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General Discussion => Q&A Section => Topic started by: sanman on 06/21/2017 04:50 PM

Title: Stage Separation (Was: Hot-Staging)
Post by: sanman on 06/21/2017 04:50 PM
Is it possible for to use hot-staging in order for an upper stage to gain extra delta-V by effectively springboarding off the lower stage?
What are the limits to the extent of which this can be done? What are the considerations here?
Why doesn't everybody do it?
Title: Re: Hot-Staging
Post by: Jim on 06/21/2017 04:54 PM
Is it possible for to use hot-staging in order for an upper stage to gain extra delta-V by effectively springboarding off the lower stage?
What are the limits to the extent of which this can be done? What are the considerations here?
Why doesn't everybody do it?

It does not provide extra delta V and it has some obvious drawbacks, especially for reusable vehicles.
See Titan II staging.

And it is called fire in the hole staging
Title: Re: Hot-Staging
Post by: envy887 on 06/21/2017 08:33 PM
The main problem is exhaust impingement. To get any "springboard" effect, the exhaust from the upper stage has to impinge twice: first on the booster, then again on the upper stage.

The biggest gain is actually from eliminating gravity losses during the few seconds of coast. But a few seconds of coast ahead of a 400 second upper stage burn isn't enough make up for the drawbacks.
Title: Re: Hot-Staging
Post by: whitelancer64 on 06/21/2017 08:44 PM
The Soyuz family of rockets actually does this. The 3rd stage ignites while the sustainer (2nd) stage is still firing, that's why the rocket has an open trusswork instead of a solid interstage.

The Proton, too.
Title: Re: Hot-Staging
Post by: Space Ghost 1962 on 06/21/2017 09:15 PM
Zenit too:
(http://spaceflight101.com/zenit-elektro-l-2/wp-content/uploads/sites/60/2015/12/0041.jpg)
Title: Re: Hot-Staging
Post by: whitelancer64 on 06/21/2017 09:27 PM
Interesting! The Long March family, as well.
Title: Re: Hot-Staging
Post by: Sam Ho on 06/21/2017 11:01 PM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.
Title: Re: Hot-Staging
Post by: ulm_atms on 06/21/2017 11:11 PM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.

I was of the thinking that this was the only reason.  By starting up while the other stage is firing, all of the fuel is in the right place to keep from getting a gas pocket in the line.

Are there any other reasons?  I just can't think of any.
Title: Re: Hot-Staging
Post by: envy887 on 06/22/2017 12:40 AM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.

Doesn't help much on stages that need 3-axis RCS/ACS already, since they double as ullage thrusters.
Title: Re: Hot-Staging
Post by: Jim on 06/22/2017 01:06 AM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.

That is the main reason for the technique. 
Title: Re: Hot-Staging
Post by: SWGlassPit on 06/23/2017 02:32 PM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.

Doesn't help much on stages that need 3-axis RCS/ACS already, since they double as ullage thrusters.
But it does eliminate the chicken-egg problem of ensuring that RCS/ullage thruster propellants are also settled. 
Title: Re: Hot-Staging
Post by: Lars-J on 06/23/2017 03:41 PM
The other benefit is that you may be able to avoid adding ullage motors for propellant settling, since the stage ignites while it is under acceleration.

Doesn't help much on stages that need 3-axis RCS/ACS already, since they double as ullage thrusters.
But it does eliminate the chicken-egg problem of ensuring that RCS/ullage thruster propellants are also settled.

RCS/ullage systems are almost always pressure-fed, so is no such chicken and egg problem in this case.

