[quote name='Arun_S' date='22 May 2012 - 10:50 AM' timestamp='1337663569' post='114939']
About thw A5 retro-cocket for FS seperation. The 16 tonne force gives an idea on how depressed first stage trajectory it can handle.
The prime cause of in atmosphere seperation is that the when old stage expends, it carries its momentum but does not suffer any appriciable drag, while the stages upstream see a retarding force due to supersonic drag, thus if expended stage is separated, the rear stage will stay into the shockwave shadow and bump into upstream stages with very high certainity.
FYI for Agni-V stage at 18 km altiude sees pressure drag of 9 tonne force, hence 16 tonne retro rocket is right sized to provide those crucitial seconds to allow upper stage to fire and ramp up thrust above 9 tonnes, while the expended stage is assured to be retarding and trailing behind.
Lower sepration altidue and the drag can be many times more.
[/quote]
I have just provided the news report to try and understand your statement.
Up to Agni-III, we ignite the upper stage first, then separate the lower stage so that there is no problem of separation.We decided to leave behind that culture of space vehicles. We now put big retro motors, which create a thrust of four tonnes each ââ¬â totally 16 tonnes of thrust ââ¬â just to separate the stages so that no dead weight is passed on to the upper stage.Correspondingly, we decided to make the mission stronger so that there are no interfaces and the separation is clean. We studied and created extensive models to simulate them on the ground in all types of disturbed conditions in wind tunnels. With all that, we could remove the inter-stages altogether.
At 18 Km altitude, the drag is 9 tonnes force on the entire two stages in question.
In Agni III
(Expended stage ---- Inter-stage-----Upper stage) This entire body has a force of 9 tonnes and is moving along due to the momentum imparted from the expended stage action.
Step1: Light upper stage. This pulls the expended stage+ inter-stage along until the inter-stage is cut.
Step2: Separate inter-stage+ expended stage. When this happens the entire body is still moving along and has a force of 9 tonnes. It decelerates as the inter-stage is broken and destroyed by explosives.
The velocity of the upper-stage increases due it being already lit. It carries the dead-weight for crucial seconds which decreases it's final altitude.The inter-stage separation is not clean as the inter-stage isn't a composite mass just a few straps of metal holding the two stages together. These can create additional drag.
Once the inter-stage is separated or blown apart the expended-stage drops down due to gravity while still travelling at 9 tonnes thrust and decelerating but separated by the interstate length. The upper stage accelerates at 9 tonnes + force of rocket engine and accelerating.
In Agni III the expended stage was eating into the fuel of the next stage with a firing rocket motor in the upper stage. The separation was also not clean due to the metal which needed disconnecting. It's difficult to ensure all the bolts and metal strips blow out and are not attached to the upper stage. So it make sense if we can detach the expended stage and inter-stage before we fire the final stage.
In Agni IV and Agni V
(Expended stage with retro-motors + Upper stage.) This entire body has a force of 9 tonnes and is moving along due to the momentum imparted from the expended stage action.
Step 1: Fire the retro-motors on the expended stage which are presumably located where the earlier inter-stage was or located inside the expended stage at the upper-reaches. The thrust here is 16 tons 4 x 4 tons.
Step 2: Fire the upper stage. This gives the upper-stage a momentum from burning fuel and it accelerates above the 9 tonnes thrust it's already been given by the expended stage. The expended stage has to counter act the 9 tonnes of force already exerted on it due to conservation of momentum and impart additional thrust to retard down the expended stage. To move in the opposite direction it's travelling in the total thrust available here is 16-9 = 5 tons.
So expended stage moves towards the earth with a decelerating thrust of 5 tons. The upper stage moves up with a thrust of 9tons + the additional thrust from the rocket motors in the final stage plus the initial separation thrust disclosed below. The retro-motors also in the initial seconds of separation stage impart an equal and opposite thrust to the upper-stage. However this initial few seconds of thrust is 9 tonnes + 16 tonnes as it's in the direction of travel of the upper stage. However the upper stage gets this only for the initial few seconds of separation as the retros go away with the expended-stage.
Since the firing sequence is reversed in Agni V, to prevent the expended stage from disturbing the direction of the upper-stage when it's still not lit and while the expended-stage is retarding due to gravity as it's not experiencing the same drag being in the wake field of the upper stage which is pushing through the atmosphere and experiencing drag the reverse motors are necessary.
So how much low can these reverse motors on the expended stage go in a depressed trajectory ?
The reverse thrust right now is 5 tons of reverse thrust given the thrust at the altitude is 9 tons.
The reverse thrust must be greater the additional drag on the upper stage. Is there too much leeway here? I will calculate these values at a later date.
![[Image: 99a6015b6a230860c9b1517b238e5de9.png]](http://upload.wikimedia.org/wikipedia/en/math/9/9/a/99a6015b6a230860c9b1517b238e5de9.png)
Added later: There is another possibility.
How long this reverse thrust is available is also not disclosed. If it's available only for the few seconds before the upper stage lights up. This thrust could be equal to the acceleration thrust provided by a lighted upper stage engine for those few seconds until the upper stage motors kicks in and provides it's own acceleration.
