Questions for real life

[Spaced Out]

Ok so I know TWR goes up in flight but ia the optimal TWR the average TWR you want to be at or your liftoff TWR? 

Also, I know that rockets throttle in flight to control TWR why is that?

Is the rocket ascent profile designed to fit the acceleration profile, or is the acceleration profile designed to fit the ascent profile? Or are they both designed to fit each other?

Do rockets want to refrain from throttling too much, and if so is there a reason to this?

Take note that this is all real life so try to use real life examples but you can use examples from KSP too.

[mikegarrison]

6 minutes ago, Spaced Out said:

Also, I know that rockets throttle in flight to control TWR why is that?

A lot of rockets can't throttle in flight. Pretty much only the newer ones. Saturn V, for instance, could not throttle the F1 engines, so they shut down the center engine of the first stage when the TWR got too high.

The main issue is that the load on the rocket and the payload is proportional to the acceleration. (Ie. people are squishy and can't take too many Gs for too much time.)

Edited September 2, 2017 by mikegarrison

[MrChumley]

50 minutes ago, mikegarrison said:

Saturn V, for instance, could not throttle the F1 engines, so they shut down the center engine of the first stage when the TWR got too high.

The F1 had a throttle range of 56%-100%. The middle gimbal was locked.

 

[Bill Phil]

26 minutes ago, MrChumley said:

The F1 had a throttle range of 56%-100%. The middle gimbal was locked.

 

Source? I highly doubt that the F-1 could throttle. Adding such a complex system to such a large (and unstable) engine seems to be a very bad idea for reliability's sake.

[mikegarrison]

See point #2? This is where the middle engine was shut down, so that the G load did not get too high.

(Point 5 is a similar middle engine cutoff. Point 6 is different -- they changed the fuel/ox ratio. So that's the only point on the chart that shows what could be called throttling.)

Edited September 2, 2017 by mikegarrison

[Bill Phil]

14 minutes ago, mikegarrison said:

See point #2? This is where the middle engine was shut down, so that the G load did not get too high.

(Point 5 is a similar middle engine cutoff. Point 6 is different -- they changed the fuel/ox ratio. So that's the only point on the chart that shows what could be called throttling.)

Even if that could be called throttling, at point 6 the second stage is active, which used J-2s.

[mikegarrison]

Just now, Bill Phil said:

Even if that could be called throttling, at point 6 the second stage is active, which used J-2s.

Yes, I know. That's why I said in my first post "Saturn V, for instance, could not throttle the F1 engines".

[Steel]

4 hours ago, Spaced Out said:

Ok so I know TWR goes up in flight but ia the optimal TWR the average TWR you want to be at or your liftoff TWR? 

Also, I know that rockets throttle in flight to control TWR why is that?

Is the rocket ascent profile designed to fit the acceleration profile, or is the acceleration profile designed to fit the ascent profile? Or are they both designed to fit each other?

Do rockets want to refrain from throttling too much, and if so is there a reason to this?

Take note that this is all real life so try to use real life examples but you can use examples from KSP too.

 1: There is no optimal value (provided it is greater than 1). It depends entirely on the rocket, and the mission profile that it is flying. For instance, you may want a low value at lift off if you know fuel is going to burn off quickly as so the TWR will rise very quickly. Also if your engines can throttle, then you can have a higher TWR early on because you can throttle down later in the flight, but if they can't throttle then you probably want a lower TWR early on to avoid excessive acceleration later.

2: 

4 hours ago, mikegarrison said:

A lot of rockets can't throttle in flight. Pretty much only the newer ones. Saturn V, for instance, could not throttle the F1 engines, so they shut down the center engine of the first stage when the TWR got too high.

Rockets tend to throttle to manage structural loads, or simply just to lower fuel burn while they're still ascending, so they can use more fuel later on when they have completed the gravity turn and are putting all the dV into increasing orbital velocity.

3: Both profiles would typically be designed to reduce losses (cosine, gravity and aerodynamics) in the early parts of the ascent as much as possible.

