What is hotstaging?

[ItsSky]

The title pretty much says it all.  Out of curiosity, what is hotstaging and what is it good for?

[Signo]

It Is a staging sequence where your next stage is lit up before your previous one flaming out. An IRL example Is the soviet N1 Moon rocket.

[Zeiss Ikon]

To expand on @Signo's answer, hot staging is something you do because of ullage.  It's manifestly unnecessary in the stock game -- stock engines and tanks don't care about ullage in any way.  If you're playing Realism Overhaul, or pretty much anything using RealFuels and real world engine parts packs, you'll need to pay attention to ullage.

"What's ullage?"

Glad you asked.  Ullage is the "empty space" in a propellant tank.  There's always some; usually filled with some kind of pressurant (even pump-fed engines need pressure in the tanks, just not as much as pressure-fed engines).  This can be bad if the pressurant winds up near the rear/bottom of the tank, where there should be propellant, when your engine needs fuel (or oxidizer).

Let's say you launch a basic sounding rocket, like a WAC Corporal (1948 technology here, hang onto your hat).  This is a primitive two-stage rocket, with a high thrust Tiny Tim booster (originally an air-to-ground unguided rocket from WWII).  Your basic WAC Corporal stack will pull something like 30 G off the launch pad with this booster -- but the booster burns out after about 0.6 seconds, leaving you falling upward at roundly 200 m/s.

If you now want to stage and ignite your (pressure fed, hypergolic liquids) upper stage sustainer, you'll find it doesn't work.  EVER.  Because the air drag at 200 m/s results in enough deceleration to immediately slosh all the propellants to the nose cone end of the tanks, and the nitrogen that's supposed to push the aniline/furfuryl fuel and red fuming nitric acid oxidizer into the thrust chamber instead runs straight out through the injectors.  This is why ullage has to be managed.  The WAC Corporal method, as of 1948, was to ignite the upper stage at the same time as the Tiny Tim, and time the staging decoupler to fire at the exact same time the Tiny Tim burned out.  That gave the sustainer engine most of a second to come up to thrust, and ensured the booster wasn't hanging on creating drag that could (even with the sustainer fully ignited) lead to pressurant in the feed lines instead of propellants.  Done this way, the WAC Corporal worked about 95% of the time, and would fly as high as about 120 km (less if it was carrying a payload, of course).

And that's hot staging.  As Signo noted, the Soviet space program used hot staging a lot -- it's the reason you see many of their rockets with open strutwork interstage structures, so that the next stage exhaust has an exit to prevent either choking the engine before it's fully ignited, or blowing out the interstage fairing with potential to lead to a structural collapse as maximum G load.  By contrast, the American space program almost never used it (can't say for certain they never did, but I think that's true).  Look at a Saturn V -- specifically at the base of the S-II second stage.  It has (relatively) small solid propellant "ullage rockets" -- there to ensure there's a (small) positive acceleration after the staging event, so the ignition of the next stage can take place successfully.  And there are no ullage rockets on the Service Module, because it was intended to fly in vacuum and needed RCS to point before a burn anyway, and RCS works just fine for ullage management.  I think there are even Apollo recordings where the crew mentions the ullage burn start, several seconds before the burn to return from Lunar orbit to Earth.

Ullage can be managed other ways, of course.  One way (used often for things like attitude jets/RCS and long-term station keeping thrusters) is a bladder tank.  This stores the fuel (usually a monopropellant, most commonly hydrazine or a derivative these days) inside a bladder, much like a water balloon inside the rigid tank structure; pressure is applied to the outside of the bladder, and the inside is carefully debubbled before launch, so that every time you open the valve, the propellant flows into the catalyst pack and turns into thrust.  In the case of Voyager 2, even if the last time was thirty years ago.

If you're launching from the ground (even if that "ground" is Lunar regolith), gravity (even if it's a fraction of a G) does an admirable job of managing your ullage.  I don't know that it's actually been done in a rocket, but another possibility would be an aerospace version of what I learned to call a "clunk tank" when I was flying radio control models (before they were electric).  This had the fuel pickup in a weight on the end of a flexible tube which would always find its way (with an audible "clunk" if you checked it on the ground with the engine stopped) to the current "bottom" of the tank.  Works great when (as with an aerobatic model) you aren't sure where you local gravity will be any given moment; won't work any better than a fixed pickup in microgravity (so probably not very useful in actual rockets).

[IkranMakto]

7 hours ago, Signo said:

An IRL example Is the soviet N1 Moon rocket.

...and almost every soviet rocket too ;} 

[AHHans]

In KSP there is also another variant of hot-staging that isn't used in real life for reasons that should be obvious. And this is to fire an upper stage while having the lower stage directly connected to it without a decoupler. That way the flames from the upper stage will heat the lower stage until that explodes and you finally get rid of the lower stage. Thus you can have a multi-stage rocket without having decoupler technology.

Edited December 3, 2019 by AHHans
Changed wording to improve clarity.

[Fierce Wolf]

To me hot staging is when i fire my boosters at the launch pad and realize RIGHT THERE that i forgot to check TWR because it struggles to go up, so i quickly click on the engines and start them before the staging secuence, saving the mission (more or less)

[Maxsimal]

On 12/3/2019 at 1:26 PM, Fierce Wolf said:

To me hot staging is when i fire my boosters at the launch pad and realize RIGHT THERE that i forgot to check TWR because it struggles to go up, so i quickly click on the engines and start them before the staging secuence, saving the mission (more or less)

I think that's known as 'hot mess' staging.  

