Reasons why rockets throttle in flight?

[Spaced Out]

When they design the rocket acceleration profile, do they make it so that as it starts gaining more and more gees of acceleration it is in a place where there is less drag or some other factor to create stress or take away delta v for that current acceleration?

[Steel]

From what I understand, the ascent profile of any launch will be designed primarily to reduce dV losses (though cosine losses, aerodynamic losses and gravity losses), while keeping within key parameters such a maximum structural load or (in the case of a manned flight) maximum acceleration.

This means that the acceleration at any given point is dependent on a huge number of factors: the aerodynamic performance the rocket, its TWR, its ability to throttle, whether or not it is manned, the thrust available, maximum structural load and many other factors. This means it is impossible to give you a general answer that applies to all rockets.

[kerbiloid]

A booster usually separates the first stage usually somewhere at 40 km, while meets the max drag somewhere at 10-12 km, i.e. with half-full first stage and half-low T/W.
As the air drag doesn't stop it completely, we can presume, that the stress from air drag deceleration is definitely less than the stress from its own full T/W before the stage separation.
So, unlikely somebody really gets bothered with this, mostly they follow "it works - don't touch".

[Spaced Out]

8 hours ago, kerbiloid said:

A booster usually separates the first stage usually somewhere at 40 km, while meets the max drag somewhere at 10-12 km, i.e. with half-full first stage and half-low T/W.
As the air drag doesn't stop it completely, we can presume, that the stress from air drag deceleration is definitely less than the stress from its own full T/W before the stage separation.
So, unlikely somebody really gets bothered with this, mostly they follow "it works - don't touch".

So are you saying that the gees from accelerating are worse than the forces you get while in max q so they throttle down as a precaution and to conserve delta v?

[Spaced Out]

What are some reasons rockets throttle in real life?

Is it like ksp when you want to pitch over a bit quicker?

I know they throttle during max q but do they typically go to full power after?

And after Max Q do they gradually throttle up or just go all out? 

[kerbiloid]

27 minutes ago, Spaced Out said:

they throttle down

Do they? Mostly they use "overthrottled" engines to take off
(say, like this

Quote

The RD-270 engine was used in all nine modules of the first and second stages, operating at 103% thrust at lift-off.

), then just throttle them a little down to "nominal" throttle to prevent overheat.

Say, planes also fly with ~80% throttle to keep them intact as long as possible..

Edited September 3, 2017 by kerbiloid

[DDE]

One of the reasons to throttle down is to keep acceleration under control, especially if you have strap-on boosters which, when almost empty, cause your TWR to go through the roof.

[Theysen]

18 minutes ago, Spaced Out said:

What are some reasons rockets throttle in real life?

Is it like ksp when you want to pitch over a bit quicker?

I know they throttle during max q but do they typically go to full power after?

And after Max Q do they gradually throttle up or just go all out? 

- IF - you have the ability to throttle in the first place it is like @DDE already pointed out to keep G-Load for crew or payload in the launcher's determined limits and prevent structural load exceeding those of the specifications. 

 

Liquid fueled rocket engines these days mostly have the ability to throttle (e.g. RD-180, RS-68A, RS-25/SSME, Merlin, etc.). SRB's have the possibility to have varying thrust by adjusting the pre-fabricated area of how much fuel is being burnt at what point of time. There are no ends to possibilites in shape, area, direction

[Vanamonde]

Some posts have been removed from this thread. This forum is for discussions of simulated and real space flight. It is not for older members to shame newbies. Can't we just be glad that there are more people to talk about this stuff with? 

[Steel]

5 hours ago, Spaced Out said:

What are some reasons rockets throttle in real life?

Is it like ksp when you want to pitch over a bit quicker?

I know they throttle during max q but do they typically go to full power after?

And after Max Q do they gradually throttle up or just go all out? 

There are several reasons, some of which have been mentioned above. However, the main reason is that any time the rocket is burning fuel when it is not at high altitude and burning directly along its velocity vector it is wasting dV. Thus, you want to burn as little fuel as possible to get up above 100km an pointing towards the horizon to minimise these losses, while still having enough thrust to get there in the first place.

[Spaced Out]

1 hour ago, Steel said:

There are several reasons, some of which have been mentioned above. However, the main reason is that any time the rocket is burning fuel when it is not at high altitude and burning directly along its velocity vector it is wasting dV. Thus, you want to burn as little fuel as possible to get up above 100km an pointing towards the horizon to minimise these losses, while still having enough thrust to get there in the first place.

