Calculating DV is "hard"??

[GoSlash27]

Not sure if i agree with that comparison. Either

(a) you design in a spreadsheet: realize the answer wont work, use your intuition to guess at a new solution, and then redesign the spreadsheet/modify the equations to tackle this new problem

or

( you design in KSP VAB itself: you realize the answer wont work, use your intuition to guess at a new solution, and then build it in the VAB using the existing spacecraft as a template.

I think ( should be faster!

Ah, but the hole in your reasoning is that you *think* it *should* be faster. You haven't actually tried it, whereas I do it all the time. I know which approach is quicker.

Thinking about it.... I suspect that the disconnect is in how you imagine the spreadsheets are used. I think you're imagining them as being used in a similar role as the mods, but they're not. I think that you picture them as being far more constrained than they actually are.

It's hard to picture the implementation of math in a context other than how you use it, but the natural progression is to *not* apply the math simultaneously as you build, but rather apply the math before you build. It's just plain more convenient, just as using a mod as you build is the most convenient way to use a mod.

Someone who's experimenting to come up with a good design (even if doing the math by hand) would *never* build random combinations of parts in the VAB and then figure out what the DV is after they've built it. They would try different combinations on paper, figure out which works best, and *then* build the ship. Building the ship is a complete waste of time in the process of figuring out the best ship, so we just plain don't do that.

Eventually, we get sick of doing the math the hard way and set up a spreadsheet to do it the easy way.

Eventually, it occurs to us that we've been doing the whole thing wrong and start working the equation backwards instead of forwards. In the process of designing, "DV" makes a much better question than an answer, right?

So we start applying the math that way... and get sick of it and build a new spreadsheet.

And now suddenly we're theoretically building the same stage with all possible different engines at the same time with just enough fuel tanks to do the DV we need. and simultaneously finding the total mass and price tag for each one. A few clicks, and we're accomplishing what would take weeks to trial-and error in the VAB.

And the process continues to feed on itself, but what doesn't change is *how* the math is used: Not during construction, but rather before.

We actually have no need for a display of things like cost, mass, DV and the like when we're building (except, perhaps, as a sanity check). We already knew all that before we started building.

Best,

-Slashy

Edited December 11, 2014 by GoSlash27

[Trentendegreth]

personally i use KE and Mecjeb, the math isn't hard just don't care to do it when i can slap on a part and guess what its done... all of it, took me like two seconds...

but if you get enjoyment out of doing the math, they hey who is stopping you, there is no wrong way to play this game...

[Slam_Jones]

So I'm curious, Slash, about how many folks you've convinced in this thread, and how many you've irritated? I mean, you seem to love math, so I'm curious what you come up with. Maybe you can finangle your slide rule in there somehow. That'll show us peons!

[klgraham1013]

So I'm curious, Slash, about how many folks you've convinced in this thread, and how many you've irritated? I mean, you seem to love math, so I'm curious what you come up with. Maybe you can finangle your slide rule in there somehow. That'll show us peons!

Honestly, I can't believe this thread is still going when the question was answered on the first page.

[Trentendegreth]

Honestly, I can't believe this thread is still going when the question was answered on the first page.

i second that thought.

[Xavven]

So I'm curious, Slash, about how many folks you've convinced in this thread, and how many you've irritated? I mean, you seem to love math, so I'm curious what you come up with. Maybe you can finangle your slide rule in there somehow. That'll show us peons!

Yeah, TBH this thread is going downhill -- just a thinly veiled ego stroke. No one is better or worse than someone else because they decided to use a spreadsheet vs. a mod.

[Freshmeat]

All the negative vibes aside, I would like to hear how you go about it in practice.

We all design for a given dV, based on mission requirements, but once you figured that number, what is your next step? Do you keep a spreadsheet with the different ISP and thrust values of different engines, as well as dry and full mass of different tanks and boosters so you can pick for a given payload?

There is no question that once you know your payload, it is in theory extremely simple to calculate fuel mass, but you still have to keep track of the additional mass of added components. So maybe a run down of you designing, say, a 120 t 5500 dV lifter as a practical example?

As you surely can read from my post, I am not convinced that predesigning is more efficient than KER, given that we all have a good intuitive grasp of mass fractions and the curse of the log function.

[PwnedDuck]

I never said that *you* said it was hard

But a lot of people around here portray it as something 3 shades deeper than deciphering heiroglyphics.

Setting aside the solving natural logs by hand (which is something nobody does), it's not "hard". Tedious for sure if you do it throughout the design process just due to sheer repetition, but not anything that could be rightfully described as difficult.

Maybe they mean that it's hard to force yourself to do unnecessary **** like this when as has been established alternatives are availible.

