity that it should be kept separate from pretty much everything. I have however seen groundies pour LOX onto little frogs hoping around the tarmac, which was cruel but fascinating. So maybe kept separate from everything except frogs...

Edited May 2, 2014 by Seret

[Z-Man]

I never said the reaction could happen on its own. It requires energy. Even the heat from the hot exhaust could cause this, but LOX requires good insulation anyways.

RIC specifically asked about an exothermic reaction, which you had implied (that's what "burn" means). Reactions that require energy are not.

[Red Iron Crown]

-How to Build Your Own Spaceship, Piers Bizony

Citing a source that is wrong does not a convincing argument make. O₂ needs to react with another substance to produce energy.

[KSK]

I think he said a wrong thing there. The danger with O2 is that it can react with almost any metal that might be used in spaceship construction: this is what happened to Apollo 13 - a short circuit in a motor used to stir the liquid in a LOX tank heated the motor enough to cause it and the O2 to burn together with an almost instantaneous explosion. But on its own, O2 can't burn with itself, despite that quote.

Incidentally, even his hyperbole that O2 is the oxidiser of oxidisers is wrong - that honour goes to fluorine which is reactive enough it can oxidise helium!

Yep - my first thought when I read that was, 'he should read about chlorine trifluoride some time.' Anything that sets fire to sand requires respect.

[KSK]

I never said the reaction could happen on its own. It requires energy. Even the heat from the hot exhaust could cause this, but LOX requires good insulation anyways.

Even then, unless there is something else to react with the oxygen, all you get is hot oxygen. Which will react quite happily with most anything else, including bits of ruptured fuel tank, insulation, fuel vapour etc. etc. but it won't react with itself.

[sgt_flyer]

Yup chlorine trifluoride is nasty stuff

http://web.archive.org/web/20060318221608/http://www.airproducts.com/nr/rdonlyres/8479ed55-2170-4651-a3d4-223b2957a9f3/0/safetygram39.pdf

It even burned through 30cm of concrete and 90cm of gravel underneath (Aliens acid blood seems to be harmless in comparison )

[shynung]

ClF3 is a very strong oxidizing and fluorinating agent. It is extremely reactive with most inorganic and organic materials, including glass and teflon, and will initiate the combustion of many otherwise non-flammable materials without any ignition source. These reactions are often violent, and in some cases explosive.

The ability to surpass the oxidizing ability of oxygen leads to extreme corrosivity against oxide-containing materials often thought as incombustible. Chlorine trifluoride and gases like it have been reported to ignite sand, asbestos, and other highly fire-ret@rdant materials. In an industrial accident, a spill of 900 kg of chlorine trifluoride burned through 30 cm of concrete and 90 cm of gravel beneath. Fire control/suppression is incapable of suppressing this oxidation, therefore the surrounding area is kept cool until the reaction ceases. The compound reacts violently with water-based suppressors, and oxidizes in the absence of atmospheric oxygen, rendering atmosphere-displacement suppressors such as CO2 and halon completely ineffective. It ignites glass on contact.

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.

[Seret]

Oh dear. I wonder why such a thing have to exist.

It probably doesn't in nature. We made it, yay us!

TBF it does seem to have some industrial uses, cleaning all the guff off the walls of CVD chambers.

[andrewas]

I never said the reaction could happen on its own. It requires energy. Even the heat from the hot exhaust could cause this, but LOX requires good insulation anyways.

The only reaction O2 can have with itself is splitting an O2 into two free radicals which can then react with other O2 to make some O3. But heat alone won't do that, it takes UV-C to split the molecules in the first place, so theres no runaway exothermic reaction with just O2. That kind of thing can only happen with unstable molecules which want to split into more stable products. High explosives, for instance, work this way, usually they have a lot of nitrogen atoms which want to form N2 molecules, which releases lots of energy. No other reactants required.

Edited May 2, 2014 by andrewas

[FREEFALL1984]

maybe its also to reduce risk of injury to the astronauts, having someone 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 as well as the hardware. At least with a hot stage like this you'll only be going from 3-4Gs back down to 2-3Gs which is much more manageable.