twr and max acceleration

[ForScience6686]

I was wandering if my twr can easily be calculated with the info displayed in map view showing ship information. It show max acceleration and mass, so I am assuming we can figure twr with the given info while in flight. I am currently against mods, but would like to be able to get a rough number with out going through a complete test of the craft.

Thanks for the help

Edited September 21, 2015 by ForScience6686

[Red Iron Crown]

Max acceleration = Mass/Thrust

TWR = Thrust/(Mass*LocalGravity)

Edited September 21, 2015 by Red Iron Crown

[Gaarst]

As Red Iron Crown said, you need local gravity to get the TWR of your ship.

From then you can use the Gravioli thing as I think it shows gravitational acceleration. Then you'll get your TWR where your stand/float.

But TWR when already in orbit isn't very useful (completely useless tbh), you'll need the surface gravitational accelerations of the bodies you're at or aiming to go. They can be found in the map wiew with all the other info about the body focussed (GM, radius...) but it is possible they are expressed in g. If so, just multiply the value by 9.81 (or 9.82 not sure in KSP) and you'll get your gravitational acceleration in m/s². This will give you the surface TWR of any body (except asteroids which have no gravity).

Also, you can divide the acceleration of your ship given in the map view by the gravitational acceleration of the body you're aiming (in m/s²), this will give you your TWR quickly and doesn't require mass or thrust on their own.

Edited September 21, 2015 by Gaarst
Mentioned surface TWR, and added more stuff

[ForScience6686]

Thanks for the quick relies guys. My hopes in this were to know what I'm looking for in interplanetary burns and captures. My usual method involves testing a burn to my destination and if I think I have enough for a capture I'll revert and load up with kerbals for the actual voyage. However, this takes a lot of time. Are there other methods of approximating how far I'll be able to go with the fuel I have? One thing I've thought of (not sure it is actually rationale), take my fuel consumption ( which I assume is fuel consumed per second) and divides by remaining fuel, then multiply by max acceleration. Is that enough to get me a rough idea? Obviously this will change as fuel is consumed, but I figure this is at least giving me my minimum available.

[Red Iron Crown]

You're looking for the Rocket Equation, and a Delta-V map.

[Gaarst]

The root of all space travel might help you. Edited September 21, 2015 by Gaarst
Ninja'd

[NathanKell]

Red, I think you mean Thrust / (mass * local gravity).

[Red Iron Crown]

I do, of course. Thanks for the catch.

[Admac]

What you are describing sounds a lot like Delta Vee (The amount of velocity change your ship can make). Delta Vee is incredibly useful for designing efficient ships and planning routes. I would say knowing the remaining delta vee is the most important stat in a vessel. There are lots of fun and mathy ways to calculate stage and vessel delta V, but none that are 'easy' in the way you do every few seconds in your head or anything.

Mechjeb and Kerbal Engineer have brilliant displays for this, and I wholeheartedly recommend either. I prefer Mechjeb, because it's a bit more flexible,but both are great. (They are informational addons, so don't really add new parts that change how you fly. Well knowing your delta v will change how you build and how you fly, but that's because you will be better at it...)

[ForScience6686]

Thanks for the literature guys, I'll do some reading tonight on this topic.

[Vectura]

Just out of interest, I assume there is such thing as thrust-mass ratio for the acceleration of your craft in 0g?

[NathanKell]

Yes. Although we usually think of TWR in terms of Earth gravity (g0, i.e. 9.80665m/s^2) even then, since it's a useful metric.

[Gaarst]

For real 0g with no gravity at all then your TWR is infinite because your weight is 0; you acceleration depends solely on mass and thrust, so it won't change anything, 0g or not.

[ForScience6686]

Does this all still apply if I'm only using this while in orbit? I basically would just like to see of I have the fuel once in orbit without running a live test. And since I am not at the proper time to do a transfer to duna, dv charts won't be accurate correct? I would also like to figure a way to calculate what is required to get into the proper orbit once to duna.

[Gaarst]

Then this is what you need.

To get in an elliptical orbit, look at the "insertion dV" number.

[Red Iron Crown]

It does apply when in orbit. If you'd rather not run the rocket equation manually you can use a mod like Kerbal Engineer Redux or MechJeb to compute the remaining dV for you. You do need to be at the proper phase angle with your target to use these values (a "transfer window"). Capture burns are shown on the map.

[ForScience6686]

Thanks for all the help guys. I I'm currently getting ready to send my 3rd ship to duna (none have arrived there yet though) and I'm looking to cut my testing time down. I'll work on getting through the literature tonight to have a better understanding. I'm just surprised that we can't determine important info such as this in game. Do the info tabs for the planets give the gravity? Sorry, I will not be getting mods, I would rather learn and use the formulas my self.

[FleshJeb]

Do the info tabs for the planets give the gravity?

http://wiki.kerbalspaceprogram.com/wiki/Main_Page

[Gaarst]

I'm just surprised that we can't determine important info such as this in game. Do the info tabs for the planets give the gravity?

The info tabs in map view do display gravity, expressed in g (as stated in one of my previous posts).

Go to map view or tracking station, focus the body you wish, click on the second icon on the right, and gravity is displayed as "gravity ASL". This is the surface gravitational acceleration expressed in g; just multiply it by 9.81, and you'll have your gravity in the right units for calculating TWR.

[ForScience6686]

Thank you gaarst. I should be all set now. I appreciate the help guys, happy science hunting to you all.

[ForScience6686]

Ok, so lets see how well I understand this (like I said, no mods please):

Kappollo 7

5 man transfer ship with a one way ticket to Duna

Transfer stage is (lets say) in 100km parking orbit around Kerbin

Wet mass: 72.4t

Dry mass: 21.7 (I am assuming the loss of drop tanks is also calculated in this)

ISP: 350 (poodle)

local g: 9.81

So if I am understanding the equation properly it goes a bit like this:

dv= ve*ln(m1/m2)

dv=3433.5*ln(72.4/21.7)

dv=3433.5*1.20489etc

dv=4137

However, once outside of Kerbins SOI this would have to be recalculated correct? So lets continue the example:

Kappollo 7

Just entered Duna SOI

um lets say:

Wet mass: 40t

dry mass: 21.7

isp: 350

Local g: 2.94(?)

dv=1029*ln(1.8433etc)

dv=629

Please don't judge the numbers as actually able to perform the trip, but let me know if I am understanding the concept correctly.

[Red Iron Crown]

g in the rocket equation is always 9.81m/s^2, it doesn't change with local gravity. It is used as a constant to convert exhaust velocity into specific impulse in seconds, which are the same in both metric and Imperial (important when German and American rocket scientists worked together after WW2). This would be less confusing if we just used exhaust velocity in m/s instead, but Isp is so well established now that it's likely not going anywhere.

[Yasmy]

If you are dropping tanks, you need to calculate the delta-v for each stage individually.

Each stage has it's own wet to dry mass ratio, which you should reevaluate after dropping tanks and separators.

Then the total delta-v is the sum of the delta-v of each stage.

[NathanKell]

Red, I _so_ hate to do this for the third time, but...as of 1.0, it's 9.80665m/s^2; in KSP versions prior to 1.0 (i.e. .90 and below) it was 9.82m/s^2 (we don't know why), although if RealFuels was installed it corrected it to 9.80665m/s^2.

That is, btw, the officially sanctioned value for g0

However, you lose next to no accuracy (except pedantry ) by using 9.81, so...

[Red Iron Crown]

I was rounding for clarity, the value is correct for the number of significant digits I used. Always happy to be corrected, never learn anything if I'm never wrong.

Just glad it isn't 9.82 anymore.