[Jouni]

There is no question that once you know your payload, it is in theory extremely simple to calculate fuel mass, but you still have to keep track of the additional mass of added components. So maybe a run down of you designing, say, a 120 t 5500 dV lifter as a practical example?

First I decide what kind of lifter I want to build. As the payload isn't too big, I'd probably design a standard 2.5-stage rocket. The first step would therefore be creating a new sheet, copying the 2.5-stage template to it, and setting the payload stage to 120 tonnes.

The second step is creating the initial upper stage. The size of the payload suggests a 3.75 m upper stage, so I'll start with an S3-14400 fuel tank (82/10 tonnes) and an KR-2L engine (6.5 tonnes, 2500 kN, estimated average Isp 380 s). I don't know anything about the payload, so preparing for the worst case, I add 0.5 tonnes for a probe core, 0.4 tonnes for a decoupler, and 0.2 tonnes for an adapter. The spreadsheet tells me that the initial (surface) TWR for this stage is 1.22, and the stage provides almost 1570 m/s of delta-v.

Next I create a placeholder for the lower stage. As the upper stage has one S3-14400 fuel tank, I start with three of them in the lower stage, and a KS-25x4 engine cluster (9.75 tonnes, 3200 kN, estimated Isp 360 s) as the engine. The 3.75 m decoupler is massless, so there is no overhead in this stage.

At this point, the total mass of the rocket is around 440 tonnes. As I'd expect that the mass of the final lifter would be over 1000 tonnes, I choose 3.75 m boosters over LFBs. I start with two boosters with KS-25x4 engine clusters (estimated Isp 330 s, which also applies to the lower stage as long as the boosters burn) and two S3-14400 fuel tanks each. The overhead from decouplers is 0.05 tonnes per booster. The spreadsheet tells me that the initial TWR is 1.20 (about right), the TWR after booster separation is 1.02 (a bit low), and the the total delta-v is 4919 m/s (too low).

As the first fix, I try adding a third booster. That changes the initial TWR to 1.32 (about right) and the total delta-v to 5302 m/s (too low). Then add an S3-3600 fuel tank to each of the boosters. The delta-v is still a bit too low, so I try adding an S3-3600 fuel tank either to the lower stage or the upper stage. Adding it to the upper stage gives better results, so I choose this as the final rocket.

The total mass of the rocket is 1068.75 tonnes, and the total delta-v is 5512 m/s. The initial TWR is 1.22 for the entire rocket, 1.01 after booster separation, and 1.11 for the upper stage. This seems acceptable, as my Isp estimates usually compensate for gravity losses from low-TWR lower stages. Designing this rocket took only a couple of minutes.

[GoSlash27]

All the negative vibes aside, I would like to hear how you go about it in practice.

We all design for a given dV, based on mission requirements, but once you figured that number, what is your next step? Do you keep a spreadsheet with the different ISP and thrust values of different engines, as well as dry and full mass of different tanks and boosters so you can pick for a given payload?

There is no question that once you know your payload, it is in theory extremely simple to calculate fuel mass, but you still have to keep track of the additional mass of added components. So maybe a run down of you designing, say, a 120 t 5500 dV lifter as a practical example?

As you surely can read from my post, I am not convinced that predesigning is more efficient than KER, given that we all have a good intuitive grasp of mass fractions and the curse of the log function.

Sure... but just a reminder, -->my goal isn't to convince anyone to slog through the math who doesn't want to. It's to convince people to not discourage newbies who want to try it.<--/disclaimer

So the process of designing a lifter to lift a 120 tonne payload with a total DV of 5500 m/sec would go like this:

1: break the launch down into phases.

Since the total DV is more than 4300, that last 1200 must be an orbital transfer stage

That leaves boost (1500), transstage (2000) and insertion (700)

I start with the payload on the insertion stage.

I plug into the spreadsheet the:

*local gravity (9.81 for Kerbin),

*required acceleration (don't care, but I'll set it at .5G)

*payload (120)

*DV (1200)

*number of engines I intend to use (1)

It now displays next to every engine how much mass of fuel and O2 I need for that stage, How much mass in loaded tanks I need, Total mass of the stage + payload, and acceleration I will get. It also displays a recommended number of engines for 1.5G/1G in atmosphere and 1G/.75G/.5G in vacuum.

Aaand that's my alarm. I'll have to pick this back up later.

-Slashy

[Red Iron Crown]

One thing about external calculation and optimization: If you take it too far your design becomes inevitable. Over-optimization results in part choices being made by the people who set the part stats rather than the player, the player just has to figure out a way to assemble the part list.

