How to plan rocket power needed for mission instead of trial and error

[sledgeweb]

Hello,

So, my normal gameplay of KSP involves settling on a mission (getting to Mun, Duna, Eve, etc) and building a rocket based on previous experience that LOOKS like it might get me there (or there and back if it is manned). I sort of like playing this way, because it seems authentic to the Kerbal way of life. I end up with Kerbonauts stranded on moons or deep orbits and then having to rescue them. That's all good fun. That, and I do a lot of "Reset to Assembly" clicking when my design fails. Lately, I've been wanting to waste less time on trial and error methods, and move into better planning my vessels for their intended journey and destination.

So, how do I go about doing this? I know I need to somehow figure out the Delta-V needed, and somehow calculate that - but I'm not sure how to do so. And, I'm not sure which resources are most applicable to 1.0. Any tips or pointers in the right direction? I'm using stock KSP only right now. Thanks!

I'm looking for a way to get a rough estimate of what I have, and what is needed without it being too overly technical or complicated. Similarly, there are various methods to calculate orbit transfers - but I generally go with the ones that show you how to align the planets and where to burn.

Edited June 2, 2015 by sledgeweb

[Enkiel]

Kerbal engineer is a must ;

http://forum.kerbalspaceprogram.com/threads/18230-1-0-2-Kerbal-Engineer-Redux-v1-0-16-6

then for Delta V ;

http://i.imgur.com/cyV43I4.png

http://i.imgur.com/Vi8H41I.jpg

Edit ;

and rule of thumb, always plan more than enough. If it takes 5050 to get to the mun, i plan for 6500. Sure, i waste ressource, but its much better to start this way, and trim the fat later, than to get kerbal lost in space all around.

Edited June 2, 2015 by Enkiel

[JoCRaM]

Kerbal engineer is a must ;

Kerbal engineer save you fiddling around emptying the tanks to get a dry weight*, and typing a few numbers into calculator. I use the windows one in scientific mode.

* and remembering to fill them again. How Jeb laughed when we found the return stage was empty

[EtherDragon]

There are mods you can install that get you a lot of that data really fast, like Kerbal Engineer. But, you'll need to pair that with a solid Delta-V map, like the ones the @Enkiel posted.

One thing that you will also want to pay attention to, while learning, is how to do what you are already doing more efficiently - that's something that no MOD can really teach you.

My play style is more like yours. I have learned from the math the kinds of things I need to do to make something work "better" (e.g. Have more Delta-V or have better TWR).

In every one of my missions, I build in fail-return points. If I have a stage and the goal of that stage is to perform X, then I fail-return the mission if that stage is not up to snuff. For instance - say I have a Mun landing mission, and my stack has an Ascent Stage, Orbital Stage, Transfer Stage, Landing Stage, and Return Stage. I'll have milestones associated with each stage, and if any milestone fails, I abort and return.

Ascent - Stack reaches ballistic trajectory of 75km or abort.

Orbital - Circularize orbit (75x75km) with fuel to spare or abort.

Transfer - Transfer to and Munar Orbit Insertion with fuel to spare or abort.

Landing - Land with fuel to spare or abort.

Return - no abort strategy given / abort stage for any previous failure. This stage, I might actually look at the math and dV-map to make sure that it has what is required by manually calculating Delta-V (it's not hard) and adjust as needed.

[Zeratul_r]

http://www.curse.com/ksp-mods/kerbal/222685-kerbal-engineer-redux

This is the link to the mod called Kerbal Engineer Redux. Install it using instructions on that page. With that when you go to VAB you will see the window which contains all the necessary data about a rocket you are currently building (if the window is not displayed press the small button which says "KER" in the lower right corner). The most important part is delta v divided by stages. This column tells you how much delta v that particular stage has and how much delta v all the stages before had.