The 5 ton net reverse thrust should move the expended stage a distance equal to the earlier inter-stage to allow the upper-stage enough space to prevent it from getting into the wake of the upper stage?
About thw A5 retro-cocket for FS seperation. The 16 tonne force gives an idea on how depressed first stage trajectory it can handle.
The prime cause of in atmosphere seperation is that the when old stage expends, it carries its momentum but does not suffer any appriciable drag, while the stages upstream see a retarding force due to supersonic drag, thus if expended stage is separated, the rear stage will stay into the shockwave shadow and bump into upstream stages with very high certainity.
FYI for Agni-V stage at 18 km altiude sees pressure drag of 9 tonne force, hence 16 tonne retro rocket is right sized to provide those crucitial seconds to allow upper stage to fire and ramp up thrust above 9 tonnes, while the expended stage is assured to be retarding and trailing behind.
Lower sepration altidue and the drag can be many times more.
[/quote]
I have just provided the news report to try and understand your statement.
Up to Agni-III, we ignite the upper stage first, then separate the lower stage so that there is no problem of separation.We decided to leave behind that culture of space vehicles. We now put big retro motors, which create a thrust of four tonnes each ââ¬â totally 16 tonnes of thrust ââ¬â just to separate the stages so that no dead weight is passed on to the upper stage.Correspondingly, we decided to make the mission stronger so that there are no interfaces and the separation is clean. We studied and created extensive models to simulate them on the ground in all types of disturbed conditions in wind tunnels. With all that, we could remove the inter-stages altogether.
At 18 Km altitude, the drag is 9 tonnes force on the entire two stages in question.
In Agni III
(Expended stage ---- Inter-stage-----Upper stage) This entire body has a force of 9 tonnes and is moving along due to the momentum imparted from the expended stage action.
Step1: Light upper stage. This pulls the expended stage+ inter-stage along until the inter-stage is cut.
Step2: Separate inter-stage+ expended stage. When this happens the entire body is still moving along and has a force of 9 tonnes. It decelerates as the inter-stage is broken and destroyed by explosives.
The velocity of the upper-stage increases due it being already lit. It carries the dead-weight for crucial seconds which decreases it's final altitude.The inter-stage separation is not clean as the inter-stage isn't a composite mass just a few straps of metal holding the two stages together. These can create additional drag.
Once the inter-stage is separated or blown apart the expended-stage drops down due to gravity while still travelling at 9 tonnes thrust and decelerating but separated by the interstate length. The upper stage accelerates at 9 tonnes + force of rocket engine and accelerating.
In Agni III the expended stage was eating into the fuel of the next stage with a firing rocket motor in the upper stage. The separation was also not clean due to the metal which needed disconnecting. It's difficult to ensure all the bolts and metal strips blow out and are not attached to the upper stage. So it make sense if we can detach the expended stage and inter-stage before we fire the final stage.
In Agni IV and Agni V
(Expended stage with retro-motors + Upper stage.) This entire body has a force of 9 tonnes and is moving along due to the momentum imparted from the expended stage action.
Step 1: Fire the retro-motors on the expended stage which are presumably located where the earlier inter-stage was or located inside the expended stage at the upper-reaches. The thrust here is 16 tons 4 x 4 tons.
Step 2: Fire the upper stage. This gives the upper-stage a momentum from burning fuel and it accelerates above the 9 tonnes thrust it's already been given by the expended stage. The expended stage has to counter act the 9 tonnes of force already exerted on it due to conservation of momentum and impart additional thrust to retard down the expended stage. To move in the opposite direction it's travelling in the total thrust available here is 16-9 = 5 tons.
So expended stage moves towards the earth with a decelerating thrust of 5 tons. The upper stage moves up with a thrust of 9tons + the additional thrust from the rocket motors in the final stage plus the initial separation thrust disclosed below. The retro-motors also in the initial seconds of separation stage impart an equal and opposite thrust to the upper-stage. However this initial few seconds of thrust is 9 tonnes + 16 tonnes as it's in the direction of travel of the upper stage. However the upper stage gets this only for the initial few seconds of separation as the retros go away with the expended-stage.
Since the firing sequence is reversed in Agni V, to prevent the expended stage from disturbing the direction of the upper-stage when it's still not lit and while the expended-stage is retarding due to gravity as it's not experiencing the same drag being in the wake field of the upper stage which is pushing through the atmosphere and experiencing drag the reverse motors are necessary.
So how much low can these reverse motors on the expended stage go in a depressed trajectory ?
The reverse thrust right now is 5 tons of reverse thrust given the thrust at the altitude is 9 tons.
The reverse thrust must be greater the additional drag on the upper stage. Is there too much leeway here? I will calculate these values at a later date.
Added later: There is another possibility.
How long this reverse thrust is available is also not disclosed. If it's available only for the few seconds before the upper stage lights up. This thrust could be equal to the acceleration thrust provided by a lighted upper stage engine for those few seconds until the upper stage motors kicks in and provides it's own acceleration.
The 5 ton net reverse thrust should move the expended stage a distance equal to the earlier inter-stage to allow the upper-stage enough space to prevent it from getting into the wake of the upper stage?