TWR Question

[Bandus]

I have been playing KSP career mode recently and slowly unlocking techs which have led to larger rockets in some cases. It has always been my understanding that TWR must be above 1.00 else the rocket will fail to go up, assuming it is being launched from KSC. 

However, I have a rocket which appears to have a TWR of 1.47 (Mass is 39,350kg & thrust is 568.75 kN) in its initial stage, however, when I launch it at full power it fails to lift off the ground at all and I'm unclear as to why. I am not using Stability Enhancers and there should be nothing else holding it to the ground. 

Any help would be appreciated!

[Gaarst]

Did you get your TWR from MechJeb or Kerbal Engineer, or calculated it yourself ?

Either way, know that engines have less thrust while in atmosphere. If your MJ/KER display is not set to "atmospheric", or if you use vacuum thrust, then your actual TWR at sea level wil be lower.

For some engines (Terrier and Poodle mostly, Rhino also) the change is thrust due to atmo is quite dramamtic, enough to take your atmospheric TWR below 1.

Edited December 3, 2015 by Gaarst

[Jovus]

Yep, unless some other mod (like Test Flight) is compromising your thrust, this is a classic case of checking the vacuum TWR and not the atmo TWR.

[Bandus]

@Gaarst I used Kerbal Engineer Redux to get the TWR. My previous variant of this same rocket weighed 34,700kg and had a thrust of 568.75 kN as well and had no problem at all getting off the launch pad.

KER is definitely set to atmospheric (learned that the hard way). I'm not sure about vacuum thrust but on the previous rocket that had no trouble I used a Skipper and this new rocket that is having the trouble is using the same.

Edit: Also, in KER altitude is set to 0.0KM so it looks properly configured to run the numbers for the KSC launchpad.

Edited December 4, 2015 by Bandus

[GoSlash27]

Could also be that your ship is clipped into the launch pad and stuck there. That's been known to happen. I gather from the numbers you're running a single Skipper? If so, try it with launch clamps.

Best,

-Slashy

 

Edited December 4, 2015 by GoSlash27

[Gaarst]

2 minutes ago, Bandus said:

@Gaarst I used Kerbal Engineer Redux to get the TWR. My previous variant of this same rocket weighed 34,700kg and had a thrust of 568.75 kN as well and had no problem at all getting off the launch pad.

KER is definitely set to atmospheric (learned that the hard way). I'm not sure about vacuum thrust but on the previous rocket that had no trouble I used a Skipper and this new rocket that is having the trouble is using the same.

Then, if KER is set to the right display, maybe putting screenshots of your rocket could help to determine the cause.

[Bandus]

Here is a screencap with the rocket and KER. I really appreciate everyone's rapid suggestions!

 

I

 

Edit: Also, incidentally, this rocket is acting super odd relative to the previous variant I bad built. The length difference is insignificant yet suddenly, when I go to the launch pad, the rocket tends to tip over.  Also, some small changes in the mass as I tried to reduce the weight a little to see if it helped.

Edited December 4, 2015 by Bandus

[Gaarst]

1 minute ago, Bandus said:

Here is a screencap with the rocket and KER. I really appreciate everyone's rapid suggestions!

-snip-

Your rocket sems fine for me. It is probably the clipping issue described by @GoSlash27: try adding launch clamps to it to avoid clipping with the launchpad.

[I_Killed_Jeb]

Something silly I often mess up is making sure the staging is set properly

[Bandus]

@Gaarst @GoSlash27 I'll definitely give launch clamps a shot ASAP.  I'll let you all know how it goes.

 

@I_Killed_Jeb I triple checked staging and all is well there. Thank you!

[GoSlash27]

Bandus,

 I'd be surprised if this one isn't resolved with the launch clamps. As an aside, you shouldn't need those fins on the second stage. You'll be around 40 km at least when you stage it. The air will be too thin to flip you and the fins won't work anyway.

Best,

-Slashy

[Bandus]

5 minutes ago, GoSlash27 said:

Bandus,

 I'd be surprised if this one isn't resolved with the launch clamps. As an aside, you shouldn't need those fins on the second stage. You'll be around 40 km at least when you stage it. The air will be too thin to flip you and the fins won't work anyway.

Best,

-Slashy

@GoSlash27 Now you have me worried. I think when I stage I'm normally more around 30Km or so. The way I have been launching it is straight up until 100 m/s, then tilting to 10. Then trying to move in 5 degree increments and holding it to a max of 300 m/s until I'm over 10k. Then slowly tilting until the horizon. Is that incorrect? 

I appreciate your insight, yet again!

Edited December 4, 2015 by Bandus

[GoSlash27]

Bandus,

 Your procedure is about like mine, but I limit my throttle by acceleration rather than speed. I design my initial stage to have 1800 m/sec and that gets me to 30 km in a gravity turn, but the difference might just be in how I calculate it. You're using KER and that's bound to be more precise than my method.

 I wouldn't worry about it. The important part is that your first stage is getting you to 30km, and if your second stage doesn't kick off until 30km then it doesn't need fins.

Best,

-Slashy

Edited December 4, 2015 by GoSlash27

[Bandus]

@GoSlash27 @Gaarst Launch clamps saved the day. Thank you both!

[GoSlash27]

Oh, 1 other thing:

 If your second stage is kicking off around 30 km, then it doesn't need that much t/w. I design my upper stages to have .7 t/w and orbital/ beyond stages to have .5. The additional t/w doesn't really gain anything up there and you could substantially reduce the mass by going with a lighter and more efficient engine. All good things

 What's the word on the launch clamp fix? Problem solved? Ninja'd!

Glad to hear the problem's solved!

Best,

-Slashy

Edited December 4, 2015 by GoSlash27