Do seconds stages have the option to be smoother in Start/Stop?

[Serenity]

Hello, i was watching the SpaceX recent crewed launch and their comments that the second stage was a lot more rough ride than the first Stage.

Can a second stage be selective and be more smoother when it starts/stops?

Or its a certain threshold they cant control and it has to be rough everytime they start to burn/stop burn?

Edited August 23, 2020 by Boyster

[DDE]

6 minutes ago, Boyster said:

Hello, i was watching the SpaceX recent crewed launch and their comments that the second stage was a lot more rough ride than the first Stage.

Can a second stage be selective and be more smoother when it starts/stops?

Or its a certain threshold they cant control and it has to be rough everytime they start to burn/stop burn?

I think you're looking at a completely wrong parameter. The roughness is usually a result of peak acceleration, determined by peak TWR. On the first stage, the engines cut out while the second stage is full and there's still a significant share of the propellant for RTLS. The second stage, meanwhile, burns to near-depletion with nothing but the spacecraft as payload.

[sturmhauke]

Also, real rocket engines generally have a minimum throttle setting that they can't go below. It has to do with the design of the fuel pumps, coolant systems, etc. This means that a second stage ignites its engines at, say, 30% throttle and immediately jumps to a higher acceleration.

[Serenity]

1 hour ago, DDE said:

I think you're looking at a completely wrong parameter. The roughness is usually a result of peak acceleration, determined by peak TWR. On the first stage, the engines cut out while the second stage is full and there's still a significant share of the propellant for RTLS. The second stage, meanwhile, burns to near-depletion with nothing but the spacecraft as payload.

So you saying the real problem is how high those parameters will reach not how fast you gonna reach peak accelearation/peak TWR?

27 minutes ago, sturmhauke said:

Also, real rocket engines generally have a minimum throttle setting that they can't go below. It has to do with the design of the fuel pumps, coolant systems, etc. This means that a second stage ignites its engines at, say, 30% throttle and immediately jumps to a higher acceleration.

Is it possible for the second stage engine to stay close to 30% or is it bound to increase a lot and then maybe throttle down?

Edited August 23, 2020 by Boyster

[wumpus]

There's also the difference between one engine and nine.  To a certain degree, I'd expect the "roughness" of each engine to cancel out to a degree.  The overall effect might have more absolute "roughness", but with nine times the mass it doesn't matter.  No idea if this makes a difference.

[sevenperforce]

1 hour ago, wumpus said:

There's also the difference between one engine and nine.  To a certain degree, I'd expect the "roughness" of each engine to cancel out to a degree.  The overall effect might have more absolute "roughness", but with nine times the mass it doesn't matter.  No idea if this makes a difference.

I think this is a very big factor. One single engine is more likely to produce high jerk transients, with a lot of sudden acceleration associated with gimbal and throttle variation. What human beings perceive as a roughness is not actually peak acceleration, but the rate at which acceleration changes. We can handle a lot of acceleration as long as it does not happen suddenly.

There are no perceptible start up transients for the Falcon 9 first stage, because all the engines are lit on the pad. Small variations in pitch and yaw are done with multiple small engine gimbal movements, and are further damped by aerodynamic forces and the overall size of the stage combination.

[Serenity]

9 minutes ago, sevenperforce said:

I think this is a very big factor. One single engine is more likely to produce high jerk transients, with a lot of sudden acceleration associated with gimbal and throttle variation. What human beings perceive as a roughness is not actually peak acceleration, but the rate at which acceleration changes. We can handle a lot of acceleration as long as it does not happen suddenly.

There are no perceptible start up transients for the Falcon 9 first stage, because all the engines are lit on the pad. Small variations in pitch and yaw are done with multiple small engine gimbal movements, and are further damped by aerodynamic forces and the overall size of the stage combination.

What if the second stage had some extra small thrusters just to smooth the transition/difference of acceleration when the big main engine starts, would that make the ride smoother?And same with the stop, if main engine cut off but the smaller ones kept running for a bit to smooth the de acceleration, thats if removing the factor of complicating things etc, would that make the ride smoother?

Edited August 24, 2020 by Boyster

[sturmhauke]

4 hours ago, Boyster said:

Is it possible for the second stage engine to stay close to 30% or is it bound to increase a lot and then maybe throttle down?

That all depends on the design of the specific engine in question.

1 hour ago, Boyster said:

What if the second stage had some extra small thrusters just to smooth the transition/difference of acceleration when the big main engine starts, would that make the ride smoother?And same with the stop, if main engine cut off but the smaller ones kept running for a bit to smooth the de acceleration, thats if removing the factor of complicating things etc, would that make the ride smoother?

It might, but it would also make that stage more complex and increase the mass, which is something that is best kept to a minimum.

[DDE]

4 hours ago, Boyster said:

What if the second stage had some extra small thrusters just to smooth the transition/difference of acceleration when the big main engine starts, would that make the ride smoother?

By necessity the second stage either has smaller ullage thrusters, or is fired before separating from the first stage. Doesn't help much.

7 hours ago, Boyster said:

Is it possible for the second stage engine to stay close to 30% or is it bound to increase a lot and then maybe throttle down?

No. The upper stage has limited thrust as is, why would you throttle it down from 100%-110% (AFAIK 109% was typical for late Shuttles)?

You only throttle down to reduce water hammer effects in the fuel lines at thrust cutoff.

[mikegarrison]

What does "rough ride" mean, anyway?

Usually upper stages have longer burns but lower acceleration. I'm not sure exactly what the Crew Dragon profile looks like, so I don't know if it fits the usual pattern.

Edited August 24, 2020 by mikegarrison

[Serenity]

24 minutes ago, mikegarrison said:

What does "rough ride" mean, anyway?

Usually upper stages have longer burns but lower acceleration. I'm not sure exactly what the Crew Dragon profile looks like, so I don't know if it fits the usual pattern.

https://youtu.be/dfA8yulRnrE?t=36

I am just wondering if in the near future space tourism becomes a thing, how can you smooth the ride to space, if its even possible.

Or its always gonna be ''rough''

Edited August 24, 2020 by Boyster

[DDE]

59 minutes ago, Boyster said:

https://youtu.be/dfA8yulRnrE?t=36

I am just wondering if in the near future space tourism becomes a thing, how can you smooth the ride to space, if its even possible.

Or its always gonna be ''rough''

Pretty sure that, given how space tourism is a type of extreme tourism, the rough ride is a feature.

[Hannu2]

As far as I know, rough means high vibrations and not high time average of acceleration. Combustion process is very turbulent and thrust magnitude and direction varies significantly in sub second time scale.

9 engines in first stage partly cancel out their vibrations (and give more complex modes). I think there is not much more to do in addition to what has been already done. Lower thrust means lower vibrations, but have other problems. More smaller engines would also do the trick but increase complexity and costs. It is better to build systems so that they can stand vibrations. Astronauts are tough and are able to stand rough ride.