Mechjeb: why limit to 40m/s acceleration?

[Spektyr]

Maybe this is posted somewhere but I couldn't find it.

I have MechJeb installed and I see this option on the ascent autopilot to limit acceleration to 40m/s. Why would that be beneficial?

First off, what the hell kind of rocket has that kind of acceleration anyway? If you're launching with that much TWR aren't you putting FAR too much rocket under the payload?

Second, does the limit to terminal velocity pretty much keep you under that anyway?

I'm just curious, what's the reason behind the 40m/s limit option?

[JonoRig]

Unless I'm remembering incorrectly. I believe where it says limit to 40, it is a text box so u can adjust it

[magnemoe]

The limit is to avoid damage to ships, say you accelerate with 4g, this will cause the strain on the ship to be four times that on the launchpad. if your ship is not well strutted and solid its an danger for the tanks to collapse. pushing the engine through the upper stage.

You can adjust or remove this if you want.

[Tex_NL]

It's 40 m/s², not 40 m/s. There is a square over the s.

Regular Earth (and Kerbin) gravity is 9.81 m/s², AKA 1 G. Most people, and presumably Kerbals, start passing out at a sustained acceleration past 4 G. Limiting acceleration to 40 m/s² prevents GILOC (G-Induced Loss-of-Consciousness), keeping them awake and able to pilot your rocket.

[Epthelyn]

I guess the reason behind the limit being so high is so that nobody can complain that their rocket designed to work at 3G cannot accelerate that fast on Kerbin. In other words, the limit is that high because no reasonable rocket will ever need to get anywhere near that; anything that does is crazy.

[Scialytic]

[...] anything that does is crazy.

You called?

[rosenkranz]

The limit is to avoid damage to ships, say you accelerate with 4g, this will cause the strain on the ship to be four times that on the launchpad. if your ship is not well strutted and solid its an danger for the tanks to collapse. pushing the engine through the upper stage.

You can adjust or remove this if you want.

This. Off the pad, you're not likely going to be accelerating at more than 20m/s2. However, by the time you reach apoapsis, you may well have a TWR of 4 or more. I've had a few ships destroyed when mechjeb started the circularization burn at apoapsis.

Under throttle control there is an option for 'smooth throttle'. This will dampen mechjebs tendance for 'instant full thrust'. But beware, leave this on during a powered landing and you'll likely crash.

[Fel]

In all honesty, I feel that unless you're doing some kind of SSTO or "nearly SSTO" your rocket design should be operating at maximum (or near maximum) throttle at all times.

Many people overplay it though and the more I realize how VERY LITTLE TWR you actually need for a launch (or that a TWR of 1 isn't always needed) which can be used to get rid of engines or trade them out for other more fuel efficient engines... the less I feel the need to actually drop throttle (again, excluding SSTO designs).

So again, unless you're doing some weird SSTO design... I don't see the reason you should limit your acceleration while FLYING the design; the building phase should have done that in the first place.

[lyndonguitar]

In all honesty, I feel that unless you're doing some kind of SSTO or "nearly SSTO" your rocket design should be operating at maximum (or near maximum) throttle at all times.

wrong, There is a thing called terminal velocity, that's why mechjeb has a auto-throttle feature. if you keep max throttle at all times especially if you have a strong rocket and high TWR, you would just waste fuel

[Eric S]

wrong, There is a thing called terminal velocity, that's why mechjeb has a auto-throttle feature. if you keep max throttle at all times especially if you have a strong rocket and high TWR, you would just waste fuel

Actually, I think his point was that if you're not flying the ship at full throttle, you shouldn't have designed it with so much thrust. I don't completely agree with this, as the TWR ratio shifts as the rocket burns fuel, and sometimes there's a big gap between the thrust of available engines (the atlas improved this so it's not quite as bad as it used to be), but if you're signficantly downthrottled at the activation of a stage for reasons other than "waiting for the SRBs to finish their burn" or "done with the burn, now I'm just fine tuning the orbit" then you might want to consider smaller engines.

[michaelphoenix22]

Smaller engines??? what blasphemy is this????

THIS IS KERBAL!!!

MORE ENGINES!!!! BIGGER ENGINES!!!! DARN THE TORPEDOES!!!!! RAMMING SPEED!!!!!!!!!!!!