Next step is to find a delta v map. KSP wiki has one: http://wiki.kerbalspaceprogram.com/images/7/73/KerbinDeltaVMap.png

It may be hard to read it at first but actually it's quite easy. Let's say you want to land your Kerbonaut on Minmus and safely get back to Kerbin. Find the Low kerbin orbit (LKO) and Minmus. Two points are connected by a line and a few numbers, add them together. It will be 930+340+160+180=1610 to land on Minmus and 180+160=340; all in all 1950 delta v. That is a rough estimate. WIth Minmus it will be probably less because 340 for inclination change is way too much but at first I'd recommend to take 10-15% more just to be safe. So if you build in your VAB a ship which has 2100-2200 delta v in vacuum (press atmospheric and move altitude slider to 70 km) according to KER it will be probably able to get your guy or gal to Minmus and back.

Of course you will need to get that ship to orbit. Build the lower stages and look at their delta v estimate (but now use 0 altitude since you will be launching from the ground). Note that Kerbin LKO delta v listed in this map is incorrect. I personally safely launch rockets with 3700+ although many people in this forum talk about 3500.

Another important piece of data the KER gives you is TWR. You can have tens of thousands delta v but if your TWR is less than 1 you will be unable to lift your rocket off the ground at all. For launch TWR higher than 1.3 is ok and higher than 1.6 is better. At the same time too high TWR is also bad. IF twr at launch is higher than 2 you should probably remove some engines from the first stage/use less powerful engines since they are basically a dead weight.

If you want to see KER window in flight you should add a piece called Kerbal Engineering System (science tab) to your rocket.

Edited June 2, 2015 by Zeratul_r
expanding on the matter

[davidpsummers]

Hello,

So, my normal gameplay of KSP involves settling on a mission (getting to Mun, Duna, Eve, etc) and building a rocket based on previous experience that LOOKS like it might get me there (or there and back if it is manned). I sort of like playing this way, because it seems authentic to the Kerbal way of life. I end up with Kerbonauts stranded on moons or deep orbits and then having to rescue them. That's all good fun. That, and I do a lot of "Reset to Assembly" clicking when my design fails. Lately, I've been wanting to waste less time on trial and error methods, and move into better planning my vessels for their intended journey and destination.

So, how do I go about doing this? I know I need to somehow figure out the Delta-V needed, and somehow calculate that - but I'm not sure how to do so. And, I'm not sure which resources are most applicable to 1.0. Any tips or pointers in the right direction? I'm using stock KSP only right now. Thanks!

I'm looking for a way to get a rough estimate of what I have, and what is needed without it being too overly technical or complicated. Similarly, there are various methods to calculate orbit transfers - but I generally go with the ones that show you how to align the planets and where to burn.

Some have pointed to Kerbal Engineer Redux (I like it) or MechJeb (never used it). I don't necessarily disagee with that. But I will point you that it isn't too hard to calculate delta V by hand (you just need to take the mass of the ship and the weight of fuel and stick them into a simple equation, if you have stages, you do them one by one with the upper stage just counting a dead weight). I set up a spreadsheet to do this.

I would recommend doing at least the calculation (if not a spread sheet) a few times before going to KER. It is instructive. I also find the spreadsheet useful even with KER. (For examples, what will my dV be if a leave 300 units of fuel at the station.)

I assume you know where to find the chart of the amount of delta V needed to reach certain places? If not, or you need a link to the equation, just ask.

[Orbital Vagabond]

Like most everyone else has mentioned, I would recommend KER to calculate the dV of your vessel. I would also recommend thinking of your vessel in two parts: The launch stage and mission playload. Figure out how much vacuum dV you need to get to orbit (vacuum dV is reported by default in the KER readout and is a convenient metric for figuring this out), and design your launch stage to supply that much dV. Then, separately, design your mission payload to have sufficient dV for your mission (more on that in a second).

For example, lets say you tend to need 4,000 m/s of vacuum dV to get into orbit, and your mission will take about 2,000 km/s from LKO. You should design your total vessel to have about 6,000 m/s in total.

To calculate your dV by hand, you need to familiarize yourself with Tsiolkovsky's rocket equation.

Now, to calculate the dV you need for your mission, there are two option: dV maps and the ol' fashioned manual method.

Using the dV maps that have been created by other players is pretty easy. The best one I've seen is Kowgan's revision of WAC's map (posted here). Just add up the values on the way to your destination.

