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Elroy Jetson

Rocket Design Help?

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I looking for some help with intermediate rocket design.  I have been successful with circular orbits of Kerbin, Mun and even landed and returned from Mimnus.  But, I have been able to get to this point only with the design help provided by YouTube videos.  I don't mind making adjustments to a basic configuration but for instance I'm trying to modify a saved design that I was able to use to get just Jeb to the Mun and back.  Now I am struggling with "my" modified design trying to bring along four passengers.  

 

What can I use to help understand how much fuel, weight and thrust I need to make initial orbit, then leave orbit to the Mun, enter orbit...and finally return.

 

At this point I have been adding  more fuel, boosters, etc but am still running out of fuel before I make the soi of mun.

 

are there some rules of thumb, or basic designs that provide a starting point?  

 

As you have probably guessed I don't understand delta v.  I have read about it a bit now, and watched a few videos, but I don't understand it enough to help me solve the above basic problem.

 

thanks!

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If you're looking for specific advice on your ship, it would be very helpful if you could post a screenshot.  There are a ton of potential issues that can limit your range, but so it's hard to which advice might or might not pertain to your project.  

That said, funnily enough, I was just starting to work on an "intermediate player tips" post with just such general advice.  It's still a work in progress but I'll post some general content below.  If you're still figuring out delta-v and the Rocket Equation, for the discussion below just keep in mind that more delta-v means longer effective range. 

34 minutes ago, Elroy Jetson said:

What can I use to help understand how much fuel, weight and thrust I need to make initial orbit, then leave orbit to the Mun, enter orbit...and finally return.

Unfortunately, there is no simple answer to this question, because it depends on a lot of other variables.  If you make a very small, efficient Mun lander and fly it perfectly, you can get away with a small rocket to start with.  But if you bring a lot of stuff (either because the mission requires it or do to design inefficiencies), your rocket will need to be bigger.  

I would very much recommend that you get used to designing from the top down.  Start with your final stage and figure out what you need on it to accomplish its goals.  Then figure out what the next stage needs to get your final stage where it needs to be, and so on and so on, until you've gotten all the way down to your launch configuration.  

I'd also suggest you keep working on delta-v, and playing around the delta-v readout (or a similar mod tool, like Kerbal Engineer Redux) until you're comfortable with the basics of what it's telling you.  For instance, you may have seen that it typically takes 3,000 to 3,500 m/s to make Kerbin orbit.  It's very helpful to be able to take that fact, and use it to see what kind of a launch configuration will give you that much delta-v. 

As far as your question about thrust, are you also familiar with thrust-to-weight ratio?  That's a more important number than total thrust.  TWR is another data point you can see in KER, or fudge by looking at the g-meter.  But as far as how much thrust you need, you probably want your ship to start with a TWR somewhere between 1.3 and 1.8.  Later stages can have progressively less TWR, but It'd keep it above 1 to start with until you're in orbit.  From that point on, anything over 0.5 should suffice.

------------------------

The Rocket Equation: Live it, Learn it, Love it.  Do you want your rocket to go farther?  It seems like a complicated proposition, but thanks to the Tsiolkovsky Rocket Equation, it’s actually a pretty simple matter.  The equation solves for the delta-v a rocket can achieve using three, and only three variables:

1.       Dry Mass: the mass of everything in your rocket other than fuel

2.       Wet Mass: the mass of the fuel in your rocket.

3.       Specific Impulse: the efficiency of your engines.

The first two can even be condensed into a single number called the mass fraction (the ratio of dry mass to wet mass), giving you only two variables to work with.  So if you want a rocket to go further, the equation gives you a few general guidelines:

1.       Decrease Dry Mass: this is arguably the most important point, and one I’ll touch on in greater detail below.  But cutting unnecessary fat can pay enormous dividends in terms of delta-v.

2.       Increase Wet Mass: that’s a fancy way of saying “add more fuel.”   Simple enough, as long as the rest of your rocket can handle it.  But be aware that adding fuel runs up against diminishing returns pretty quickly, because that extra fuel equals extra dead weight up until the point it’s burned.  This is the core of the “Tyranny of the Rocket Equation.”

3.       Increase Specific Impulse: this just means selecting an engine with favorable ISP numbers for the task at hand.  Again, simple enough on the surface.  But (again as discussed below) ISP is just one of many factors you’ll have to balance, and can be a bit of a trap if over-prioritized.

There’s one more range factor that’s not expressed in the classic Rocket Equation, but obviously has a large effect on your ship’s range.  That’s staging, and while it’s an important topic, I’m not going to get too far into the theory here.

 

Keep it Simple: This goes back to the “reduce dry mass” point above.  It is almost impossible to overstate how large an effect dry mass can have on your ship’s delta-v.  I don’t want to get too technical, but a very significant fraction of your ship’s delta-v comes when your fuel level is low, at which point the rocket is very light, and every unit of fuel burned can produce a lot of acceleration.  Keeping dry mass light really helps amplify this effect.  This is actually the major reason why real-life rockets can achieve much greater delta-v per stage than the KSP equivalents – their fuel tanks and engines are much lighter, giving you much better mass fractions.