So rockets mainly throttle to keep a TWR that minimises drag and gravity losses. But do they ever have to throttle in order to follow the most efficient ascent profile? (Such as throttling down so their gravity turn pitches down faster, or does that increase gravity losses?)

[Pand5461]

28 minutes ago, Spaced Out said:

So rockets mainly throttle to keep a TWR that minimises drag and gravity losses. But do they ever have to throttle in order to follow the most efficient ascent profile?

Probably not. But there are other reasons. Sometimes it's just good to have another control variable (throttle level) for guidance algorithm.

Imagine burning a maneuver node in KSP, and the full-thrust burn is 30 seconds long. Ideally, you want to start approximately 15 seconds before node. And, let's say, you are really impacient and accidentally start burning 20 seconds before the right time, or 35 seconds before node. In LKO, such an error in timing can lead to your orbit being quite different from what you want it to be. BUT you can correct if you burn not at full thrust but set a throttle level that makes your burn 70 seconds instead of 30, and it's still going to be split in half by the node.

For powered landings, throttle control is absolutely essential. There is the "ideal" suicide burn trajectory which needs precise knowledge of gravity field, atmosphere, engine stats etc. and starting engine at very precise moment. If engine is lit a split second later, rocket is going to come to stop a few meters below ground. A split second earlier - and it's going to stop a few meters above the ground. For Falcon 9 stages that cannot hover the latter is not much better than the former. And you cannot start an engine at a very precise time, so throttling during powered descent allows to compensate for errors, deviations and disturbances.

[sevenperforce]

Since most modern launch vehicles do not restart to circularize, throttling down is essential to hitting a precise orbit...essentially maintaining a consistent time to apogee right up until the point that you circularize.

[Spaced Out]

2 hours ago, Pand5461 said:

 

Imagine burning a maneuver node in KSP, and the full-thrust burn is 30 seconds long. Ideally, you want to start approximately 15 seconds before node. And, let's say, you are really impacient and accidentally start burning 20 seconds before the right time, or 35 seconds before node. In LKO, such an error in timing can lead to your orbit being quite different from what you want it to be.

Is there any examples of this during ascent with the first stage? (And by the way thank you for answering my questions )

[YNM]

Rockets that can throttle throttles itself...

 

Because it can.

The priorities that should be taken by a rocket are :

- Avoid RUD

- Avoid failures

- Carry out trajectory

As rockets generally carry forces close enough (or even at, or exceeding what's required) to shred itself into bits, sometimes they can't cope if continued at maximum and hence there should be an action taken to reduce the forces. Down-throttling is one of them. Though the reverse also occur, for example in SSME. The later shows that they need to stay on a trajectory, no matter whether they're capable of exceeding it or not.

Some rocket stages does ineffective burns. One of the most prominent to me is Ariane V. All of which is to ensure the trajectory is achieved.

Edited September 4, 2017 by YNM

[Vanamonde]

Alright, since people are determined not to stop bugging the poor newbie about this, a couple of similar threads have been merged. 

Now talk about the subject rather than each other's posting habits or we will start handing out warns.  

[Hannu2]

12 hours ago, Spaced Out said:

So rockets mainly throttle to keep a TWR that minimises drag and gravity losses. But do they ever have to throttle in order to follow the most efficient ascent profile? (Such as throttling down so their gravity turn pitches down faster, or does that increase gravity losses?)

Gravity losses are the lower the faster acceleration is. Drag losses are relatively small in normal launches (about 100 m/s or less). More important reasons to throttling is avoiding too large stresses in launcher's and payload's structures. Details depend on many things, as already mentioned, but as far as I know all rockets lift off at full thrust because they want to clear the tower as soon as possible. Lift off is dangerous phase. The craft can hit launch structures or if thrust fails it does not give much time for abort. When craft gets speed the drag increases. About after one minute speed is transsonic and soon after that the aerodynamic pressure is highest. Aerodynamic forces exert to shrouds and surfaces of craft and they are also turbulent and may cause torques. At that phase many rockets throttle down to avoid too high speeds in too high pressure air. Also if launcher has SRB's they are light and push at high thrust. Thrust profile of SRB's can be changed somewhat but limits are quite narrow.

Soon after max Q aerodynamic stresses decrease rapidly and typically SRBs burn out and are jettisoned. Typically rockets burn at nominal thrust because it is the most fuel economic way. Near the end of 1st stage it gets very light and acceleration increases and causes stress to payload and rocket structures. In that phase engines are throttled down to limit acceleration to largest allowed value of rocket or payload.