[John FX]

Didn't read past the first page.

I play games to have fun. After being buried in math all day, gaming is an escape especially since my free-time is highly limited. Sure, I could sit down and spend time to do all these calculations but I'm wasting my time. Plus, I don't like math.

I can do it, but I don't want to do it. However I do kind of want to do it by hand now that MJ & VOID give me 2 different ÃŽâ€v readings (VOID is 10dv smaller than Mechjeb) so I can find which is more accurate. However, on the other hand I don't care about 10 ÃŽâ€v difference is that significant enough for me to sweat over.

The math that is hard is "Taxes", but that's an entirely prohibited topic...

Out of MJ and VOID, KER gives the most accurate Dv readout...

(they did a huge fuel logic overhaul a little while ago)

[John FX]

Honestly, I allowed myself to get sucked into the discussion of which approach is "better" and that wasn't the point of why I brought this up.

You set your own criteria for which is the best way to play the game for you. It's not my place to dictate that to you.

Me *personally*, I get the most enjoyment from this game by achieving the most difficult goals in the most efficient manner possible with no guesswork, trial and error, or failures.

Turning the tables on that thought, would you find the game more enjoyable by not using KER and having no idea what your DV is, or less enjoyable? Some people actually prefer playing that way.

Your mileage may (and should) vary

Best,

-Slashy

Well the gist of the OP was, to paraphrase, "Why do people advise to use a mod instead of a spreadsheet? I think using the Dv formula is better" which naturally raises the issue of which is better/more fun for most players and sort of sidesteps the issue of how hard the maths is (spreadsheet or mod, you are not doing the maths yourself anyway)

I think there is a consensus in the thread that it is not how hard the maths is that is the issue.

Some people find fun in a well designed spreadsheet that gives them the optimal design criteria for a craft. That`s good.

A lot more people it seems see KSP as a game and they want to get on with building rockets and flying little green men to fantasy planets. That`s also good.

I`m not sure you`ll convince the second group to promote using a spreadsheet in the forum as they just don`t want to (for their own game so that is the advice they give). They want to stay ingame all the time to retain immersion. If you stay ingame then the only way to get a Dv readout on screen is with MJ, VOID or KER (AFAIK) The people who want to stay ingame generally don`t want to use a pen and paper either (Obviously there are exceptions)

This seems to be the core of the issue. The difficulty of the maths is second to that.

As Horseman says, why not write an ingame thing that does what you say is good about doing things external to the game?

win-win surely?

KSP, Kerbal Spreadsheet Program FTW

To recap, it seems most people do not think it hard, they think it not fun and a break to immersion.

Why do you play a game? To immerse yourself in it and have fun.

EDIT : although I agree with the posters who have said that context is king when it comes to what you say to new players who are asking how to find out the performance of their craft.

Asking how to calculate Dv? Tell them the formula.

Asking how to get a Dv readout ingame? Point them to the mods.

That is how people should be responding (To actually post On Topic)

Slashy, why not write a tutorial, make a thread for it, have it in your Sig and point people towards it? Like a spreadsheet it only needs doing once then you have a resource that will help new players be guided to where they would like?

Edited December 12, 2014 by John FX

[GoSlash27]

Okay, I'm back.

Thinking about this on my way to work, I have to add another disclaimer: 120 tonnes is a huge payload! I don't normally design rockets that big, so my lack of experience may cause me to wind up designing a sucky rocket.

I'm just doing it to illustrate the process itself. /disclaimer

So... 120t and 1200 m/sec. Don't care about the acceleration.

My spreadsheet says that the LV-N is best for that job and I need 23.2 tonnes of loaded tanks. That's an x200-32 and an x200-8.

24.8 tonnes for the stage, 145 tonnes for the preceding stage to lift.

I plug the numbers in and it says a mainsail will be ideal for this.

38.3 tonnes for the loaded fuel tanks, which is a Jumbo 64 and an x200-8. Total stage mass 46.5 tonnes, total vehicle mass 192 tonnes.

Now we need 1G minimum acceleration, 192 tonnes of payload, and 2000 m/sec DV.

Plug them in, and...

This one's very close, but 6 skippers can do the same job a few tonnes cheaper than 3 mainsails, so we're going asparagus.

I move into my asparagus model

I can asparagus these around my mainsail. Each radial booster is a single skipper with 1 jumbo 64.

Total mass is up to 473 tonnes.

This leaves our boost phase.

payload 473 tonnes, DV 1500 m/sec, atmospheric Isp, and at least 1.5G acceleration.

Plugging in the numbers, it says 7 mainsails is the way to go. So another round of asparagus staging (or something similar). Without the asparagus staging, I'm looking at a stage mass of 426 tonnes and a total vehicle mass of 899 tonnes, but I can improve that with staging.

