Calculating necessary thrust and fuel?

[Nairou]

The two biggest problems I have when building spacecraft are getting it off the launch pad (too heavy, not enough thrust), and running out of fuel (usually before achieving orbit, but sometimes later in more complex missions). I've launched many successful rockets, but it's usually just trial and error after several failed attempts.

I've watched videos of people doing really fancy missions with equally fancy rockets, and they never have these problems...

How do you calculate how much thrust or fuel you will need for a given spacecraft? I can only assume that's what people are doing, but I don't know where they get the necessary information to make those calculations...

[TheDarkStar]

The wiki has a good page for it here. Your thrust to weight ratio has to be more than 1, and how fast you can go (velocity change, or delta-v) depends on engine efficiency, fuel mass, and dry mass.

[Dispatcher]

http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

Another place of info.

[UmbralRaptor]

For fuel, the rocket equation is a good place to start. For thrust, you just want to make sure your thrust/mass is higher than the surface gravity of whatever body you're landing on / taking off from. Values are in the wiki, but you'll tend to have an easier time if you go for 1.5 - 2.5x that.

[capi3101]

For any mission plan, the best method of attack is to work backwards. You have your payload; great. What is its mass? What engine combination do you want to try to launch it with? What is the combined mass of the engines? How much thrust do they output, and what is the resultant stage Isp?

You can then work the Tsiokolvsky rocket equation backwards to figure out how much fuel you need for a particular part of yoyr mission. Go with the standard assumption that the full mass of a fuel tank is nine times that of the dry mass (a valid assumption for every fuel tank in KSP except the Round-8 and the Oscar-, and then solve for the dry mass. You change the equation to look like this:

target delta-v = ln((Full Mass + Deadweight)/(Dry Mass + Deadweight))*9.81*Stage Isp

where deadweight is the combined mass of the payload, engines and any other equipment.

When you've figured up your Full Mass, add it to the payload, multiply the resultant amount by 9.81, and divide your thrust by that result. If the final result is less than one and you're dealing with a launch from Kerbin, you're going nowhere; try again. If it's greater than one, you're going to space. 1.6-1.7 is optimal, but anywhere from 1.2-2.2 will do the trick. Greater than 2.2 and you've got too much thrust for your stack; you run the risk of telescoping your stack, especially if you're running with a heavy payload.

I did a set of calculations last night...here. It's what I just posted in action; I turned around and built the craft I suggested in this thread (though I ultimately went with four LV-T30s due to the extra mass of the chutes) and it got into orbit; an SSTO rocket.

Edited September 26, 2013 by capi3101

[roosterr]

I will second the link Shared by Dispatcher, its all about specific impulse, aka ISP, which is just fancy for the fuel efficiency of a motor.

High large thrust motors are not usually fuel efficient. And the ISP is different per your altitude. Some are great in a vacuum but terrible in atmosphere.

One approach I like is using the tri and quad couplers to cluster smaller high ISP motors together to combine their thrust.

Another good way is to strap on multiple liquid fuel asparagus boosters, like six of them, but chain their fuel lines so it burns off and drops two motors/tanks at a time and when the last two boosters drop, the center motor/tank (core) is still full of fuel.

See this: https://www.youtube.com/watch?v=o0pMDWu3FgQ

Edited September 26, 2013 by roosterr

[capi3101]

One more bit of information you could use: the Cheat Sheet. It's not actually a cheat, just an accumulation of really useful information. The delta-V map is probably the most useful thing on the page; it gives optimal delta-V values for planning specific parts of missions around the Kerbol system. It does assume the optimal case, so you should add a fudge factor to the numbers (on the assumption that something somewhere will be screwed up); the amount you add really depends on your level of skill.

[SRV Ron]

The School of Hard Knocks, Kerbal Science, is the heavier the payload, the more rocket will be needed to get it to where you want it. And that second part of the equation rises rapidly for a small additional increase in weight. Example, this bare bone lightweight payload reached Mun on this small two stage design from Nova Punch mods;

While adding lander legs, an RCS package, and two parachutes required this to reach Mun, land on it, and return to Kerban;

Even with the calculations, you will find major variations in design are possible as you look for the right combination of staging and engine fuel tank sizes to build the lightest rocket possible for a given payload.