4: As before, most rockets can't throttle. Even the ones that can often can only drop down to 70% or 80% of max thrust, and most likely can only do that in fairly big jumps (i.e either in jumps of 10% or more). There are definitely no rockets that can throttle continuously like in KSP. One of the most throttle-able 1st stage rockets rockets (we're ignoring things like the super Draco here) was the Space Shuttle Main Engine (SSME) which could throttle from 109% to 67% rated power (so basically 100%-58%) in 1% increments. Also throttling, especially on older engines, comes with a certain degree of risk that the engine will stop burning due to the drop of fuel pressure, chamber pressure e.t.c

 

3 hours ago, MrChumley said:

The F1 had a throttle range of 56%-100%. The middle gimbal was locked.

 

Also to clarify, The F1 could not throttle at all.

Edited September 2, 2017 by Steel

[MrChumley]

You are right. I was wrong. The F1s did not throttle.

[magnemoe]

Note that on the other side SRB is easier, in KSP you can just set trust, in real life you can shape the burn profile so the SRB reduces trust over time.
This has to be done during production. but reduces lots of the need for throttling of the main engines. 

[wumpus]

13 hours ago, mikegarrison said:

See point #2? This is where the middle engine was shut down, so that the G load did not get too high.

(Point 5 is a similar middle engine cutoff. Point 6 is different -- they changed the fuel/ox ratio. So that's the only point on the chart that shows what could be called throttling.)

Just out of curiosity, were they flying more or less horizontally before they hit 4g?  If they were (perfectly horizontal), the total would be ~4.1 (thanks to Pythagoras) while straight up would be  5.  I'd expect that even at 3g they weren't all that close to vertical and being squished.  Micheal Collins (in his highly recommended book/autobiography) didn't seem to worry to much about such acceleration, but I think he tested fighters for the Air Force.

[magnemoe]

4 minutes ago, wumpus said:

Just out of curiosity, were they flying more or less horizontally before they hit 4g?  If they were (perfectly horizontal), the total would be ~4.1 (thanks to Pythagoras) while straight up would be  5.  I'd expect that even at 3g they weren't all that close to vertical and being squished.  Micheal Collins (in his highly recommended book/autobiography) didn't seem to worry to much about such acceleration, but I think he tested fighters for the Air Force.

Its not only the astronauts to worry about, also the structure of the rocket, been some talk if falcon heavy can support an 60 ton payload in disposable mode. 
solution here is to reduce trust, you would want to ramp down core stage as fast as practical anyway to leave more after you drop the boosters. 

[Iskierka]

5 hours ago, wumpus said:

Just out of curiosity, were they flying more or less horizontally before they hit 4g?  If they were (perfectly horizontal), the total would be ~4.1 (thanks to Pythagoras) while straight up would be  5.  I'd expect that even at 3g they weren't all that close to vertical and being squished.  Micheal Collins (in his highly recommended book/autobiography) didn't seem to worry to much about such acceleration, but I think he tested fighters for the Air Force.

Irrelevant, because the g-force measured is the acceleration of the rocket; as it is in no way connected to the ground, gravity does not influence experienced forces, as all things are falling uniformly, and the only non-body-force is that of the engine's thrust.

The F-1 engine would also be capable of throttling with minimal modification; the required pieces of flow control valves, flow sensors, and control feedback systems were already all established in the engine to maintain proper operation at a fixed throttle. The only modification required to get some throttle capability would be a control unit that could accept a requested throttle and adjust the feedback loop accordingly. While the original engine could not throttle, it was not a physical limitation of the design, simply a practical matter.

The CECO shutdown was also not to limit g-forces, although the pilots may have appreciated that factor more. It was to limit pogo oscillation, as the vehicle becoming lighter, as well as the increased duration of flight and reduced atmosphere damping meant the centre engine would start to bounce back and forth, restricting fuel flow as it came forward and compressed its supply line and increasing it as it fell back from reduced thrust. The thrust structure supporting the engines was only mounted to the main body around the outside of the vehicle, so the centre was not fully secured to prevent this; rather than add structural mass to do so, they stopped the engine before it became a problem.

The reason is pretty much the exact same in stage 2, where you may note that the g-force CECO occurs at is significantly lower than stage 1 CECO, or especially stage 1's peak acceleration. This acceleration is clearly tolerable, so it's an engineering reason for stopping the engine.

[mikegarrison]

56 minutes ago, Iskierka said:

Irrelevant, because the g-force measured is the acceleration of the rocket; as it is in no way connected to the ground, gravity does not influence experienced forces, as all things are falling uniformly, and the only non-body-force is that of the engine's thrust.

