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During the PSLV (India's Polar Satellite Launch Vehicle) C24 launch on 4 April 2014, once stage 3 (PS3) had burned out, it wasn't immediately jettisoned, but instead there was a coasting phase for over a minute before it was jettisoned and stage 4 (PS4) ignited:

    enter image description here

    PSLV C24 Flight Profile. Image: Indian Space Research Organization, Image source: Spaceflight 101 PSLV C24 Launch Updates

Event                       Time      Alt. (km)  Vel. (m/s) 
-----------------------------------------------------------
PS1 Ignition                00.00.0        0.02       452.0 
Booster 1,2 Ignition        00:00.5        0.02       452.0 
Booster 3,4 Ignition        00:00.7        0.02       452.0 
Booster 5,6 Ignition        00:25.0        2.60       606.5 
Booster 1,2 Separation      01:10.0        23.3      1424.6 
Booster 3,4 Separation      01:10.1        23.4      1429.3 
Booster 5,6 Separation      01:32.0        39.4      2026.7 
PS1 Separation              01:51.5        56.4      2389.9 
PS2 Ignition                01:51.7        56.4      2389.9 
Payload Fairing Separation  03:24.5        112.8     3709.6 
PS2 Separation              04:23.5        129.9     5376.7 
PS3 Ignition                04:24.7        130.1     5376.3 
PS3 Separation              10:08.7        184.3     7734.1 
PS4 Ignition                10:18.7        186.1     7732.0 
PS4 Cutoff                  18:48.8        454.2     9638.4 
IRNSS-1 Separation          19:25.0        506.3     9598.9 

The PS3 burn time is quoted at only 83 seconds (1.38 min), and the flight profile shows PS3 ignition to separation time of 344 seconds (5.73 min). This doesn't make sense to me, especially after hearing the quote on a previous launch that launches always avoid carrying any mass for longer than they have to.

What is the benefit to carrying this 3rd stage for some time?

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For reference, here's the video of the mentioned PSLV C24 launch from the PS3 burnout time, starting the coasting phase. Commentators discuss this coasting phase to some detail in it. It also includes t/alt and t/v flight profile / telemetry graphs. –  TildalWave Apr 5 at 12:43

5 Answers 5

up vote 20 down vote accepted

Keeping a burnt-out stage attached doesn't hurt until it's time to start the next stage. In fact, keeping it attached until shortly before it's time to ignite the next stage can improve the total launch delta V.

Stage three separation occurred at an altitude of 184 km. There's still air up there. It's not very dense air, but for a vehicle going at close to orbital velocity, there's going to be a decent amount of drag on the vehicle. Drag acceleration is proportional to drag cross section and inversely proportional to mass.

Since rockets fly a zero angle of attack profile, that burnt-out stage won't increase the drag cross section by much. Keeping it attached obviously increases the mass compared to that of the vehicle without that stage attached. Keeping the stage attached reduces deceleration due to drag.

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This does explain why it would be better, during a coast, to keep the stage attached rather than jettison it immediately; but doesn't answer the question of why there is a coast period at all (instead of jettisoning the previous stage and lighting the next immediately)? –  dvnrrs Apr 4 at 19:29
    
@dvnrrs Did you read the whole question? The gist of it is why the 3rd stage would not separate from 4th stage as soon as possible. The question stems from our Space Exploration Chat event for the PSLV C24 launch, I know because I've been there chatting with Rory when the question popped up, but please suggest a new edit for it, if you think that part was not made clear enough. Thanks! –  TildalWave Apr 4 at 20:02
    
@TildalWave I did read the question. To someone not familiar with the chat, wondering why there's a coast seems a natural extension of the question. –  dvnrrs Apr 4 at 20:11
    
@dvnrrs Fair enough, I just thought that you missed that the question ends with "What is the benefit to carrying this 3rd stage for some time?", which is what David was answering here. –  TildalWave Apr 4 at 20:20
1  
BTW I'm still trying to figure it out if the separation boost also provides acceleration for 4th stage liquid propellants (MMH/MON). Since they would be in freefall together with the rest of it, the expansion gas (usually Helium) pushing propellant in the fuel (MMH) and oxidizer (MON) tanks would need to know somehow what's "up" and what's "down". Slight acceleration in the velocity vector could sort that out. Not sure if it applies to PSLV's PS4 tho. –  TildalWave Apr 4 at 20:35

The coasting period is certainly to perform a Gravity Turn.

It is a trajectory optimization that uses gravity to steer the vehicle onto its desired trajectory. ... the thrust is not used to change the ship's direction, so more of it is used to accelerate the vehicle into orbit.

Once the vehicle has coasted into the right angle, P4 ignition takes place completing the journey to orbit.