Second stages have usually much weaker TWR and do not always need throttling at all.

 

[Spaced Out]

4 hours ago, Hannu2 said:

At that phase many rockets throttle down to avoid too high speeds in too high pressure air

How long do they wait after max-q to throttle back up? Is there a certain safe amount of air pressure for them to throttle back up?

And one more thing, do they have to create the ascent profile to accomodate the faster gravity turn if they throttle down? Or is it not enough to matter? Because if you slow down the rocket will pitch over faster in that area from the gravity turn.

Edited September 4, 2017 by Spaced Out

[Pand5461]

13 hours ago, Spaced Out said:

Is there any examples of this during ascent with the first stage?

No, I believe throttling on ascent stages is primarily for other reasons:

• To limit g-loads and drag, as pointed out many times
• If there are boosters, it's beneficial to save fuel in the core stage for longer burn after booster separation (Delta IV Heavy, Soyuz)
• To equalize the wear on engines (landing engines of reusable Falcon 9)

[Steel]

17 hours ago, Spaced Out said:

So rockets mainly throttle to keep a TWR that minimises drag and gravity losses. But do they ever have to throttle in order to follow the most efficient ascent profile? (Such as throttling down so their gravity turn pitches down faster, or does that increase gravity losses?)

The faster you turn, the further away you are from pointing along your velocity vector, so you incur cosine losses (losses caused by not thrusting through your velocity vector).

[sevenperforce]

I've been working through a long-duration challenge that replicates the development of the Falcon 9 family by requiring thrust-limiting of the first-stage engines to make the dV requirements more challenging. It's very neat to see how the ascent profile has to change depending on TWR and throttling capability.

 

[Hannu2]

On 9/4/2017 at 6:51 PM, Spaced Out said:

How long do they wait after max-q to throttle back up? Is there a certain safe amount of air pressure for them to throttle back up?

And one more thing, do they have to create the ascent profile to accomodate the faster gravity turn if they throttle down? Or is it not enough to matter? Because if you slow down the rocket will pitch over faster in that area from the gravity turn.

I do not know details. Safety limits depend on launcher type and some payloads may have own restrictions. This video shows main engine throttle of Shuttle. You can see a dip at around 1 minute to avoid excess aerodynamic stress but it runs at full thrust over burn out and jettison of SRBs.

They make massive simulations for ascent profiles and take every known things into account. They have also huge number of emergency profiles for situations in which thrust is anomalous for some reason. Some of them may lead to intended or wrong orbit and some of them makes just abort more safe.

 

 

[wumpus]

On 9/3/2017 at 11:54 AM, DDE said:

One of the reasons to throttle down is to keep acceleration under control, especially if you have strap-on boosters which, when almost empty, cause your TWR to go through the roof.

This only matters if it creates a [new] maxQ or your engines and cargo (especially astronauts) have issues with acceleration.  The structural supports will bear a weight equal to the thrust of the rockets, and the actual mass & acceleration simply fit the needed ratio.  Of course if your balloon fuel tanks supply more support when full, this could be a problem.  But it doesn't add support issues by losing mass.

The above post was where I derived an equation for efficiency TWR vs. velocity of the rocket.  As far as I can tell, there's no reason throttle unless you are approaching terminal velocity.  This was a big thing in pre-release KSP and the "souposphere" but now only likely considered for places like Eve, Venus, and any gas giants you get too close to.  Furthermore, you are almost certainly not reaching these speeds in a liquid fueled rocket, so you would be "preshaping" the solid rocket boosters to reduce thrust to avoid such high speeds at insufficient altitude.

That leaves reducing maxQ and reducing max acceleration (especially for manned flights and delicate scientific flights).  And probably things like "rely more on rockets you will stage first and less on fuels you will stage later" and other efficiency issues directly related to the rocket equation.

Edited September 10, 2017 by wumpus
hopefully grammering better

[ARS]

To keep the acceleration, atmospheric heating and G-force during launch within an acceptable limit. Yes, you can say we can make a stronger craft that can withstand more, but that cannot be said with their crew or payload. There are several reasons why engines can be throttled:

- Prevent excessive atmospheric drag which causes overheating (usually leads to RUD when excessive heat causing structural failure)

- Prevent excessive G-force to protect the crew and payload (Remember, they are still human being, no matter how tough they are. Keep G-force reasonable to avoid turning them into bloody paste in cockpit)

- Better gimbaling (If you gimbal a nozzle at full power, the sudden change of heading might cause a structural failure, especially when the craft is still within the atmosphere)