The F-1 engine would also be capable of throttling with minimal modification; the required pieces of flow control valves, flow sensors, and control feedback systems were already all established in the engine to maintain proper operation at a fixed throttle. The only modification required to get some throttle capability would be a control unit that could accept a requested throttle and adjust the feedback loop accordingly. While the original engine could not throttle, it was not a physical limitation of the design, simply a practical matter.

The CECO shutdown was also not to limit g-forces, although the pilots may have appreciated that factor more. It was to limit pogo oscillation, as the vehicle becoming lighter, as well as the increased duration of flight and reduced atmosphere damping meant the centre engine would start to bounce back and forth, restricting fuel flow as it came forward and compressed its supply line and increasing it as it fell back from reduced thrust. The thrust structure supporting the engines was only mounted to the main body around the outside of the vehicle, so the centre was not fully secured to prevent this; rather than add structural mass to do so, they stopped the engine before it became a problem.

The reason is pretty much the exact same in stage 2, where you may note that the g-force CECO occurs at is significantly lower than stage 1 CECO, or especially stage 1's peak acceleration. This acceleration is clearly tolerable, so it's an engineering reason for stopping the engine.

Let me just point out that the pogo oscillation is related to the relationship between the stiffness of the engine mount and the forces on it -- so for any given engine mount stiffness, "shutting it down before the pogo mode starts" is functionally the same thing as "limiting the g-forces". As in, if they wanted the mount to be able to go to 5 gs without the pogo oscillation, they could have done it. But instead, they went with what was about a 4-g limit. (And on at least one flight they shut it down early.)

That also means they limited the stresses in all the other parts of the entire rocket too, including the crew.

What I'm saying is that you are talking about what the specific failure mode is, but it's also true to say that in the bigger picture they were limiting the acceleration forces.

Edited September 3, 2017 by mikegarrison

[Iskierka]

14 hours ago, mikegarrison said:

Let me just point out that the pogo oscillation is related to the relationship between the stiffness of the engine mount and the forces on it -- so for any given engine mount stiffness, "shutting it down before the pogo mode starts" is functionally the same thing as "limiting the g-forces". As in, if they wanted the mount to be able to go to 5 gs without the pogo oscillation, they could have done it. But instead, they went with what was about a 4-g limit. (And on at least one flight they shut it down early.)

That also means they limited the stresses in all the other parts of the entire rocket too, including the crew.

What I'm saying is that you are talking about what the specific failure mode is, but it's also true to say that in the bigger picture they were limiting the acceleration forces.

The problem with simplifying it this way is that g-force did not cause the pogo oscillation; it would not be prevented by shutting down two outboard engines, or shutting down one and gimballing to compensate. It's specifically the inboard engine that suffers and must be shut down, independent of what the acceleration is, and was a problem but tolerable from ignition to CECO.

While limiting stress would potentially be an argument for the lower stage, it does fall flat noting, again, that the second stage has CECO at very low acceleration. This is a localised problem to just that engine that must be addressed.

[mikegarrison]

6 hours ago, Iskierka said:

The problem with simplifying it this way is that g-force did not cause the pogo oscillation; it would not be prevented by shutting down two outboard engines, or shutting down one and gimballing to compensate. It's specifically the inboard engine that suffers and must be shut down, independent of what the acceleration is, and was a problem but tolerable from ignition to CECO.

While limiting stress would potentially be an argument for the lower stage, it does fall flat noting, again, that the second stage has CECO at very low acceleration. This is a localised problem to just that engine that must be addressed.

The second stage is easily explained because the forces on that second stage engine are in different directions when the first stage is firing versus when the second stage is firing.

I still think we are talking about the same issues using different words, but I would need to know more details to be sure.

Let me put it this way -- the force from the engine is roughly the same the entire time (discounting effects due to the external atmospheric pressure changing). Why would pogo be any worse as the fuel load burned down if the overall g-forces on the rocket as a whole were not involved with it?

Edited September 3, 2017 by mikegarrison

[Spaced Out]

On 9/2/2017 at 1:20 AM, Steel said:

2: 

Rockets tend to throttle to manage structural loads, or simply just to lower fuel burn while they're still ascending, so they can use more fuel later on when they have completed the gravity turn and are putting all the dV into increasing orbital velocity.

When you said lowering fuel burn do you just mean putting it at a good TWR to conserve fuel the rest of the flight until you are focusing on orbital velocity or need to throttle again?