More relevant information from Wikipedia regarding the coasting period (emphasis added):

If the rocket were not producing thrust, the flight path would be a simple ellipse like a thrown ball (it's a common mistake to think it is a parabola: it is only true if you consider Earth is flat, and gravity always points in the same direction, which is a good approximation for short distances), leveling off and then falling back to the ground. The rocket is producing thrust though, and rather than leveling off and then descending again, by the time the rocket levels off, it has gained sufficient altitude and velocity to place it in a stable orbit.

If the rocket is a multi-stage system where stages fire sequentially, the rocket's ascent burn may not be continuous. Obviously, some time must be allowed for stage separation and engine ignition between each successive stage, but some rocket designs call for extra free-flight time between stages. This is particularly useful in very high thrust rockets, where if the engines were fired continuously, the rocket would run out of fuel before leveling off and reaching a stable orbit above the atmosphere. The technique is also useful when launching from a planet with a thick atmosphere, such as the Earth. Because gravity turns the flight path during free flight, the rocket can use a smaller initial pitchover angle, giving it higher vertical velocity, and taking it out of the atmosphere more quickly. This reduces both aerodynamic drag as well as aerodynamic stress during launch. Then later during the flight the rocket coasts between stage firings, allowing it to level off above the atmosphere, so when the engine fires again, at zero angle of attack, the thrust accelerates the ship horizontally, inserting it into orbit.

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+1. This is a high thrust rocket (compared to mass). 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, Whoosh! and it's gone. Big huge rockets carrying a big huge payload don't do this because Whoosh! and they're still there. Apollo and Shuttle launches were lumbering beasts during takeoff. –  David Hammen Apr 4 at 20:04
1  
@DavidHammen That's I believe how I got Rory to watch the launch with us, I promised it'll achieve 4 km/s even faster than in 4 minutes like the yesterday's Soyuz / Sentinel-1 launch from Kourou did. And Soyuz isn't even all that big, but PSLV beat it for about 30 seconds to 4 km/s. :) –  TildalWave Apr 4 at 20:09

Kerbal Space Program, the video game, teaches us that it is most effective to thrust at apoapsis (highest point of orbit) to increase our periapsis (lowest point of orbit). Waiting to fire the next stage there will make better use of fuel. Note that you want all the force applied at apoapsis, so you want to burn a little before and after it, since it is a moving target as you thrust.

Coasting to get further out of atmosphere can also increase the efficiency of some engines as they will have a higher specific impulse in vacuum compared to an atmosphere.

The other answers also point out that it helps minimizing deceleration due to drag to leave empty stages and their mass attached during ascent.

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-1 because 1. video games are generally no reliable source, even when they are simulation-oriented and 2. it's called Kerbal Space Program. –  Philipp Apr 5 at 13:32
3  
@Philipp You could have suggested an edit then, couldn't you? Haven't had the time to play it yet, but if I'm not mistaken, Kerbin is the home planet of Kerbals, so it's at worst just a simple confusion over the name of something that you later argue isn't even relevant. The apoapsis burn is actually relevant and a good call (point at which inclination change is the easiest to achieve, kinda inverse to Oberth maneuver), regardless of where this knowledge has been learned from. Also, please, try to be more courteous to new members, attitude of our senior members reflects on all of us. Thanks! –  TildalWave Apr 5 at 17:13
    
The physics underlying Kerbal is actually pretty good. –  Rory Alsop Apr 6 at 10:29
1  
For comedic citation xkcd.com/1356 –  EnabrenTane Apr 17 at 19:04

Any rocket flight will involve a burn in it's final orbital position. If you want to enter a 500km orbit your engine will shut down at 500km up. The profile shows it needs nearly 19 minutes to reach altitude but the rocket burn times only add up to 15 minutes. You can't magically make it get there in 15 minutes because you don't have extra fuel. Thus the engines must be quiet for 4 minutes. You can either accomplish this by restarting an engine or by delaying the ignition of an engine in the first place.

This is more typically done by a small engine used for the final burn (for example, the Shuttle jettisoned the main tank and used it's orbital maneuvering engines for the final burn) but in this case it obviously made more sense to use the 4th stage.

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Some "coasting" phases are designed to limit the aerodynamic stresses on the spacecraft. The US space shuttle is an example of this. After around 26 seconds the main engines are throttled back for the next 34 seconds. In actuality this is called a "Thrust Bucket".

While this isn't a true "coast phase" because the solid rocket boosters are still burning, it is in some sense a "coast" because you are trying to limit acceleration to reduce heat and aerodynamic stress on the spacecraft.

While other answers here accurately describe your particular use-case for coasting, this answer notes that there are other reasons for the general question of "Why is there a 'coasting' phase in some space launches" for the benefit of future visitors.

Credit to TildalWave for this reference and naming/clarifying what I was going on about.

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