Real world throttle up (and down) times

[Aser]

In KSP you have access to instant changes in throttle (x and z). I'm wondering, what are the real world rates of throttle change? I expect it takes some amount of time to go from zero thrust to max and from max back to zero.

[Nibb31]

Real world rocket engines hardly throttle at all. Most of them are on-off designs. They certainly have trouble deep-throttling below 50%. I think SSMEs could throttle between 70 and 110%. Only the SpaceX Merlin seems to be able to throttle down to something like 30% for landing. The Apollo LM engine could throttle down to 10%, but it could really only operate at either 10 to 50% or 100%.

When throttling down or switching off, they typically have a few seconds of residual thrust. It's not instantaneous. Edited November 24, 2015 by Nibb31

[Theysen]

ninja'd ^

[RainDreamer]

I imagined setting fire to several tons of explosives is not something you can just dial down or up easily at will.

[AeroGav]

KSP treats all rocket engines the same in this regard - instant thrust.

That is probably close to the truth when it comes to solid rocket motors, and also lower thrust liquid engines which are pressure fed. Basically , the whole fuel tank is pressurised with a helium cylinder and releasing the fuel into the combustion chambers is much like squirting an aerosol can - instant. These pressure fed engines are used for orbital maneuvering and some upper stages.

However, higher power liquid engines pump their propellants in with turbopumps. The fuel tank is unpressurised and they are reliant on a turbine to do the pumping. Some of the fuel stream is diverted to a power turbine to drive this turbopump. These are much more like jet engines and take a while for the pumps to rev up and reach max power. I think the space shuttle main engines were ignited 7 seconds before launch, because they needed to spool up. At zero seconds on the countdown, an incendiary charge is fired within the solid rocket boosters. Thing happen pretty quickly after that...

[Nibb31]

Actually, on solid rockets, the thrust varies with burn time depending on mission requirements. Getting the proper thrust curve is a hard part of designing a solid rocket. They do this by using different castings for the solid fuel inside the booster. Also, they don't stop instantly. They tend to burn out gradually, which is why they seem to still be burning when they are jettisoned. Edited November 24, 2015 by Nibb31

[Hcube]

[quote name='Nibb31']Real world rocket engines hardly throttle at all. Most of them are on-off designs. They certainly have trouble deep-throttling below 50%. I think SSMEs could throttle between 70 and 110%. Only the SpaceX Merlin seems to be able to throttle down to something like 30% for landing. The Apollo LM engine could throttle down to 10%, but it could really only operate at either 10 to 50% or 100%.

When throttling down or switching off, they typically have a few seconds of residual thrust. It's not instantaneous.[/QUOTE]
iirc the SSMEs can throttle from something like 68 to 109% of rated power, but most importantly in 1% increments. That's really impressive ! So much precision for that amount of thrust... Edited November 24, 2015 by Hcube

[Streetwind]

[quote name='Nibb31']Only the SpaceX Merlin seems to be able to throttle down to something like 30% for landing.[/QUOTE]

Hah, SpaceX [I]wishes[/I] it would :P That would make their first stage landing so much easier, because the F9 first stage could hover. Alas, the Merlin 1D as has flown so far can throttle only down to 60% of its 650 kN sea-level thrust, which is still a TWR >1 even with a single engine, and so the stage must perform a "hoverslam" (suicide burn).

Of course, that "60%" figure is an old hat - now that the Merlin 1D is uprated to its "full thrust" of 756 kN sea-level, it still retains the same minimum thrust from before, which is now roughly 50% of the maximum.

@Topic: SpaceX has a much more throttle-able engine though, namely the SuperDraco. It can go at least as low as 20%, maybe even lower, and more importantly: it switches between throttle states almost instantaneously. The time delta between ignition and 100% thrust, for example, is just 0.1 seconds. You can see them showing off with the throttle response in [URL="https://www.youtube.com/watch?v=0zOzk0keqU8"]this recent video[/URL]. Edited November 24, 2015 by Streetwind

[Shpaget]

It appears it still takes several seconds to cut the power.

Anyway, the suicide burn doesn't seem to be a problem for powered landing. Their timing was quite good the last time they got to that phase.

[SomeGuy12]

So, TLDR, to land a [I]real [/I] Moon lander you need computer engine control. You cannot throttle to 31% thrust so you are descending slowly and gracefully.

So when Armstrong "went to manual", the computer must have still be in the loop to help him control the craft, he was just telling it to move laterally because the boulder field was a bad spot.

[Shpaget]

Engines on the Apollo landers were throttleable to about 10%.

[Nuke]

i know that jet engines also take a few seconds for changes in their throttle settings to take effect. ksp seems to model that but with much shorter adjustment times.

[Aser]

Thanks for all the replies!

I was thinking about trying to use real world throttle profiles in KSP, but it sounds like it would [B][U]significantly[/U][/B] increase difficulty. Just in case I'm feeling brave, does anyone know of a mod(s) that model the inability to instantaneously change throttle and/or minimum thrust rates?

[fredinno]

[quote name='Aser']Thanks for all the replies!

I was thinking about trying to use real world throttle profiles in KSP, but it sounds like it would [B][U]significantly[/U][/B] increase difficulty. Just in case I'm feeling brave, does anyone know of a mod(s) that model the inability to instantaneously change throttle and/or minimum thrust rates?[/QUOTE]
Just don't use the shortcut keys for thrust on/off.
:P

[Streetwind]

[quote name='Aser']Thanks for all the replies!

I was thinking about trying to use real world throttle profiles in KSP, but it sounds like it would [B][U]significantly[/U][/B] increase difficulty. Just in case I'm feeling brave, does anyone know of a mod(s) that model the inability to instantaneously change throttle and/or minimum thrust rates?[/QUOTE]

KW Rocketry's engines have laggy throttle responses, and its SRBs cannot be throttled to below 50%. Though be sure to get the community patches as well, because there's not been an official update since 1.0.2.

If you install Realism Overhaul, you'll have plenty of this too - engines that don't throttle at all, engines that fail to ignite mid-flight because the fuel hasn't settled, engines that can only be turned on once, all that jazz :P Edited November 25, 2015 by Streetwind