Is this web KSP thust calculator wrong?

[Brainlord Mesomorph]

Trying to figure out if my latest lander has enough delta V fro Tylo.

According to this web site:http://iyates.co.uk/ThrustCalc/

It has enough delta V for escape of Kerbin!

But it does not.

Suborbital apogee of 48 km. Is the website wrong? Or is it just the atmosphere of Kerbin?

Edited October 26, 2014 by Brainlord Mesomorph

[Cmdr. Arn1e]

Without knowing what engines and tanks you are using, it's hard for us to say... It could well be out of date, as various engines have had small tweaks over time, most notable one recently was the buff to the Mainsail Engine applied with patch 0.25...

[Starman4308]

Trying to figure out if my latest lander has enough delta V fro Tylo.

According to this web site:http://iyates.co.uk/ThrustCalc/

It has enough delta V for escape of Kerbin!

But it does not.

Suborbital apogee of 48 km. Is the website wrong? Or is it just the atmosphere of Kerbin?

Pretty sure that thing is only telling you about TWR (Thrust-to-Weight Ratio), not delta-V. You can stick an Oscar-B on top of a Mainsail and get a TWR of 25.14, but the fuel won't last even a tenth of a second, and you will have an apogee of ~80 meters.

You need enough TWR to resist gravity (for stock, around 1.6-2.0 launch TWR is the sweet spot for efficiency), plus enough delta-V to reach orbital velocity (for stock, around 4400 m/s, around half of which is spent just fighting gravity and atmosphere).

I strongly recommend using Kerbal Engineer or MechJeb, which will give you combined TWR/dV readouts. You can let TWR slip a bit as you go upwards: it is plenty possible to circularize on TWR < 1, but launch absolutely requires you fight the full force of gravity and still accelerate upwards.

EDIT: Just to be thorough, delta-V is how much you can change your velocity, and is related mostly to fuel mass vs. dry mass (look up the Tsiolkovsky rocket equation), while TWR is how quickly you can change your velocity. In vacuum, there is little reason to have much TWR: while it's nice to not have to burn so long, and the Oberth effect favors short burns, most of the time, you get more overall efficiency by having lighter and higher Isp engines.

Edited October 26, 2014 by Starman4308

[Brainlord Mesomorph]

Pretty sure that thing is only telling you about TWR (Thrust-to-Weight Ratio), not delta-V. > ... < delta-V is how much you can change your velocity, and is related mostly to fuel mass vs. dry mass (look up the Tsiolkovsky rocket equation), while TWR is how quickly you can change your velocity. In vacuum, there is little reason to have much TWR: while it's nice to not have to burn so long, and the Oberth effect favors short burns, most of the time, you get more overall efficiency by having lighter and higher Isp engines.

OK wrong calculator.

Looking for a delta V calculator to tell me if this thing has enough engines and fuel to make orbit of Tylo.

Without knowing what engines and tanks you are using, it's hard for us to say... It could well be out of date, as various engines have had small tweaks over time, most notable one recently was the buff to the Mainsail Engine applied with patch 0.25...

its basically a trash can; the big gray fuel tank, a two-man lander can, and three Rockmax 55 radials, plus gear and a tri-sci module adds up to 32 tons.

With the aid of a drop-tank, can do a Munar Mission (from LKO) by itself.

[Pecan]

Grab a spreadsheet (LibreOffice is free).

deltaV = (Isp * G) * LN(Full_Mass / Empty_Mass)

Where:

Isp = engine ISP reported in the VAB/SPH (right-click on the engine in the parts menu if you haven't made a note of it. The Mark 55 is 320).

G = gravity of the body you're launching from (9.81 - or 9.82, I can never remember - for Kerbin).

Full_Mass = the total mass of your vehicle (32 tonnes, you say? Check by launching - but not going anywhere - switching to map mode and getting the figure from the 'i'nformation button on the .right).

Empty_Mass = the mass of the vehicle without any fuel (ie; once it's been burnt-off. Check this by right-click tweaking the fuel out of the tank(s) and getting the mass as above).

[LN() is the natural logarithm function which, in LibreCalc and Excel at least, is written as 'LN']

[Brainlord Mesomorph]

Grab a spreadsheet (LibreOffice is free).

deltaV = (Isp * G) * LN(Full_Mass / Empty_Mass)

Where:

Isp = engine ISP reported in the VAB/SPH (right-click on the engine in the parts menu if you haven't made a note of it. The Mark 55 is 320).

G = gravity of the body you're launching from (9.81 - or 9.82, I can never remember - for Kerbin).

Full_Mass = the total mass of your vehicle (32 tonnes, you say? Check by launching - but not going anywhere - switching to map mode and getting the figure from the 'i'nformation button on the .right).

Empty_Mass = the mass of the vehicle without any fuel (ie; once it's been burnt-off. Check this by right-click tweaking the fuel out of the tank(s) and getting the mass as above).

[LN() is the natural logarithm function which, in LibreCalc and Excel at least, is written as 'LN']

math?

