Timing second stage in atmosphere on Kerbin

[JoeSchmuckatelli]

Just a bit of idle curiosity: should I fire the second stage immediately after the last stage runs out of fuel, or give it a second or two?  

[Pronoes]

Depends on numerous things. Mostly whether you're using RSS/RO or not.

In general stock KSP, IIRC, you can have get away with decouple 'last' stage and fire engine for 'next stage' all in one stage. I haven't played stock for a while so can't be 100%

With RO, the general issue of just firing straight away is ullage. You may need ullage rockets. Also, big chunky engines firing on the previous stages remains is not the best idea.

[GoSlash27]

Joe Schmuckatelli,

 It won't hurt anything if you fire the second stage immediately. Me personally... I don't like things going all 'splodey around my rockets.

Best,

-Slashy

[JoeSchmuckatelli]

Thanks.  I've always waited a bit as it just seemed prudent, but didn't know if I lost speed doing so.

 

[Raptor9]

Another consideration is rocket tumble.  If you wait too long before you light the next engine, you may lose attitude control, especially in the transonic range (300-400 m/s roughly).  As a general rule of thumb, I always design my rocket so the first stage can get me at least past the 450m/s mark.

[KerBlammo]

i used to separate and ignite the next stage at once, but in 1.0.5 it caused the first stage to explode.  Like GoSlash27 I try to avoid the 'splodey stuff so I put some separtrons on the next stage and fire those for a short period before igniting the second stage.

 

[GoSlash27]

1 hour ago, JoeSchmuckatelli said:

Thanks.  I've always waited a bit as it just seemed prudent, but didn't know if I lost speed doing so.

 

 Joe Schmuckatelli,

 You do lose at least a little speed if you're suborbital, but how much depends on where you are when you stage. If you're slow, vertical and in thick atmosphere, staging may cause you to tumble and lose the whole vehicle. If you're fast, horizontal, and in thin atmosphere, taking the time to stage won't cost you enough to care about or even notice.

I design my lifters to have 1800 m/sec DV in the first stage. This gets me right about 30 km in a gravity turn, which is high enough so that aerodynamics is no longer a concern. I can concentrate any fins on the first stage and treat the second stage as essentially being a full vacuum low thrust stage.

Best,

-Slashy

 

Edited December 8, 2015 by GoSlash27

[WuphonsReach]

I also prefer my primary stage (not including strap-on boosters or asparagus-style side pods) for about 30km altitude (1500-2000 dV).  Gets me up out of the thickness before I need to wait 2.5 seconds for that second stage to fire.

Side-mounted boosters do not count because the main engines on the primary stage do not stop firing while they disconnect.  So there's no loss of attitude control.  Same thing with asparagus boosters, as they just fall away without affecting the main engines.

Note: I always put at least four vernor RCS around the base of the main stage for attitude control as well as some sort of controllable fin (such as the Mercury fins from FASA).  Plus there's usually a SAS somewhere in the stack of the primary stage.

[FancyMouse]

I only do delayed staging when deorbiting lower stage while I'm already in orbit (so that upper stage thrust provides retro force to lower stage)

[Rocket In My Pocket]

My rule of thumb is typically a quick "3,2,1..." under my breath between each stage, which amounts to about a second delay. Not sure if it's required but I don't think it hurts to be safe!

[swjr-swis]

From having played with StageRecovery, I am in the habit of building mostly recoverable stages. That gives me a reason not to make them explode by firing the next stage too quickly.

Stock does not really require a delay though, other than as said before, to maintain attitude control. I don't think I've ever had my remaining vehicle damaged due to parts exploding off the just-released stage (*).

(*: well, when a released stage is generally under/behind the rest of the craft. Kerbals contraptions don't always follow that rule, in which case all bets are off)

[numerobis]

If your second stage has a big enough rocket, you can save a few coins (and kilos) by foregoing the decoupler and staging just by firing your engine into the spent lower stage. What you lose in realism you gain in Jebediacal explosions.

[numerobis]

Example: My early-game workhorse was a first stage of three BACCs; above the central one I attach the second stage, another BACC. The third stage is a T30. No decoupler in all that. With its thrust limiter above 50% the second stage instantly vaporizes the booster attached to it. Set the throttle to 100% with the T30 and you'll barely be able to see the second stage overheat.

Foregoing the decouplers knocked off $800 per launch, and it allowed me two extra parts, and it was silly. (Also saves 100kg on launch, 50kg from the second stage, but those aren't that much.)

[OhioBob]

I usually do a "hot separation", meaning that decoupling and ignition are grouped together in the same staging event.  Never had any problem with this.

[KerikBalm]

As do I, unless you want to recover the stage below it... otherwise, depending on what part of your ascent, you've just wasted dV due to gravity drag (only really true when you are thrusting up to raise Apoapsis).

Also, Ullage is not a problem in real life for "hot staging" the lower stage thrust will keep the propellants settled... if you ignite the upper stage just as/before the lower stage thrust runs out, ullage is not a problem. If the lower stage is a SRB, the thrust doesn't cut off all at once anyway (I don't know how much "rebound" there is as the tanks may flex under the force of the propellant being accelerated if the thrust cuts off suddenly).

And then there is hot staging where the upper stage ignites before the lower stage is gone:

 

[monstah]

Just be sure to wait until whatever has liquified due to exteme G and vibration within the rocket to re-solidify, otherwise it might hit a strut in the wrong way and have your whole ship precessing out of control.

And, no, that doesn't happen in KSP. Just a nerd vent