Plugging the requirements in my asparagus model...

Each radial booster is an S3-7200 with a mainsail. Total vehicle mass 755 tonnes.

That's pretty much how the process works using a spreadsheet.

From there, I could juggle my requirements, design for different objectives and schemes, etc.

Not saying this is a great rocket design (I just did it off the cuff), but illustrating how it's done with a spreadsheet as opposed to building in the VAB with a DV readout.

Best,

-Slashy

Edited December 12, 2014 by GoSlash27

[GoSlash27]

EDIT : although I agree with the posters who have said that context is king when it comes to what you say to new players who are asking how to find out the performance of their craft.

Asking how to calculate Dv? Tell them the formula.

Asking how to get a Dv readout ingame? Point them to the mods.

That is how people should be responding (To actually post On Topic)

This is exactly what I'd set out to get across (badly)

If the newbie is asking how to use the formula, don't say "it's really tricky, use a mod"

Likewise, if the newbie is asking how to use a mod, don't say "suck it up and learn the maths".

Thanks,

-Slashy

[arkie87]

This is exactly what I'd set out to get across (badly)

If the newbie is asking how to use the formula, don't say "it's really tricky, use a mod"

Likewise, if the newbie is asking how to use a mod, don't say "suck it up and learn the maths".

Thanks,

-Slashy

A few questions:

(1) How exactly does your spreadsheet work? The best way i can think is to have one worksheet for all the part names and specs (wet and dry mass, thrust, ISP etc..). And each subsequent worksheet are stages, starting from uppermost. In each worksheet, do you type in a part name and it looks up specs from first worksheet, or do you have to find them yourself (a HUGE time killer)? (Maybe a screenshot of your spreadsheet word help?) After that, each sheet can sum up wet/dry mass and calculate deltaV, TWR etc...

(2) What is the formula for calculating deltaV for two engines with differing ISP's and burn times? Is there a simple equation, or do you have to make simplifications/assumptions to arrive at a formula in the first place?

[Red Iron Crown]

I can answer some of that. I keep a table of parts and use the VLOOKUP function in Excel to automatically populate the relevant data fields (I do stages on one sheet instead of several, though).

Net Isp of a heterogenous cluster is calculated using a thrust-weighted average of the Isp of each engine, burn time is not relevant in most designs I use because I fuel line. If there are multiple engines in a stage with different burn times, it is better to treat it as two stages even if nothing is jettisoned.

[GoSlash27]

^ What he said.

And I'm not sure what you mean by "huge time waster". The alternative would've been assembling and disassembling boosters in the VAB with various combinations and taking notes until I'd decided I'd confirmed that I had the best option.

*That* is my opinion of a huge time waster.

IAC, that's not how it works (at least not on mine).

All of my engines are on the same sheet and the results of the calculations are next to every engine assuming I'm using that engine, so the problem is solved for every engine simultaneously, allowing me to tell at a glance which engine I should use.

Plugging the fuel required into the tank section will have it instantly tell me how many tanks of what type to use.

The various rocket models require that I manually transfer specs into the stages for the engines and tanks I want.

I have models for various rocket stack types, such as a layer cake stack, 1-2-4 layer cake, several types of asparagus, twisted candle, and 3 stage drop tanks.

They will keep track of the minimum acceleration of each stage, DV for that stage, and cumulative DV for the entire vehicle.

Best,

-Slashy

Edited December 12, 2014 by GoSlash27

[arkie87]

A few questions:

(1) How exactly does your spreadsheet work? The best way i can think is to have one worksheet for all the part names and specs (wet and dry mass, thrust, ISP etc..). And each subsequent worksheet are stages, starting from uppermost. In each worksheet, do you type in a part name and it looks up specs from first worksheet, or do you have to find them yourself (a HUGE time killer)? (Maybe a screenshot of your spreadsheet word help?) After that, each sheet can sum up wet/dry mass and calculate deltaV, TWR etc...

(2) What is the formula for calculating deltaV for two engines with differing ISP's and burn times? Is there a simple equation, or do you have to make simplifications/assumptions to arrive at a formula in the first place?

So I figured out #1: Making a sheet for parts, and stages seems most elegant to me, and you can dynamically call all part specs by looking them up in the first worksheet easily by using "Match" and "Index" functions in excel

[arkie87]

I can answer some of that. I keep a table of parts and use the VLOOKUP function in Excel to automatically populate the relevant data fields (I do stages on one sheet instead of several, though).

