Jump to content

The best TWR!


Ferdoni

Recommended Posts

Hello fellow orbiters/landers/crasher/weird other people!

 

So i did a little expirement with TWR and I thought to share some results with your, and they are interesting!

Okay, what i did is slap a kickback SRB under a probe core, with nothing else exept a nose cone to figure out what the best TWR setting was. 

I set the kickback on the following TWR settings and the results are in metres.:

TWR Result
2 506.150
3 643.879
3.5 660.000
4 663.452
5 643.909

So we see that the best setting, a TWR of 4 is! suprising since i thought that it was lower.

I will post more of this kind of tests in the future for your convenience!

 

Kreetings!

Ferdoni

Link to comment
Share on other sites

What does the "result" column represent ? Is it the altitude reached by the rocket when fired verticaly ?

I suspect a curved trajectory, as in a gravity turn which is what should be done, would have a lot of influence on the result by having your rocket stay longer in the atmosphere. 

Also, your TWR setting is the TWR at launch time, right ? Rockets TWR increases as they burn their fuel, this is why it is recommended to throttle down as you go up (obviously, not possible with SRBs).

Link to comment
Share on other sites

The result column represents the amount of metres vertically archieved. 

The trajectory is indeed a little bit curved, due to the rotation of kerbin. And yes, the rocket remains longer in the atmosphere but when you go for orbit, you still need a vertical movement. 

TWR is at launch time, right. 

Link to comment
Share on other sites

You reduced the amount of solid fuel, right? Because a Kickback surely can go much further than few hundred meters when fully fueled... (They can get to LKO on their own) Did it go supersonic, where most of the aerodynamic drag comes from?

Also, would you post the screenshots of the test rockets?

Link to comment
Share on other sites

2 hours ago, Abastro said:

You reduced the amount of solid fuel, right? Because a Kickback surely can go much further than few hundred meters when fully fueled... (They can get to LKO on their own) Did it go supersonic, where most of the aerodynamic drag comes from?

Also, would you post the screenshots of the test rockets?

no, that would be inconsistent. I changed te thrust level in de VAB

and its not a few hundred metres, its about 600 kilometres

I will give more and better test results soon!

Link to comment
Share on other sites

Just now, Ferdoni said:

600 kilometres

Hmm... then, please update the OP to include the unit (km).

This one was confusing as you said:

18 hours ago, Ferdoni said:

the results are in metres.:

I thought this means the result was in meter.

 

Besides, screenshots would be great for verification.

Link to comment
Share on other sites

41 minutes ago, Ferdoni said:

no, that would be inconsistent. I changed te thrust level in de VAB

and its not a few hundred metres, its about 600 kilometres

I will give more and better test results soon!

Oh, so a constant dv but at higher TWR goes further than a 1.01 TWR?...you just proved the Oberth effect

For what you are testing, using the same Kickback IS inconsistent. Who would throttle at 10% when they have a Vector engine attached? If you are only going to use 10% thrust, then rather use a 1/10 of the engine (that is really hard to calculate with SRB's), thus saving on engine mass.

Please don't take this the wrong way, this is meant as constructive criticism. Here comes the constructive part :D:
Use a LFO fuel tank, and an engine cluster underneath. Don't change the fuel level.
Use a single type of engine to remove the Isp variable. Leave the throttle 100%.
TWR comes at a price of dry engine mass.
Add more engines to the cluster for higher TWR
Remove engines for lower TWR
The engine parts will add drag (in stock KSP). Aero-drag is another variable here that muddles the TWR results, but aero-drag is also a cost of more TWR.

That should give you much better results, but you still need some "to-orbit" trajectory else you are just testing the Oberth effect with a 20km layer of atmosphere. What i mean is the Kinetic Energy Equation(1/2 * m * v^2): 
Throw a 1kg ball upwards at 10m/s. Calculate the energy
Now throw a 1kg ball upwards at 11m/s. Calculate the energy

The 10m/s ball could achieve 5.097m altitude with that speed
The 11m/s ball could achieve 6.167m altitude with that speed
Thats 20% more height, by just adding 10% more speed

The faster they go, the more that ratio differs even by 1m/s changes. That is what you are seeing in the rocket, and it IS really cool that you went and did that in KSP...no better way to see stuff like this than with your own eyes! :wink:

Edited by Blaarkies
Link to comment
Share on other sites

3 hours ago, Abastro said:

Hmm... then, please update the OP to include the unit (km).

This one was confusing as you said:

I thought this means the result was in meter.

 

Besides, screenshots would be great for verification.

I think this is a communication problem.

In some places they use "." to separate thousands. 

Link to comment
Share on other sites

5 hours ago, Blaarkies said:

The 10m/s ball could achieve 5.097m altitude with that speed
The 11m/s ball could achieve 6.167m altitude with that speed
Thats 20% more height, by just adding 10% more speed
 

That is right.. but not what i did. 
let see it as a car. You can drive at 110 miles an hour. or 60 miles an hour. Within an hour you don't go further at 60 miles than driving at 110 miles. But with a limited amount of fuel you go actually further while driving at 60 miles an hour! how weird is that?