The manual method is substantially more complex. Do really do it well, having a year or so of high school/secondary level physics really helps. Basically, you need to calculate the desired velocity for your desired orbit at the location where you intend to burn, and subtract from that velocity at which you will be traveling at the location you intend to burn given your current orbit. Repeat this process for each maneuver you intend to make to reach your mission target.

Since velocity is speed and direction, this REALLY is vector math, so being familiar with vectors is helpful as well. A good summary of the equations you'll use is available here. It's a great site that I've been using since I found it. I think it's maintained by someone on these forums. You'll start with the vis-viva equation, and work your way up.

Hope the helps.

Edited June 2, 2015 by TestPilotTheta

[StrandedonEarth]

I like using VOID. It used some code from KER so it gives you most of that info. It also has a nice little HUD for the most important info, like TWR, dV, altitude above surface, and what biome you're in/over

[GoSlash27]

I calculate the DV on my own. It's not as difficult as it appears, but the math isn't for everyone.

Step #1: refer to the DV maps for basic mission planning. For all else, there's Basics of spaceflight

Step #2: Plan the mission from end to beginning. Each stage is payload for the preceding stage

Step #3: Apply the rocket equation backwards

Step #5: ???

Step #6: Profit!

Best,

-Slashy

[sledgeweb]

Okay, I downloaded KER and am trying to use it, but having trouble understanding a few things. Here's one problem I'm encountering:

http://imgur.com/a/6m0aT

In these two images you can see that in the first I have DV 3,173 in Stage 7.

Now, in the next image, all I did was drag the large central engine from Stage 7 to Stage Stage 6. This results in 3,943 DV in Stage 5 (why stage 5?) and 1,669 DV in Stage 7. So, Stage 5 and 7 have more DV in this configuration? Also, Stages 0 and 3 DV increase because of this change?

Also, all the DV readouts have ###/### format. What is the number before the "/"? And the number after?

And, the readout at the bottom, seems to say the vessels total DV is the same as Stage 0?

Thanks for the help, everyone!

[AlexinTokyo]

Not at my KSP PC, so going from memory here. Caveat Lector.

ÃŽâ€v readouts in KER are given as Current Stage/Total at Current Stage, so you can tell how much you have and each individual stage's contribution.

For an example (simplified) Mun mission:

S0 - 2,000/7,200 - Lander/Return vehicle has 2,000 m/s ÃŽâ€v, and the entire stack combined has 7,200 m/s

S1 - 1,500/5,200 - Transfer stage has 1,500 m/s ÃŽâ€v, for a total of 5,200 m/s.

S2 - 1,500/3,700 - Second (orbital insertion) stage has 1,500 m/s ÃŽâ€v, bringing the total to 3,700 m/s.

S3 - 2,200/2,200 - Launch/boost stage has 2,200 m/s ÃŽâ€v.

You may find that it skips some stage numbers (for example if you have a decoupler and the engine to fire after decoupling it in different stages), and it follows the KSP convention of the final staging event to fire being "stage 0".

Regarding the seemingly strange behaviour of the ÃŽâ€v in each stage, that you describe, it's very hard to say exactly what's causing it without a picture of the rocket (which is there, but I can't see it as imgur is blocked at work). However, especially with complex staging, the correct numbers can be quite counter-intuitive.

I'd suggest starting with a few small, simple launches (single stage, two-stage serial, central stack with boosters, etc.) to get used to how the numbers are displayed, and to get a bit of a feel for what they mean, in terms of capability.

When starting out, I also found it very useful to calculate the ÃŽâ€v by hand and compare it to KER's results, which forces a check of (sometimes basic) assumptions when the results disagree. As Slashy said, however, the maths may not be for everyone.

[Harry Rhodan]

So, Stage 5 and 7 have more DV in this configuration?

Yes. The whole point of staging a rocket is to shed mass you don't need. In your first configuration your rocket is severely overpowered which results in too much TWR initially and having to drag around the burnt out central stack and its Mainsail until your boosters are also empty. In your second configuration your boosters will carry the second stack as payload which results in much more delta v on paper because it assumes that it can use the Mainsail later into the flight. But also in way too less TWR to actually move you off the pad.