[Monkeh]

Grab this little plug in/mod/thingy from here, put the little tape recorder thingy on your craft somewhere and let it do all the hard work for you.

It will tell you your thrust to weight ratios for each stage in your craft as well as how much delta v you've built in as well.

Having a trust to weight of around 1.5 or slightly more is a good way to start and just over 4000 m/s of delta v will get you to orbit. Another 700-800 or so to get the Mun, 2,500 or so to go see Jool.

It will show you in real time as you build as well so you can see exactly what happens when you change a nuke engine for a mainsail. It will also work when flying too so you can drag your maneuver node out to the exact amount of delta v you have left in that stage or whatever. It's great.

Go get it, easy to install and use and sooooooooooooo very useful. Also, as it's just basically a calculator it isn't even cheating either.

You can see it on the left in this screenshot, so good, did I already say get it? Go get it.

You can also set it to tell you the values for sifferent bodies in the system, so how it would work on the Mun or Laythe and the different gravities and ISP depending on atmosphere.

Build a lander for a particular mission, set the engineer to tell you how it works on the body it shall visit and save it with the sub assembly mod. Build a lifter, save with sub assembly, build a transfer stage and save with sub assembly, put them all together, win the game.

Edited September 26, 2013 by Monkeh

[blizzy78]

Actually calculating how much thrust you need given a specific payload mass is rather easy:

thrust = (payload mass) / (desired payload fraction) * 100 * (desired TWR) * (gravity constant)

For example, if your payload mass is 44 t, the payload fraction is 15%, the desired TWR is 1.7, and the gravity constant is 9.81 m/s, you would need at least 4892 kN of thrust:

44 / 15 * 100 * 1.7 * 9.81 = 4892

To save the tedium, I've made a handy engine layout calculator that can be used to produce central and booster stack layouts given a plethora of parameters. Of course, it's still up to you to assemble the boosters in an asparagus-style setup, and make sure you provide enough fuel. But that's easy enough when using the Stretchy Tanks mod.

[Nairou]

Thanks for the awesome replies and links! I didn't realize there was so much technical information in the wiki.

Two remaining questions:

- As far as calculating the mass of a rocket, which everyone refers to, I assume the only way to do that (without using mods) is to go through each component you use, look up it's mass in the parts listing, and add them up?

- Everyone talks about deltaV, and the wiki mentions that it is a change in velocity. That doesn't quite explain what it is or why it's important. What exactly does a high deltaV represent? A rocket that can increase it's speed very quickly? Is that just another representation of thrust? Why do some tasks require a higher deltaV than others?

[blizzy78]

- As far as calculating the mass of a rocket, which everyone refers to, I assume the only way to do that (without using mods) is to go through each component you use, look up it's mass in the parts listing, and add them up?

Exactly. It might be possible to click the little buttons on the right in the map view, perhaps the mass is listed there, too.

- Everyone talks about deltaV, and the wiki mentions that it is a change in velocity. That doesn't quite explain what it is or why it's important. What exactly does a high deltaV represent? A rocket that can increase it's speed very quickly? Is that just another representation of thrust? Why do some tasks require a higher deltaV than others?

Because that's what "change in velocity" means - it is the potential to change your velocity. Imagine you're going forwards (whatever direction that is) at 100 m/s. Now to come to a complete stop (with respect to that direction), you need to change your velocity by going 100 m/s in the opposite direction. Hence, the stopping maneuver has a cost of 100 m/s. Or the other way around: To not only go at 100 m/s, but 200 m/s, you need to execute a maneuver that will change your velocity by 100 m/s in the same direction. That maneuver will again cost 100 m/s delta-v.

So essentially delta-v means how far you can go, or how much maneuver potential your rocket has. Kind of like the volume of your average car fuel tank. Higher is of course better, but comes with the tradeoff of more mass to push.