--- sigh ---

(thanks, I will add that to my KSP excel workbook)

Edited October 26, 2014 by Brainlord Mesomorph

[Starman4308]

Looking for a delta V calculator to tell me if this thing has enough engines and fuel to make orbit of Tylo.

You can make a spreadsheet, but to save you the hassle of manually recalculating delta-V each time you change something, I strong suggest either the Kerbal Engineer or Mechjeb mods, which will give you TWR and delta-V as you're building the rocket.

Really, once you think about it, KSP kind of is rocket science, with some of the fiddly engineering details abstracted away.

its basically a trash can; the big gray fuel tank, a two-man lander can, and three Rockmax 55 radials, plus gear and a tri-sci module adds up to 32 tons.

One suggestion: make your lander wider. Put some fuel tanks radially around the bottom (connected with fuel lines) or something, because the wider the base of your lander, the more stable it is and more likely to not tip over. One little trick if your lander needs a lot of delta-V* is to put those radial fuel cans and the lander legs on radial decouplers, so that when it's time to leave, you can shuck the mass of both the empty fuel cans and the lander legs.

*And if you don't plan on re-using the lander. That's a rather important consideration if you planned multiple landings.

EDIT: Also, I think Pecan may have messed up a little bit in his explanation: for the purposes of calculating delta-V, G is always Kerbin gravity (9.82 m/s^2). Isp is exhaust velocity, and the form reported in KSP is exhaust velocity divided by surface gravity. That was done way back in the day to facilitate communication between those using SI and Imperial units, because an Isp reported in seconds is independent of whether you're using meters, feet, parsecs, etc.

One way to think about Isps given in units of time is that it would take Earth/Kerbin surface gravity this much time to cancel the velocity of the exhaust gases.

Edited October 26, 2014 by Starman4308

[Brainlord Mesomorph]

One suggestion: make your lander wider. Put some fuel tanks radially around the bottom (connected with fuel lines) or something, because the wider the base of your lander, the more stable it is and more likely to not tip over. One little trick if your lander needs a lot of delta-V* is to put those radial fuel cans and the lander legs on radial decouplers, so that when it's time to leave, you can shuck the mass of both the empty fuel cans and the lander legs.

*And if you don't plan on re-using the lander. That's a rather important consideration if you planned multiple landings.

totally reused. Lander CM and return lander.\

you'd be surprised what a low CoG that has. Its down where the landing gear is. ( the thing on top is light, and I use a drop tank during decent so it lands full)

EDIT: Also, I think Pecan may have messed up a little bit: for the purposes of calculating delta-V, G is always Kerbin gravity (9.82 m/s^2). Isp is exhaust velocity, and the form reported in KSP is exhaust velocity divided by surface gravity. That was done way back in the day to facilitate communication between those using SI and Imperial units, because an Isp reported in seconds is independent of whether you're using meters, feet, parsecs, etc.

will that work as a formula in XL, or not?

(I was just trying that)

[Starman4308]

totally reused. Lander CM and return lander.\

you'd be surprised what a low CoG that has. Its down where the landing gear is. ( the thing on top is light, and I use a drop tank during decent so it lands full)

will that work as a formula in XL, or not?

(I was just trying that)

For a delta-V spreadsheet, just plug in 9.82 (Kerbin gravity) * Isp * ln(fueled mass / dry mass), and the output will be in m/s of delta-V.

I'd use sea-level Isp values* for the first stage, and vacuum Isp values for the second stage and onwards. It's not 100% accurate, but short of simulating your flight path and integrating the rocket equation yourself with variable Isp, it's reasonably safe to assume your first stage will get you into near-vacuum.

*Rockets are less efficient when they have to push against atmosphere. This is particularly noticeable with the LV-N atomic rocket, whose Isp goes from 200s at sea level to 800s in vacuum. I don't think KSP will model continued Isp loss in atmosphere thicker than Kerbin sea level (such as Eve's atmosphere of doom), but I'd have to double-check that.

[Pecan]

math?

--- sigh ---

(thanks, I will add that to my KSP excel workbook)

Yeah, but that's the point isn't it - YOU don't have to do it, just know what equation to use.

The rocket equation (which I gave you) isn't actually that bad when someone explains it - it boils down to "you can do more with more efficient engines or more fuel", which makes sense ^^.

deltaV (how much you can do) means:

Step 1: (Isp * G) -> how efficient your engines are

Step 2: (Full_Mass / Empty_Mass) -> how much (what proportion of the ship's mass) is available fuel

Step 3: LN(Step 2) -> unfortunately there's diminishing returns because you need to burn more fuel to lift the rest of the fuel as well as the rest of the rocket

Step 4: Step 1 * Step 2 -> more efficient engines and more (usuable) fuel means you can do more

As Starman says, it's a lot easier with a mod.

[Starman4308]

Yeah, but that's the point isn't it - YOU don't have to do it, just know what equation to use.

The rocket equation (which I gave you) isn't actually that bad when someone explains it - it boils down to "you can do more with more efficient engines or more fuel", which makes sense ^^.