Anyway, hot staging appears to be going out of favor in newer Chinese and Russian launch vehicles. And there is no other nation that fields a current hot stage launch vehicle as far as I know. (India perhaps?)
Title: Re: Hot-Staging
Post by: Jim on 06/23/2017 04:00 PM
But it does eliminate the chicken-egg problem of ensuring that RCS/ullage thruster propellants are also settled. 

those have bladders
Title: Re: Hot-Staging
Post by: JAFO on 06/23/2017 07:45 PM
Let's not forget the ultimate Fire in the Hole staring, the N-1. Dan shame we never got to see if it would have worked.
Title: Re: Hot-Staging
Post by: Space Ghost 1962 on 06/23/2017 08:48 PM
Let's not forget the ultimate Fire in the Hole staring, the N-1. Dan shame we never got to see if it would have worked.

Wrong - we did see that it (N1) didn't work. (Not for such staging.)

Could it have been made to work? Eventually.

(And note that the upper blok's (above the third stage) are enclosed, so not necessarily handled the same.)

The last stages have the flight computer/avionics/GNC. The sole benefit is for the lower stages to potentially be more mass/propulsion efficient as in less mass in the expendable LV. A two stage vehicle does not get a benefit, especially if the US restarts, as it needs RCS/GNC/ullage (also, it is less efficient due to propulsion start causing the vehicle's direction to wander and more loss (cosine) induced in recovering the inertial state of the vehicle).
Title: Re: Hot-Staging
Post by: JAFO on 06/23/2017 09:24 PM
Let's not forget the ultimate Fire in the Hole staring, the N-1. Dan shame we never got to see if it would have worked.

Wrong - we did see that it (N1) didn't work. (Not for such staging.)

Could it have been made to work? Eventually.


I was speaking more in general terms of the N1 FitH staging itself, not the success of the entire vehicle. That's a whole 'nother thread. 
Title: Re: Hot-Staging
Post by: Arch Admiral on 06/24/2017 06:09 AM
The first design to use this technique was Titan I, and it is known that the Soviet ICBM design teams got a detailed data package on Titan I from a GRU informant working for Martin. It was incorporated into the R-9A with some other Titan technology and spread from there to many Soviet designs. Oddly, it never appeared in later US boosters. For instance Saturn V had at least a dozen small solid-fuel motors for separation and ullage, each of which could have exploded and killed a mission. This is the only area where N-1 actually looks good by comparison.
Title: Re: Hot-Staging
Post by: MP99 on 06/24/2017 07:41 AM
ISTM that one disadvantage is that any impulse from startup through to separation is lost.

Cheers, Martin
Title: Re: Hot-Staging
Post by: sanman on 06/24/2017 01:03 PM
The main problem is exhaust impingement. To get any "springboard" effect, the exhaust from the upper stage has to impinge twice: first on the booster, then again on the upper stage.

But why isn't it possible to engineer a vigorous piston effect, above and beyond mere exhaust impingement? Like a gas piston, or a bullet launching from a gun barrel. The idea would be for the separating stages to push away from each other with the highest possible impulse. Do the newer methods, like the pusher-rod on Falcon 9, offer the highest possible impulse at separation?

If you're not flinging your lower stage backward with maximum possible force, then you're missing an opportunity for useful momentum transfer to benefit upper-stage Delta-V.
Title: Re: Hot-Staging
Post by: Jim on 06/24/2017 01:21 PM
The first design to use this technique was Titan I

Wrong.  Titan I did not use fire in the hole staging.  It had staging solid motors that separated the stages. 
Hence, the rest of your post is in doubt. Also, the Russians flew upper stages on the R-7 before the first Titan I launch.
Title: Re: Hot-Staging
Post by: Jim on 06/24/2017 01:46 PM

If you're not flinging your lower stage backward with maximum possible force, then you're missing an opportunity for useful momentum transfer to benefit upper-stage Delta-V.

Wrong.  That is not useful


But why isn't it possible to engineer a vigorous piston effect, above and beyond mere exhaust impingement? Like a gas piston, or a bullet launching from a gun barrel. The idea would be for the separating stages to push away from each other with the highest possible impulse.


wrong.  Because

1.  the upper stages, full of propellants and payload, are likely heavier than the discarded lower stage.  It would a more velocity to the lower stage than the upper stage.