Edited September 26, 2013 by blizzy78

[capi3101]

Some mods also add up the mass of your rocket - KER in particular is good for that. You can also just put a rocket under construction on the pad, call up the map view and look in the info window; mass information is there. And then there's the ol' pencil-and-paper method; time consuming but satisfying if you're into math.

As blizzy said, delta-V is change in velocity, and it's important when considering a rocket's delta-V budget (look that up on Wikipedia) - the amount of delta-V that a particular piece of a rocket can contribute to the rocket as a whole. High delta-V means the rocket can make more changes in its velocity (both magnitude - i. e. speed - and direction) under its own power, which in turn usually means that it can reach particular destinations.

[Nairou]

So essentially delta-v means how far you can go, or how much maneuver potential your rocket has. Kind of like the volume of your average car fuel tank. Higher is of course better, but comes with the tradeoff of more mass to push.

Thanks, that part makes sense. The thing that doesn't make sense is when people talk about engine parts as having high or low delta-v. Like, the ion engine. Very low thrust, but I've heard repeatedly that it has a high delta-v. I know it has a very high specific impulse, but I would have thought that it had a very low delta-v, given the low thrust. But then, I keep thinking in terms of thrust. If an engine has high thrust, I assume it has high delta-v since it can increase velocity more quickly. Somewhere in here I'm sure my assumptions are incorrect...

[blizzy78]

I think the main problem people have when initially trying to understand what delta-v is is that its unit is in distance divided by time (m/s.) But in space, you just don't go somewhere and then stop. You're always moving. Moving around Kerbin, moving around the Mun, moving around Kerbol. Moving, moving, moving. But to go from a planet to another planet you will need to go faster to leave the source's sphere of influence. Hence, a change in velocity. To orbit the target, you need to go at a certain velocity. Again another change in velocity.

So all in all, it's the best way to describe a rocket's fuel tank in quantities of m/s rather than units of volume or mass.

(Come to think of it, for cars it would probably best to describe the tanks in units of distance. But that's another story.)

[Kerbart]

So all in all, it's the best way to describe a rocket's fuel tank in quantities of m/s rather than units of volume or mass.

(Come to think of it, for cars it would probably best to describe the tanks in units of distance. But that's another story.)

It goes actually a bit further than that. In the end, what really matters is energy. It takes energy to change speed, and energy to move radially in a gravitational field. There are various ways you can express that energy budget, and delta-V is one of them. The reason it's used a lot is because delta-V is a measurement that is independent of the vehicle mass, or the celestial body you're circling

Of course you could say "you need 25 MJ/kg to get into orbit" or "It takes 75 Ns per kg in one g" or whatever, but in the end delta-v is a measurement that allows you to quickly figure out what you'll need with the only variable left being the mass of your spacecraft (and a few more things about your spacecraft if you want to figure out how much fuel you need to burn).

What you have to keep in mind is that it's a shortcut for energy calculations. A low Kerbin orbit needs on orbital speed of, say, 2200 m/s, and a higher orbit only needs 1500 m/s. But you'll also be in a higher orbit and that takes energy too, which is why going from one to the other takes for instance 200 m/s delta-V, despite the fact that you end up going slower when all is said and done.

[Lego8_bit]

ok Delta T Delta A Delta V are Time, Acceleration, and velocity this is basic physics and physical science on a college level if you want to calculate for speed its for expamle V=D/T

[Diazo]

Thanks, that part makes sense. The thing that doesn't make sense is when people talk about engine parts as having high or low delta-v. Like, the ion engine. Very low thrust, but I've heard repeatedly that it has a high delta-v. I know it has a very high specific impulse, but I would have thought that it had a very low delta-v, given the low thrust. But then, I keep thinking in terms of thrust. If an engine has high thrust, I assume it has high delta-v since it can increase velocity more quickly. Somewhere in here I'm sure my assumptions are incorrect...

In the type of conversation I assume the 'high delta-V ion engine' was happening is that it was in terms of fuel capacity.

Because the Ion engine is very efficient it produces magnitudes more delta-V for the same amount of fuel as a rocket engine (poodle/skipper/etc.)

Therefore the ion engine is high delta-v in comparison.