There is one other conclusion of the rocket equation:

The ratio of full to empty mass determines what your delta-V is. If you have 0 payload, and ignoring the mass of the rocket motors, there is a theoretical limit, determined by the mass of a full fuel tank divided by its empty weight. The weight of rocket motors also plays into this: in order to maintain a given minimum TWR, you must have X mass of rocket motor for every Y mass of fuel tank. The fact that you do generally have payloads further reduces this ratio.

The solution is staging: ditching empty fuel tanks (and the rockets powered by them) to start the rocket equation anew. While it does mean you must add more mass for the new stage's rocket engines and for the staging equipment, removing the prior stage's empty fuel tanks lets you reset that above ratio. It additionally has other benefits: once you're in upper atmosphere, you generally don't need as much TWR, so you can get away with proportionally less powerful engines, and sometimes more fuel-efficient vacuum engines*.

*Hydrogen-liquid oxygen is the fuel mix of choice for many upper-atmosphere rocket stages, but hydrogen-burning engines tend to have poor sea-level TWR and efficiency. As such, you will often see an RP-1 or other propellant in the first stage, with hydrogen used in the upper stages where the atmosphere is negligible and the rockets can operate with near-100% efficiency. The closest analog of this in stock KSP is the aforementioned LV-N, with its poor atmospheric performance and superb vacuum efficiency.

Edited October 27, 2014 by Starman4308

[Brainlord Mesomorph]

I've heard of the Rocket Equation (Robert Goddard, right?).

I get this:

[TABLE=width: 566]

[TR]

[TD][/TD]

[TD]Isp[/TD]

[TD]WetWgt[/TD]

[TD]DryWgt[/TD]

[TD]G (Kn)[/TD]

[TD]DeltaV (Kn)[/TD]

[/TR]

[TR]

[TD]Gamma II [/TD]

[TD]320[/TD]

[TD]32[/TD]

[TD]18[/TD]

[TD]9.81[/TD]

[TD]1806.183124[/TD]

[/TR]

[/TABLE]

But now I'm confused. That is the max Delta V for a rocket launching from the surface of a body with gravity "G" right?

But what about in space?

[Red Iron Crown]

Gravity is not relevant to delta-V calculation. G is used as a convenient constant so Isp can be expressed in seconds, which work in both metric and imperial systems of measurement. It is always Earth's gravity in real life, and 9.82 in KSP.

The calculated delta-V value is the same for all gravity situations (excluding some differences to Isp for atmosphere).

[Brainlord Mesomorph]

Gravity is not relevant to delta-V calculation. G is used as a convenient constant so Isp can be expressed in seconds, which work in both metric and imperial systems of measurement. It is always Earth's gravity in real life, and 9.82 in KSP.

The calculated delta-V value is the same for all gravity situations (excluding some differences to Isp for atmosphere).

So i *don't* swap that out for the gravity of Tylo? (When do calculations for Tylo)

---- (light slowing dawning) OOOOHHHHHHH, so I can use either feet or meters -- if I put in G as feet per second^2, I get deltaV in ft/s ---

(DAMN this has got to be THE most educational game I ever played)

[Red Iron Crown]

Right on all points: Don't swap out g values for local ones, g's units determine the result's units, KSP is the most educational game ever.

[lincourtl]

So i *don't* swap that out for the gravity of Tylo? (When do calculations for Tylo)

---- (light slowing dawning) OOOOHHHHHHH, so I can use either feet or meters -- if I put in G as feet per second^2, I get deltaV in ft/s ---

(DAMN this has got to be THE most educational game I ever played)

Isn't it just? KSP is really unique in how it's so much fun, and yet manages to be incredibly educational if you want it to be. I tell my friends that they don't have to do math to have fun with the game, but if they do learn the math it actually makes the game even more fun.

BTW: What does change with gravity is your surface thrust to weight ratio.

[phunk]

OK wrong calculator.

Looking for a delta V calculator to tell me if this thing has enough engines and fuel to make orbit of Tylo.

its basically a trash can; the big gray fuel tank, a two-man lander can, and three Rockmax 55 radials, plus gear and a tri-sci module adds up to 32 tons.

http://benmargolis.com/ksp/screenshot950.png

With the aid of a drop-tank, can do a Munar Mission (from LKO) by itself.

Just FYI, with that tri-sci module you've built, whichever 3-way adapter you added second will only be connected to one of the science modules. Won't make much difference in that ship, but if you ever connect that top docking port to anything, it may be a bit more flexible than you expected...

[Brainlord Mesomorph]

Just FYI, with that tri-sci module you've built, whichever 3-way adapter you added second will only be connected to one of the science modules. Won't make much difference in that ship, but if you ever connect that top docking port to anything, it may be a bit more flexible than you expected...

They seem to have fixed that in 0.25 - I tested it, I hung a weight from one those from a gantry - try it

in 0.24 I had to build them around scaffold. the new ones look much better

EDIT: I also built a spaceplane that involves a MK2 Two-way adapter connected to a two-way to large tank adapter - then to a large tank. it works too.

Edited October 27, 2014 by Brainlord Mesomorph