jonpfl

Engineering redux question

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All,

just bougth KSP a few weeks ago and installed this mod after reading many positive things about it.  When I am in the VAB and it shows delta V and TtW ratio, is that calculated at sea level with a fully loaded vehicle?

I assume those values change dramatically as fuel is burned off, correct?

Thx

jonpfl

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Posted (edited)

It depends whether or not you click the atmo button on the top of the readouts. If atmo is enabled, than deltaV and TWR will be calculated assuming you're at sea level on Kerbin (you can change it to calculate for any height on any planet with an atmo). If atmo is disabled, it will show you stats like you are in space.

In response to your other question, yes, when fuel is burned off TWR rises and deltaV drops. 

Edited by TheRagingIrishman

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They also change progressively as you go through the atmosphere - though how they change can be quite dependent on the engine.  (In general: Atmosphere-optimized engines will have fairly small gains as you exit the atmosphere.  Vacuum-optimized engines will have large gains - especially to ISP/dV - as you go through the atmosphere towards space.  However this can be tweaked quite a bit, and some engines have specific speed/altitude ranges where they are most effective.)

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Posted (edited)

This add-on discussion has been moved to Add-on Discussions. :)

Edited by Dman979
Punctuation!

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8 hours ago, TheRagingIrishman said:

It depends whether or not you click the atmo button on the top of the readouts. If atmo is enabled, than deltaV and TWR will be calculated assuming you're at sea level on Kerbin (you can change it to calculate for any height on any planet with an atmo). If atmo is disabled, it will show you stats like you are in space.

In response to your other question, yes, when fuel is burned off TWR rises and deltaV drops. 

So what is this telling me at the moment?  At sea level on Kerbin in or orbiting Kerbin?

screenshot11.png

Thx
jonpfl

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22 minutes ago, jonpfl said:

So what is this telling me at the moment?  At sea level on Kerbin in or orbiting Kerbin?

screenshot11.png

Thx
jonpfl

That pic shows in space with full tanks.

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17 minutes ago, Jas0n said:

That pic shows in space with full tanks.

So, if I am trying to figure out if I can get into orbit on Kerbal, how do I determine that?  Look at atmosphere AND in space and ?

Thx
jonpfl

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2 hours ago, jonpfl said:

So, if I am trying to figure out if I can get into orbit on Kerbal, how do I determine that?  Look at atmosphere AND in space and ?

Thx
jonpfl

Simplest answer:  click on that little book icon in the bottom right of your screen.  There should be something titled "Planet Wiki" or similar.  Click on the planet you like (Kerbin) and it will have all of the delta V info you need..... approximately.

So, to build orbit capable rocket, look up "delta V needed to orbit" on that planet's page, and give your rocket a bit extra  (I add 200 dV minimum).

After that, trial and error (loads of errors) are you best friends!

Once you are good with that, there are many other tools that you can use to navigate throughout the Kerbol system. You'll find that math becomes your friend!

Good luck!

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13 hours ago, jonpfl said:

So, if I am trying to figure out if I can get into orbit on Kerbal, how do I determine that?  Look at atmosphere AND in space and ?

Thx
jonpfl

I typically start with d/v from here:

Then I build a rocket which has at least that (as those are reasonable minimums, and the amount you need can vary depending on your launch profile).  Whether I'm checking for atmosphere or vacuum d/v in a specific stage depends on the design of the rocket and my launch profile - in the 'simple case' of a single-stage-to-orbit with an atmosphere-optimized engine, I tend to use atmosphere d/v, but if I'm doing a more complex launch I may have a circlization stage where vacuum d/v (and thrust!) is more important.

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16 hours ago, jonpfl said:

So, if I am trying to figure out if I can get into orbit on Kerbal, how do I determine that?  Look at atmosphere AND in space and ?

Thx
jonpfl

I normally build a rocket with a starting vacuum TWR of 1.7ish and 3500m/s of delta-v. That will get to orbit if flown well unless you have made very bad engine choices. If you use a vacuum engine as a first stage it's performance will be different enough from predicted that you might not make it.

If you click the atmosphere button on KER it will show you the TWR and delta-v for the current body and altitude that you have selected.

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Try to give the booster a burn time of about a minute and with a dV of app.1/3 to orbit. Ground level TWR of about 1.3-1.5, less will waste fuel (gravity and atmospheric losses) and too much as well (fighting against your terminal velocity).

The upper stage can be setup to burn the remaining 2/3 of dV to Orbit, a relatively low initial vacuum TWR of 0.8 (grows up to like 1.5, depends on the payload) should do the trick, if your trajectory isn't completely off. The US usually gets staged at an altitude of ~50km, where dynamic pressure is low enough to not flip your rocket (and invoke unplanned rapid disassembly^^).

 

That way simplifies the dV and TWR guesstimations: Booster/first stage is read from atmo TWR/dV only and the second/upper stage from vac TWR/dV only. Together, they should be around 3.4k dV and a little less when using FAR. Maybe add a hundred or two for error margins.

 

Have fun playing around :)

 

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I always check my total vacuum delta-V, then pull the altitude slider all the way down to check the sea level TWR for launch.  Back and forth, back and forth, back and forth as I make changes.  Otherwise I pretty much follow the numbers outlined above.

MechJeb also has a delta-V readout panel that shows both at the same time.

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