[ItsSky]

On 12/2/2019 at 6:49 PM, Zeiss Ikon said:

To expand on @Signo's answer, hot staging is something you do because of ullage.  It's manifestly unnecessary in the stock game -- stock engines and tanks don't care about ullage in any way.  If you're playing Realism Overhaul, or pretty much anything using RealFuels and real world engine parts packs, you'll need to pay attention to ullage.

"What's ullage?"

Glad you asked.  Ullage is the "empty space" in a propellant tank.  There's always some; usually filled with some kind of pressurant (even pump-fed engines need pressure in the tanks, just not as much as pressure-fed engines).  This can be bad if the pressurant winds up near the rear/bottom of the tank, where there should be propellant, when your engine needs fuel (or oxidizer).

Let's say you launch a basic sounding rocket, like a WAC Corporal (1948 technology here, hang onto your hat).  This is a primitive two-stage rocket, with a high thrust Tiny Tim booster (originally an air-to-ground unguided rocket from WWII).  Your basic WAC Corporal stack will pull something like 30 G off the launch pad with this booster -- but the booster burns out after about 0.6 seconds, leaving you falling upward at roundly 200 m/s.

If you now want to stage and ignite your (pressure fed, hypergolic liquids) upper stage sustainer, you'll find it doesn't work.  EVER.  Because the air drag at 200 m/s results in enough deceleration to immediately slosh all the propellants to the nose cone end of the tanks, and the nitrogen that's supposed to push the aniline/furfuryl fuel and red fuming nitric acid oxidizer into the thrust chamber instead runs straight out through the injectors.  This is why ullage has to be managed.  The WAC Corporal method, as of 1948, was to ignite the upper stage at the same time as the Tiny Tim, and time the staging decoupler to fire at the exact same time the Tiny Tim burned out.  That gave the sustainer engine most of a second to come up to thrust, and ensured the booster wasn't hanging on creating drag that could (even with the sustainer fully ignited) lead to pressurant in the feed lines instead of propellants.  Done this way, the WAC Corporal worked about 95% of the time, and would fly as high as about 120 km (less if it was carrying a payload, of course).

And that's hot staging.  As Signo noted, the Soviet space program used hot staging a lot -- it's the reason you see many of their rockets with open strutwork interstage structures, so that the next stage exhaust has an exit to prevent either choking the engine before it's fully ignited, or blowing out the interstage fairing with potential to lead to a structural collapse as maximum G load.  By contrast, the American space program almost never used it (can't say for certain they never did, but I think that's true).  Look at a Saturn V -- specifically at the base of the S-II second stage.  It has (relatively) small solid propellant "ullage rockets" -- there to ensure there's a (small) positive acceleration after the staging event, so the ignition of the next stage can take place successfully.  And there are no ullage rockets on the Service Module, because it was intended to fly in vacuum and needed RCS to point before a burn anyway, and RCS works just fine for ullage management.  I think there are even Apollo recordings where the crew mentions the ullage burn start, several seconds before the burn to return from Lunar orbit to Earth.

Ullage can be managed other ways, of course.  One way (used often for things like attitude jets/RCS and long-term station keeping thrusters) is a bladder tank.  This stores the fuel (usually a monopropellant, most commonly hydrazine or a derivative these days) inside a bladder, much like a water balloon inside the rigid tank structure; pressure is applied to the outside of the bladder, and the inside is carefully debubbled before launch, so that every time you open the valve, the propellant flows into the catalyst pack and turns into thrust.  In the case of Voyager 2, even if the last time was thirty years ago.

If you're launching from the ground (even if that "ground" is Lunar regolith), gravity (even if it's a fraction of a G) does an admirable job of managing your ullage.  I don't know that it's actually been done in a rocket, but another possibility would be an aerospace version of what I learned to call a "clunk tank" when I was flying radio control models (before they were electric).  This had the fuel pickup in a weight on the end of a flexible tube which would always find its way (with an audible "clunk" if you checked it on the ground with the engine stopped) to the current "bottom" of the tank.  Works great when (as with an aerobatic model) you aren't sure where you local gravity will be any given moment; won't work any better than a fixed pickup in microgravity (so probably not very useful in actual rockets).

Ah thanks a ton for clearing that up!  I use mechjeb a ton, saw the option "hotstaging" in the ascent guidance, and didn't know what that meant.  The only thing I still don't know from this is how would this benefit you or your rocket?  Whats its use?  Is it better then normal decoupling/staging?  Otherwise thanks for taking the time to clarify that!

[Zeiss Ikon]

Once you're beyond the sounding rocket level, hot staging benefits mainly if you have to stage at relatively low altitude, when depending on gimballed engines for attitude control.  If you're flying at Mach 1 or faster, there's a lot of dynamic pressure on the nose of your rocket, and if you won't have large (heavy) fins on the second stage (and even larger, heavier ones on the booster to keep the rocket from swapping ends almost immediately after launch), that stage is going to tend to tumble between MECO and second stage start, even if you have ullage rockets powerful enough to overcome air drag and get a reliable start.  By hot staging, you avoid having a gap of a few seconds in your vectored thrust.

American rockets, historically, handled this by not staging in thick air -- a modern example is the Falcon 9, which stages above 100 km, where there isn't enough air to matter.  Early Soviet launchers, by contrast, had shorter burning engines.  One of their methods was to ground-start a core with a longer tank and a set of side boosters with shorter tanks, and stage away the side boosters while allowing the core to burn longer, but even then they still used hot staging, likely because ullage rockets added complexity and reduced reliability (or possibly because they simply didn't have suitable small solid propellant engines, though engines from air-to-ground or air-to-air missiles are very suitable for this).