The issue is that the ion has a very low thrust so its maximum acceleration, or rate it can generate delta-V, is very low.

(Random numbers to make a point)

Ion engine:

2 m/s at maximum throttle, consumes 1L of fuel a minute.

Rocket engine:

50 m/s at maximum throttle, consumes 1L of fuel a second.

On a 10L fuel tank the Ion burns for 10 minutes, generating 1200 m/s delta-v total, but only 120 delta-v a minute.

On the same 10L fuel tank, the rocket engine burns for 10 seconds, generating only 500 m/s delta-v total, but it generates 3000 m/s delta-v a minute (if the fuel did not run out after 10 seconds).

So, trade offs.

Bottom line, delta-v is how much a ship can change it's velocity, which determines where you can go.

It is rough, but about 4500 delta-v is used to make kerbin orbit, so if your rocket has that much delta-v you can make orbit (assuming everything else is fine).

D.

edit: An older version, but still reasonably accurate to my knowledge, here is the delta-v required to get places.

http://www.skyrender.net/lp/ksp/system_map.png

Edited September 26, 2013 by Diazo

[blizzy78]

Thanks, that part makes sense. The thing that doesn't make sense is when people talk about engine parts as having high or low delta-v. Like, the ion engine. Very low thrust, but I've heard repeatedly that it has a high delta-v. I know it has a very high specific impulse, but I would have thought that it had a very low delta-v, given the low thrust. But then, I keep thinking in terms of thrust. If an engine has high thrust, I assume it has high delta-v since it can increase velocity more quickly. Somewhere in here I'm sure my assumptions are incorrect...

Thrust does not have anything to do with delta-v. Thrust is a measure of how much force your engine can produce. Delta-v, as explained, is a measure of how far you can go or how much velocity change potential your tanks contain.

In that sense, it does not make any sense at all to say, "an ion engine has high delta-v." Only in combination with its high specific impulse and a given amount of fuel would it make sense to say, "this ion engine will make my rocket have more delta-v than that other rocket that uses an engine with lower Isp and the same amount of fuel" (of course implying both engines can use the same type of fuel.) There's a simple reason for this: The higher the specific impulse, the more fuel efficient an engine is, so the more delta-v is it able to produce with your tank's contents.

Edited September 26, 2013 by blizzy78

[Specialist290]

Most of the other users here have already done a good job explaining things, but like I always do when this topic comes up, I'm going to recommend that you read these four handy pages over at Atomic Rockets, which try to explain the basic principles of rocket science in layman's terms. The author mainly writes with an eye towards real-world rocketry, but many of the things he discusses still hold true for Kerbal Space Program.

I'd also highly recommend you check out MyKSPCareer.com, which also tries to break things down in an easily accessible manner from a KSP-specific perspective.

Hope this helps

[capi3101]

Thanks, that part makes sense. The thing that doesn't make sense is when people talk about engine parts as having high or low delta-v. Like, the ion engine. Very low thrust, but I've heard repeatedly that it has a high delta-v. I know it has a very high specific impulse, but I would have thought that it had a very low delta-v, given the low thrust. But then, I keep thinking in terms of thrust. If an engine has high thrust, I assume it has high delta-v since it can increase velocity more quickly. Somewhere in here I'm sure my assumptions are incorrect...

The assumption you're making is that thrust and delta-V are co-dependently related upon one another; they are not.

blizzy78's already covered this well; he's right in saying that "it does not make any sense at all to say, 'an ion engine has high delta-v.'" Engines can high or low Isp values, which in turn affects the delta-V they can produce given x amount of fuel. The only other kind of part that will affect delta-V is fuel tanks - the more fuel you have, the more a rocket's potential delta-V in general (because that part of the equation is logarithmic, there does come a point where adding more fuel - or more specifically more mass - actually starts to decrease delta-V).

EDIT: I actually hit that point (more fuel = less delta-V) with a rocket once; I might have to dig up the calculations on that one...I know I have them somewhere.

[Nairou]

Thanks all! Your replies have been extremely helpful! I've now got several promising articles to read. You guys rock!