#how to compare engines

85 messages · Page 1 of 1 (latest)

void cape
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So,
I know what mass is, what dry mass is, I know what Thrust is, and I know what cost is, but that alone is not enough to compare engines is it?
how do I go about deciding if I should just add some more fuel tanks to my swivel engine or add a stage with a hammer booster?
what will get my rocket higher up? 2x T100 fuel parts or solid hammer + decoupler?
The metric I'm missing here is something in the realm of how long with the engine run for, since then I believe I could do something like:
(EngineA thrust - gravity force of rocket A) * time before rocket A runs out of fuel VS (EngineB thrust - gravity force of rocket B) * time before rocket B runs out of fuel. Sure I'm missing air drag, and I suppose the rocket mass changes over time, but I'd imagine those have small impacts and are unlikely to alter the decision, right?

Q1) Is this a good reasoning? is there some formula for comparison of some kind?
Q2) Is that drag important to consider in the calculation?
Q3) what is that metric that I'm missing? what's it's name and where do I find it/calculate it in the engine specs?
Q4) is there other types of metrics I should be paying attention to?
Q5) any good resources at explaining this things out?

steady rune
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do you lnow about isp?

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and twr?

void cape
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no, neither

steady rune
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isp is engine efficeny

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twr is thrust to weight ratio

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isp is the reason why terrier is the muchz better engine as terrier for space

steady rune
silk flame
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Also watch out for the engine mass

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Although vector has good enough isp and high thrust, using it on light spacecraft actually bad since vector is quite heavy

void cape
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ok, I might know twr, correct me if wrong:
my twr would be:
swivel = 168kn
rocket mass = 7.49t
twr = 168 / (7.49*9.8) = 2.288

steady rune
void cape
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that deltaV has a checkbox for TWR, but not an actual number, am i in the wrong place?

steady rune
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but you know wwhat delta V is or?

void cape
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kind of know... very important after you are in orbit, but is it also while on the ground?

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but I still don't know where that TWR metric is... any screenshot?

steady rune
silk flame
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Delta v is the range given current atmosphere pressure

void cape
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and... was that calculation of TWR i made accurate?

silk flame
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I think so

void cape
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also, correct me if wrong, but that ISP would be enough to compare the options if they had the same amount of fuel, but how would you compare them if they don't?
OptionA: 2.81t fuel with IPS of 170
OptionB: 0.98t fuel with IPS of 250
would I just go about multiplying them? It doesn't feel right... specially since they also have different masses. Is that ISP really helpful to compare the options? or is there a easier way?

silk flame
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It would be enough unless the engine mass difference is high

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Or it use different fuel

void cape
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it uses difrent fuel, and it is high

silk flame
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Different fuel type

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Fuel amount doesn’t matter for comparing engine, at that stage you’re comparing rocket instead

void cape
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yeah, comparing 2 rockets, that differ on the engine, but how would you compare 2 engines if not for a specific rocket/situation?

silk flame
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Vector (and maybe nerv) is really the exemption as they have high engine mass for its size, I usually check isp then

void cape
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so you would simply use the highest ISP engine that you can get your hands on?

silk flame
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No, since I would consider specific situation but you exclude it sooo

void cape
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when did I exclude specific situation? 😄 I'm trying to learn what factors to pay atention to and how to make decisions on a case by case basis

silk flame
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“If not for a specific rocket/situation”

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If you allows talking for specific situation then you also have to consider gravity

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nerv and ion engine has highest isp but you never use it on Kerbin ground since they have low twr, not enough to lift off the ground so you should tick them off away first

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But on the other hand gravity doesn’t matter that much at orbit, so they have big advantage there

void cape
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in that example I mentioned, how would you decide between optionA and optionB?

silk flame
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You check delta v

void cape
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how?

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or do I calculate it?

silk flame
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You don’t judge by how much fuel a rocket contains, you check for same twr you wanted, whether the delta v is higher

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You shouldn’t calculate it unless you want spend 1 hour to calculate it lol
Unless you’re playing old version of ksp, it should be displayed at the stage tab

void cape
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idk what version i'm playing... let me check

silk flame
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you shouldn’t play old ksp to use old mods most of the time btw

void cape
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1.12.5.3190

silk flame
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you’re on newest

void cape
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I just started the game yesterday

silk flame
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Check your right bottom corner when building rocket

void cape
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so... where's that delta v tab?

silk flame
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Move ur decoupler to stage 0

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You should watch tutorial if you don’t know what that means

void cape
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same as chute, not a seperate one?

silk flame
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Either that or separate one

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Both options are ok

void cape
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alright, so i'm guessing the blue is delta v for that stage and orange for the full thing?

image.png
silk flame
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Yes

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Click on the blue bar for twr

void cape
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ohhhhh

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alright, that is going to be VERY usefull

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even has that burn time

silk flame
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I guess M should take over since I gonna sleep lol

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2 am moment

void cape
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oh boy

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well, thanks a lot!

silk flame
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bye

void cape
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next question is: "You don’t judge by how much fuel a rocket contains, you check for same twr you wanted, whether the delta v is higher" what if the 2 options don't have the same TWR

steady rune
void cape
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you lower TWR to avoid drag and heat?

steady rune
void cape
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ok, so i'd limit the liquid fuel option thrust to match the TWR of the solid fuel, right?

steady rune
void cape
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ok, so would this make the option to the right better then the left one?

image.png
steady rune
amber kelp
steady rune
vapid panther
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don’t calculate it. There are only two terms that you ever really need to know

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Delta V and TWR

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Eventually you’ll get the hang of feeling out what’s best for the job, ISP being usually the most important stat to look at

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For launch stages, you want a decent TWR, around 1.2-2.0. You also want to look at max TWR for each stage, which idk if KSP tells you by default. This can be an indication of if you’re building inefficiently because if MAX TWR and initial TWR are really far apart, you’re likely building inefficiently

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For orbital stages TWR is less of a constraint so you can go below 1

zealous vine
vapid panther
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Or just use a less powerful engine

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You can get two similar TWRs and delta Vs with extremely different masses