[Corw]

In 2nd scenario your second stage has to move less mass, because you've dropped side boosters, so same amount of fuel has to do less work. That is why you get more delta v. And that extra delta v is propagating to total delta v of the upper stages. Left side of the / is delta v of the stage, which is unchanged for upper stages, but total of that stage with lover stages included is higher.

Basically, you have discovered the most crucial thing that KER can show you = less is more. Just enough engines and fuel can get you further with lower mass and parts count than bloated design, which at first had is counter-intuitive. You don't have to go "moar boosters!" to go to space, you can just reshuffle staging, add small bits here, remove small bits there and get much better results.

For me personally, KER was such a revelation and I would never go past LEO without it.

[Crazy_Redd]

Are there any Excel or Google Sheets files that let you calculate DeltaV? If yes, where can I find them? I have the same question as OP but I do not want to install Mods or addons

[KerbMav]

Where lies the difference between reading numbers and typing them into a calculator/spread sheet - and letting the calculator/spread sheet (KER) read those number itself?

Mods do not necessarily change the game itself, especially KER just feeds you numbers. It does not give engines way better specs than the stock ones.

Not telling you how to play the game, just wondering.

[Lukaszenko]

Are there any Excel or Google Sheets files that let you calculate DeltaV? If yes, where can I find them? I have the same question as OP but I do not want to install Mods or addons

I don't know, but it's simple enough to calculate (I assume, I've never actually done it ) that if I was going to go that route I would just make the spreadsheet myself. That way I'd have a better understanding and trust of the numbers.

[Crazy_Redd]

Where lies the difference between reading numbers and typing them into a calculator/spread sheet - and letting the calculator/spread sheet (KER) read those number itself?

Mods do not necessarily change the game itself, especially KER just feeds you numbers. It does not give engines way better specs than the stock ones.

Not telling you how to play the game, just wondering.

Maybe the "figure things out yourself" part in the gameplay? (Ok a premade spread sheet is not very selfmade, but it's more self figured out than a mod who tells you right away).

Also I don't like the feel of having a mod on a game, I don't know it gives me a kind of "cheating" feeling, even when it's accually not. Besides a spread sheet can be chosen to use or not (So first trial and error and when all that doesn't work, you use the spreadsheet), a mod gives you the information anyway I presume.

Hard to explain, mostly a feeling of wanting to play a clean game perhaps?

[Ferdoni]

You can use the formulas to calculate your everyday kerbal needs:

http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

a few examples of formulas wich you can use:

Thrust to Weight ratio (TWR) = to calculate if you are actually ascending. a TWR of less than 1 means you are not escaping from the celestial body on wich you are. A TWR of more than 1 means logically that you are actually going to take off!

DeltaV = to calculate the required change in speed. To orbit Kerbin, you have to go at a speed of around 2250 metres per second. Escaping from Kerbin's atmosphere costs around 1250 metres per second. So if you want to go from the surface to a orbit you need 1250 + 2250 = 3500 m/s.

The formula: "9,81 x Isp x LN(weight full/weight empty)" calculates how many 'change in speed' your vessel has. (Isp is the specific impulse wich counts for the engine that is thrusting your vessel, can be found in the engine info in the VAB/SPH)

It's really not rocket science once you understand it.. it's just basic math

For me, KSP would be incredible harder without these formulas

also: there are mods, like MechJeb, wich calculate these number automatically, and per stage. But you need to understand them first

[Crazy_Redd]

You can use the formulas to calculate your everyday kerbal needs:

http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet

a few examples of formulas wich you can use:

Thrust to Weight ratio (TWR) = to calculate if you are actually ascending. a TWR of less than 1 means you are not escaping from the celestial body on wich you are. A TWR of more than 1 means logically that you are actually going to take off!

DeltaV = to calculate the required change in speed. To orbit Kerbin, you have to go at a speed of around 2250 metres per second. Escaping from Kerbin's atmosphere costs around 1250 metres per second. So if you want to go from the surface to a orbit you need 1250 + 2250 = 3500 m/s.

