Do you throttle-limit engines that are too big?

[wumpus]

On 5/4/2018 at 12:02 PM, bewing said:

This is true ... however, when you start talking about supersonic flight regimes, it gets a lot more complicated. As long as you are below transonic speeds, drag goes pretty closely as v-squared. Once you are transonic it gets a lot worse than that. Once you go supersonic it levels out a lot. In the KSP drag model, in fact, drag drops a significant amount once you break the sound barrier. These effects combine to make it true that you want to go quite fast, fairly quickly, and get supersonic.

Reality check: Passenger jets fly somewhere between mach .8 and mach .9, after that you get the nasty transonic region with the high drag levels.  Drag won't go below those levels, which is why nothing has replaced the concorde flying at its high drag levels.  If you are launching rockets, you aren't interested in flying at passenger jet speeds (SSTO might hang around there for quite some time as they gain altitude) so it is less relevant.  this may have lead to the confusion, but KSP is modeling things accurately (it is only when you get into spaceplanes that KSP's aero modeling starts to lie: you might want to install the Feram mod if that is a problem for you, and probably RSS:RO [the fundamental issue is that jet engines can't go fast enough to get into orbit on Earth, but they can on the much smaller planet of Kerbal]).

My understanding is that the most efficient speed is always going to be your terminal velocity.  Back in the pre-release souposphere version, this was equal to TWR=2.0.  Unfortunately, this little tip appears to have been forgotten as the terminal velocity has largely been corrected.  The important point is that it is almost impossible to hit your terminal velocity (at least while packing enough delta-v for orbit), so once you leave the transonic region (that bewing was talking about, at ~500m/s) you should hit the "Z" button to completely open up the throttle.

PS: this means that you should never set the throttle limit on the engine during launch (only for delicate maneuvers) so you can max thrust when needed, just use the main throttle.  You probably shouldn't set the throttle on SRBs (more thrust is mostly the point of the things, just stop adding them if TWR=2).

[Legacy600]

I mainly use thrust limiting for landing in Gilly, but they occasionally come in handy on liftoff.

[RizzoTheRat]

I've played around with limiting max Q (both in MechJeb and KOS) and have generally found unless your vessel is way overpowered (TWR>2) gravity losses are more of an issue than aerodynamic losses so it's best to go full chat.  However where it comes in useful is with odd shaped draggy payloads, limiting Q keeps your drag down in the thicker atmosphere which keeps the vessel a lot more stable, allowing me to launch payloads that would cause the ship to flip I launched at full power.

I usually aim for a TWR around 1.6, and will limit solid boosters to achieve that but don't usually limit liquid engines.