[Holo]

The blasphemy of going where you want cheaply?

[Malaugrym]

The blasphemy of going where you want cheaply?

Giant columns of fire > balanced budgets.

[RoboRay]

Hint: TWR increases during engine burns as fuel is consumed.

A stage with 2:1 TWR when ignited may reach dangerous gee loads just before burn-out. This setting mitigates that problem.

[Fel]

Actually, I think his point was that if you're not flying the ship at full throttle, you shouldn't have designed it with so much thrust. I don't completely agree with this, as the TWR ratio shifts as the rocket burns fuel, and sometimes there's a big gap between the thrust of available engines (the atlas improved this so it's not quite as bad as it used to be), but if you're signficantly downthrottled at the activation of a stage for reasons other than "waiting for the SRBs to finish their burn" or "done with the burn, now I'm just fine tuning the orbit" then you might want to consider smaller engines.

Exactly... while you CAN use KSP's fairly overpowered engine scheme (KSP Launch compared to Earth Launch) to hang onto engines longer than NASA would... that is a large amount of weight you're dragging behind you.

We do need far more engines that "DO NOT Fill Niche Spots" but just "Offer increased thrust / weight / ISP variety." Atlas is great, but... what you're saying XD. Dropping down a few ticks near the end of the stage is one thing (although it can be planned against, it is hard without engine variety [and to further explain, you can design so your acceleration fairly closely matches the delta-terminal velocity]), but hanging onto the mainsail only to supply 30% thrust is something else.

Edited May 11, 2013 by Fel

[rosenkranz]

I don't suppose you'd need this when using a normally staged rocket because you'd not have enough thrust to drive the engine up through the ship at apoapsis. However, on SSTO rockets you still have all your lift engines and considerably less fuel. So unless you have agtion groups to turn off some engines, a full thrust of 5g or more at apoapsis could well destroy your ship.

However, in many cases, turning off some engines can through off balance and fuel consumption. This is where being able to limit the throttle to certain gee loads is good. Thus if you know 5g will kill your ship (which seems common) set the throttle to 40m/s2 (which happens to be default) and you don't have to worry about MJ killing your ship. Since it's setable, if you want to set it lower for a more fragile ship, so be it.

[Orphican]

This needs to be in the final game. In the "campaign mode" you should be able to balance between safe, cost effective, and dull designs that lose public interest versus . . . well a "Kerbal" design that nearly fails every other second. That said if you using KSP's stock aerodynamics model its a choice really. But if you using say Ferram Aerospace Research and Deadly Reentry . . .

[mushroomman]

Maybe it's just because of FAR, but when I use mechjeb with a really fast rocket, I have to limit the acceleration or else the drag it creates forces it into a horrible end-over-end spin.

[Tarrow]

Never needed to use it for launches but the setting carries over to the other orbital maneuvers. Asking mechjeb to do a 70m/s delta burn when he's only got a mainsail can be tricky because he'll overshoot - lock it to a max acceleration of about 10m/s2 and it's much more precise.

[MaHuJa]

Note: MJ2's function of burning half before and half after a node doesn't take this setting into account, in my recent experience.

[Tarrow]

Hmmm - I've just realised I'm on the -39 dev build from the link in the mechjeb thread not the main release. In that release the node burn timings do correctly take into account the acceleration limiter set in the ascent pilot panel.

Just relocated the option to be on my delta v display (I tend to always have this one open). I've been putting it to good use with high power-to-unladen-weight tugboat ships.

Edited May 21, 2013 by Tarrow

[Eric S]

Maybe it's just because of FAR, but when I use mechjeb with a really fast rocket, I have to limit the acceleration or else the drag it creates forces it into a horrible end-over-end spin.

It is. With FAR enabled, normal aerodynamic drag is removed, so MechJeb can't compute the terminal velocity and goes all out, the same way it would on an airless body.

[Teh_SAUCE]

Real life probe launches have a section of sustained 5 gs (roughly 50m/s^2) . Orbiting Kerbin isn't quite like the real thing, but 40m/s^2 isn't so unreasonable. The faster you get up the less gravity losses you have, so +1 for efficiency.

[SSSPutnik]

I've had ships where I've had to balance max acceleration or get structural failures.

If you set to about 22 it pretty much balances with terminal velocity for most of launch.