The formula: "9,81 x Isp x LN(weight full/weight empty)" calculates how many 'change in speed' your vessel has. (Isp is the specific impulse wich counts for the engine that is thrusting your vessel, can be found in the engine info in the VAB/SPH)

It's really not rocket science once you understand it.. it's just basic math

Thanks for your reply, I'll figure it out with the cheat sheet. Btw sorry OP for taking over your question

[Captain H@dock]

Are there any Excel or Google Sheets files that let you calculate DeltaV? If yes, where can I find them? I have the same question as OP but I do not want to install Mods or addons

I made my own one as I couldn't find anything like what I wanted, and I'd rather not install a mod for that.

I enter wet mass, tanks type and count, and it gives me the deltaV (this way i don't have to measure empty tank weight).

It also lists the deltaV for any of the alternative engines, which is a good way to trim down your lander.

It won't do multistage, but this is mostly designed for orbital tugs and Landers. While it also displays atmospheric and kerbin TWR, I don't use it for reaching LKO. I've got a a range of launcher that I know can bring xx tons to orbit, and I always refuel up there.

I'd encourage you to do it yourself. It's fairly easy (except the boring part of copying all the Isp/mass/tank capacity from the VAB screen), and if you're familiar-ish with Excel (mostly VLOOKUP) you can come up with nice features that suit your specific needs.

I'll try to post an image later today.

Edited June 3, 2015 by Captain H@dock

[Ferdoni]

Are there any Excel or Google Sheets files that let you calculate DeltaV? If yes, where can I find them? I have the same question as OP but I do not want to install Mods or addons

There is an app called DeltaV wich helps you relatively easy

[Crazy_Redd]

I made my own one as I couldn't find anything like what I wanted, and I'd rather not install a mod for that.

I enter wet mass, tanks type and count, and it gives me the deltaV (this way i don't have to measure empty tank weight).

It also lists the deltaV for any of the alternative engines, which is a good way to trim down your lander.

It won't do multistage, but this is mostly designed for orbital tugs and Landers. While it also displays atmospheric and kerbin TWR, I don't use it for reaching LKO. I've got a a range of launcher that I know can bring xx tons to orbit, and I always refuel up there.

I'd encourage you to do it yourself. It's fairly easy (except the boring part of copying all the Isp/mass/tank capacity from the VAB screen), and if you're familiar-ish with Excel (mostly VLOOKUP) you can come up with nice features that suit your specific needs.

I'll try to post an image later today.

I'm looking forward to your image. I'm fairly good with Excel (Only not maths ) so filling in and using the formulas is not that hard. Only thing I find hard is the maths

[sledgeweb]

Yes. The whole point of staging a rocket is to shed mass you don't need. In your first configuration your rocket is severely overpowered which results in too much TWR initially and having to drag around the burnt out central stack and its Mainsail until your boosters are also empty. In your second configuration your boosters will carry the second stack as payload which results in much more delta v on paper because it assumes that it can use the Mainsail later into the flight. But also in way too less TWR to actually move you off the pad.

Okay. Makes sense. So, I hadn't set up the asparagus staging yet at this point. If I did that, does KER take into account how the fuel lines are set up for asparagus staging? So, I would shed two outer tanks, then two more, etc. leaving the large central tank and mainsail full to the end but burning the whole time? That should help my DV? I'll test this when I get home.

[Harry Rhodan]

If I did that, does KER take into account how the fuel lines are set up for asparagus staging? So, I would shed two outer tanks, then two more, etc. leaving the large central tank and mainsail full to the end but burning the whole time? That should help my DV?

Yeah. KER can do that. But you'd still have too much TWR at launch and that tiny upper stage is basically worthless.

[sledgeweb]

Yeah. KER can do that. But you'd still have too much TWR at launch and that tiny upper stage is basically worthless.

Hmm, I'll have to recheck it. I thought when I looked at KER with just the upper stage, and 70KM setting - it had enough DV to get back from Minmus on it's own. This thing is supposed to just sit on Minmus as a station, but I figured I'd provide an escape vehicle for one person to come home. Lol. Plus it fulfills the contract requirement of having enough space for 5 Kerbals. Still, at this point, I was just monkeying around trying to figure out how KER works and not really building for feasibility.