2.  The interstage would have to be reinforced to handle the pressure so that it could be harnessed.  This would increase mass more than any benefit derived from the impulse.  Titan II did every thing it could to release the pressure.  That is why Russian vehicles have lattice interstages.  The pr

Do the newer methods, like the pusher-rod on Falcon 9, offer the highest possible impulse at separation?


no

The point is not impulse but to provide separation.  The pusher rod is not accelerating the upper stage.  It is just pushing it clear.  The design does not look at staging as a place to get delta V out of the event, but just to provide clearance from the lower stage.
Title: Stage Separation (Was: Hot-Staging)
Post by: sanman on 06/24/2017 03:58 PM
Hi - I'm just going to change the thread title to "Stage Separation (Was: Hot-Staging)", if you all don't mind - just for a little broader about separation dynamics, and the tradeoffs of different methods.



If you're not flinging your lower stage backward with maximum possible force, then you're missing an opportunity for useful momentum transfer to benefit upper-stage Delta-V.

Wrong.  That is not useful


But why isn't it possible to engineer a vigorous piston effect, above and beyond mere exhaust impingement? Like a gas piston, or a bullet launching from a gun barrel. The idea would be for the separating stages to push away from each other with the highest possible impulse.


wrong.  Because

1.  the upper stages, full of propellants and payload, are likely heavier than the discarded lower stage.  It would a more velocity to the lower stage than the upper stage.

But even that Delta-V for lower stage can be beneficial in the case where lower stage wants to do RTLS (eg. F9R booster) Then the boostback requirement could be reduced.

Furthermore, why couldn't the mass balance between upper and lower stage be shifted to help momentum exchange favor a higher proportion of Delta-V for upper stage?
Take the case of a winged flyback booster, which features the added mass of wings for the purpose of reusability on the lower stage.
 
2.  The interstage would have to be reinforced to handle the pressure so that it could be harnessed.  This would increase mass more than any benefit derived from the impulse.  Titan II did every thing it could to release the pressure.  That is why Russian vehicles have lattice interstages.

In the winged flyback booster example I gave above, the nose of the booster might already have to be reinforced for re-entry purposes anyway.

Do the newer methods, like the pusher-rod on Falcon 9, offer the highest possible impulse at separation?


no

The point is not impulse but to provide separation.  The pusher rod is not accelerating the upper stage.  It is just pushing it clear.  The design does not look at staging as a place to get delta V out of the event, but just to provide clearance from the lower stage.

Fine - I get that - the main concern has always been about getting the upper stage clear of the lower one, so that it could continue on unhindered.

But surely stage separation can accomplish more than just that, can't it?
Has it ever been considered?
Title: Re: Stage Separation (Was: Hot-Staging)
Post by: Jim on 06/24/2017 05:14 PM

1.  But even that Delta-V for lower stage can be beneficial in the case where lower stage wants to do RTLS (eg. F9R booster) Then the boostback requirement could be reduced.

2.  Furthermore, why couldn't the mass balance between upper and lower stage be shifted to help momentum exchange favor a higher proportion of Delta-V for upper stage?

3.  In the winged flyback booster example I gave above, the nose of the booster might already have to be reinforced for re-entry purposes anyway.

4.  But surely stage separation can accomplish more than just that, can't it?


1.  No, because the velocity to be gained is insignificant

2.  no, because the stages are sized for other reasons that have magnitudes more effect on performance

3.  So you are proposing an winged flyback booster with an open cylinder for a nose?

4.  No, see #1 and because any changes would have a negative effect.     The duration of the separation effect is so short there is no need to look for any gains, the point is to minimize losses.  The best thing is to get the upperstage away from the booster and start operating.  You aren't going to get any thing more efficient than operating the upperstage.   Anything else is adding mass and would basically have less equivalent performace than the upperstage or the booster. 

Here is the puzzle.  How would adding pistons, pushers, reinforced interstage, etc provide more velocity than the equivalent mass of propellant in the upperstage?