It is because, when you go faster, you recieve more resistance. But in KSP lowering the TWR to the lowest point at around 1.01 TWR means that you need to burn faaar to long to get to space than with an TWR of 3, even with the same amount of fuel. 
 

starting to get it what I'm testing? I'm basicly testing the fuel economy at certain TWR's (or speeds as you like) Going faster doesn't mean going further nor going slower. But going efficiently does. 
 

And what I'm going to do is test different SRB's. The Kickback has an other ideal TWR setting than the Hammer SRB. And don't get me wrong by thinking that I want SRB's to get you to orbit. But the Hammer, Thumper and Kickback represent an certain height to wich you can burn within a stage. That's why :wink: 

Link to comment
Share on other sites

6 hours ago, Ferdoni said:

That is right.. but not what i did. 
let see it as a car. You can drive at 110 miles an hour. or 60 miles an hour. Within an hour you don't go further at 60 miles than driving at 110 miles. But with a limited amount of fuel you go actually further while driving at 60 miles an hour! how weird is that?

It is because, when you go faster, you recieve more resistance. But in KSP lowering the TWR to the lowest point at around 1.01 TWR means that you need to burn faaar to long to get to space than with an TWR of 3, even with the same amount of fuel. 
 

starting to get it what I'm testing? I'm basicly testing the fuel economy at certain TWR's (or speeds as you like) Going faster doesn't mean going further nor going slower. But going efficiently does. 
 

And what I'm going to do is test different SRB's. The Kickback has an other ideal TWR setting than the Hammer SRB. And don't get me wrong by thinking that I want SRB's to get you to orbit. But the Hammer, Thumper and Kickback represent an certain height to wich you can burn within a stage. That's why :wink: 

I understood you want to test the aero-drag and gravity-drag variables. It seems you are more interested in the aero-drag for this, as if you want an answer about "what is the perfect TWR to set this rocket to?" 

The short answer is: As fast as the craft's terminal velocity.
Thats all, it is the perfect speed where the sum of gravity-drag and aero-drag are at a minimum. To estimate this speed, get your fully fueled craft to about 40km altitude and let it fall nose first. That should give you a good idea of how much it's terminal velocity is (keep in mind, the massive booster you need to get the craft up there should be detached. Also, terminal velocity changes with air pressure and the mass of your craft)

I would advise removing the "space" part in the experiment as that can easily add to confusion (above 40km up to the 600km result). You want to know how fast you should go through the atmosphere on your way to orbit, so that you save as much fuel as possible, right? So then, decrease the fuel amount to about 1/4. The rocket should burn out before exiting the atmosphere. (because if you burn outside the atmosphere, you could just as well do this on the Mun...less variables there)

Why i am mentioning all this is because using different SRB's will lead to using different Isp values on the experiment...you will need to add another column to the results table for that. Going straight up makes the engine's Isp value change with respect to altitude, and this rate of change depends on the TWR.

Also remember KSP has a weird aerodynamics model where drag is created based on open nodes (the bottom node on engines). This shouldn't be a problem for you as that is part of what makes each engine good/bad, but keep it in mind when doing aero experiments.

Link to comment
Share on other sites

On 25/04/2017 at 2:27 AM, Ferdoni said:

The result column represents the amount of metres vertically archieved. 

The trajectory is indeed a little bit curved, due to the rotation of kerbin. And yes, the rocket remains longer in the atmosphere but when you go for orbit, you still need a vertical movement. 

TWR is at launch time, right. 

Actually you need vertical movement to go outside the region where drag* can reduce your velocity, to orbit you only need 'horizontal' movement.

*we can just consider collision with the terrain a particular kind of drag for this discussion.

On 25/04/2017 at 6:19 PM, Ferdoni said:

And what I'm going to do is test different SRB's. The Kickback has an other ideal TWR setting than the Hammer SRB. And don't get me wrong by thinking that I want SRB's to get you to orbit. But the Hammer, Thumper and Kickback represent an certain height to wich you can burn within a stage. That's why :wink: 

 

I have a LV that basically does just that. There is a 2nd stage for the last 400m/s but most of the work its done by a kickback. The problem is, the orbit is 400km x 100km. This is not a big deal since it launch small satellites  to much higher orbit. If instead my target was something like 80km x 80km the deltaV used to raise the apoapsis from 80km to 400km apoapsis would be a waste, simple put a lot of unnecessary vertical movement.

 

 

 

Edited by Spricigo
Link to comment
Share on other sites

  • 2 weeks later...
On 28/04/2017 at 7:01 PM, Spricigo said:

*we can just consider collision with the terrain a particular kind of drag for this discussion.

Spoken like a true kerbal!

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...