Net Isp of a heterogenous cluster is calculated using a thrust-weighted average of the Isp of each engine, burn time is not relevant in most designs I use because I fuel line. If there are multiple engines in a stage with different burn times, it is better to treat it as two stages even if nothing is jettisoned.

Thanks! That makes sense regarding thrust-weighted average ISP. I suppose I could do the math and prove that it is the correct formula. Regarding how to treat different burn times-- it is a good approximation though it is an underestimate if you are able to jettison parts before the other runs out.

Would you mind sharing the part sheet? Pretty please?

Edited December 12, 2014 by arkie87

[RuBisCO]

When I calculated Delta-V needed to take off of the moon or 10X mun I get 1857.6 m/s, that is 1817.8 m/s to go from the surface at 2000 km from the core to 2060 km Elliptic orbit (that goes from 2000 km to 2060 km, or 0 to 60 km from the surface) and then another 39.8 m/s to circularize at apoapsis. In actually I needs several hundred more m/s then this, with safe margins 2500 m/s, and 2300 m/s at least, so that is 500 m/s more. For my accent does not perfectly follow the supposed elliptic orbit, there is gravity loses in having to simply fight against gravity from falling down, etc, are there equations for calculating gravity loses and accent profile performance?

[GoSlash27]

When I calculated Delta-V needed to take off of the moon or 10X mun I get 1857.6 m/s, that is 1817.8 m/s to go from the surface at 2000 km from the core to 2060 km Elliptic orbit (that goes from 2000 km to 2060 km, or 0 to 60 km from the surface) and then another 39.8 m/s to circularize at apoapsis. In actually I needs several hundred more m/s then this, with safe margins 2500 m/s, and 2300 m/s at least, so that is 500 m/s more. For my accent does not perfectly follow the supposed elliptic orbit, there is gravity loses in having to simply fight against gravity from falling down, etc, are there equations for calculating gravity loses and accent profile performance?

Sorry, but I'm afraid not. The math cannot predict (and you cannot mathematically define) how imperfectly you will follow the ideal path.

Your best bet is to pad the budget a certain percent based on how much more you need by your experience, which is what you're doing.

You should also look at your technique and figure out *why* you're not matching the expected figures.

In your case, I'm guessing you're not gravity turning as early or aggressively as you could. You need to be going horizontal as fast as you can as low as you can (without smacking into the terrain) to get the best results.

Best,

-Slashy

[Red Iron Crown]

Thanks! That makes sense regarding thrust-weighted average ISP. I suppose I could do the math and prove that it is the correct formula. Regarding how to treat different burn times-- it is a good approximation though it is an underestimate if you are able to jettison parts before the other runs out.

It's not an approximation at all, it's an exact calculation. It can be calculated relatively easily, mass flow rates are derived from thrust and Isp. The mass flow rates will give stage depletion time, and with that you can determine how much will be burned out of a partially expended stage. (I must admit that I haven't spreadsheeted this calculation as I rarely if ever use that sort of design).

Would you mind sharing the part sheet? Pretty please?

Can you give me a bit of time? I only have a subset of the parts in it and you've given me a good excuse to update and complete it.

[arkie87]

It's not an approximation at all, it's an exact calculation. It can be calculated relatively easily, mass flow rates are derived from thrust and Isp. The mass flow rates will give stage depletion time, and with that you can determine how much will be burned out of a partially expended stage. (I must admit that I haven't spreadsheeted this calculation as I rarely if ever use that sort of design).

Sorry, i think you misunderstood me; or perhaps i misunderstood you. It's an "exact" solution, but only when additional calculations are done: to calculate deltaV for two sets of thrusters with different ISP and burn time, you have to first calculate burn times of each, and then make one deltaV calculation for when they are both burning and one deltaV calculation for when only the non-jettisioned stage is burning. That would be an exact calculation.

What i thought you meant it just ignore burn times and calculate thrust-weighted average ISP and use that to get deltaV, ignoring burn times (assume you dont jettison spent engines when they run out of fuel). This is a conservative estimate, as real-world deltaV will be higher. This is not an exact calculation; it is a conservative estimate.

Can you give me a bit of time? I only have a subset of the parts in it and you've given me a good excuse to update and complete it.

Thank you so much! Right now, i have placeholder parts, such as "banana" and "butt" with made up stats (butt obviously has the most thrust )

[Red Iron Crown]

Part list in xlsx format.

I don't have any of the massless parts included on the list, but I did update it to include all the wing parts (I had left them out before). I also don't have air-breathing Isps included, the calculations for those make my head hurt and vary greatly with both altitude and speed.

Edit: I did include one massless part, the O-10, because its needed for RCS powered designs.

Edited December 12, 2014 by Red Iron Crown