Reducing dry mass can pay other dividends as well.  You will get better TWR out of your engines, which at a minimum can improve maneuver efficiency, but can sometimes allow you to switch to lighter (or fewer) engines – further reducing dry mass in a virtuous cycle.

So with that as background, it’s always good practice to approach a rocket with the question of “what can I cut?” rather than “what can I add?”  

 

 

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1 hour ago, Elroy Jetson said:

As you have probably guessed I don't understand delta v.  I have read about it a bit now, and watched a few videos, but I don't understand it enough to help me solve the above basic problem.

With 1.6 as you design the rocket, each stage will display the amount of DeltaV on the right staging panel. At the bottom is the total for the whole rocket.

Use that plus This Delta V Map to design rockets that will achieve what you want.

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Thanks for the links and info!  I dug into the math today and it is starting to make sense!  I enjoy a launch much more now that I can calculate if the rocket will actually leave the pad, and I can calculate if I have enough delta v to make the initial orbit of kerbin.

The "subway" delta v map is making sense now, but not totally.  

 

So, to land on the mun leaving from KSC, I need 3400,  860, 310, 580 total delta v right?  So I add all those numbers to load that much delta v to land on the mun?  I recognize that I need to do the individual calculations to account for changing mass due to ejection of primary stages along the route and subsequent mass change.

 

What about getting home?  Does it take the same amount to return home?  I would guess not since I don't need to lift the weight of anything out of kerbin gravity.

 

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7 hours ago, Elroy Jetson said:

Thanks for the links and info!  I dug into the math today and it is starting to make sense!  I enjoy a launch much more now that I can calculate if the rocket will actually leave the pad, and I can calculate if I have enough delta v to make the initial orbit of kerbin.

The "subway" delta v map is making sense now, but not totally.  

 

So, to land on the mun leaving from KSC, I need 3400,  860, 310, 580 total delta v right?  So I add all those numbers to load that much delta v to land on the mun?  I recognize that I need to do the individual calculations to account for changing mass due to ejection of primary stages along the route and subsequent mass change.

 

What about getting home?  Does it take the same amount to return home?  I would guess not since I don't need to lift the weight of anything out of kerbin gravity.

 

 

Getting home is exactly the same amount of delta-V, except that you can use aerobraking to slow down when you get back to kerbin. That will eliminate most of the 3400, and I think some of the 860, depending on how sturdy your heat shield is. If there wasn't air, you'd have to use exactly the same amount to slow down to zero and land. This is what makes Tylo such a hard moon, it has kerbin's gravity, with no air.

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9 hours ago, Elroy Jetson said:

So, to land on the mun leaving from KSC, I need 3400,  860, 310, 580 total delta v right?  So I add all those numbers to load that much delta v to land on the mun?  I recognize that I need to do the individual calculations to account for changing mass due to ejection of primary stages along the route and subsequent mass change.

 

What about getting home?  Does it take the same amount to return home?  I would guess not since I don't need to lift the weight of anything out of kerbin gravity.

 

Exactly right for landing on the Mun. 3400 to LKO, 860 from LKO to flyby the Mun, 310 to turn a flyby into low orbit around the Mun, then 580 to land.

Going home, take it in reverse: 580 to return to low Mun orbit. 310 to turn that orbit into a flyby heading back towards LKO. That's when you can stop using your engines, since you can hit the atmosphere and aerobrake instead.

 

However, the dV map necessarily makes "average use" calls, and Your Mileage May Vary. One of the big YMMVs is the 310 m/s requirement to turn a flyby into a capture around the Mun. As far as I can remember, I've never had to spend 310 getting into low Mun orbit or to return from the Mun. It's more like 280 m/s. On the other hand, it takes a whole lot of skill to get a Mun landing down to 580 m/s: you essentially need to do a picture-perfect, nearly-horizontal suicide burn. The sort of burn that leaves you ramming into the surface at high speed with no possible escape route if you start the burn a touch too late...

Averaging out the YMMVs, that does mean that you basically need a total of 6.1k m/s dV to get there and back.

While possible (and there are challenges to prove it), that is very much at the extreme limit of what is feasible in career mode without upgrading buildings. It becomes a lot easier if you can start with 2.5m rocket parts to get up to LKO.

 

I've found it easier to divide my rocket designs into two separate parts. "What do I need from LKO?" and "How do I get this to LKO?"
For the Mun, therefore, you need about 2.8km/s dV from LKO. That is unfortunate, since the very easy setup is a 1.25m command pod, heatshield with 20% ablator, "400"-sized fuel tank and Terrier engine... which gives around 2.3km/s dV (if I remember correctly). Using the simplest and most obvious upper stage can get you down to the surface of the Mun, but will never get you home.
Therefore, the design for doing a full Mun-surface-and-back trip needs a touch more thought. The most obvious is to have drop-tanks on your upper stage. An Apollo-style approach is possible but is actually a lot more complicated (separate lander, docking in low Munar orbit, etc.). Or simply making sure that your lifter stage gets you to orbit with another 500 m/s still in the tanks.

The "how to get this to LKO" therefore needs a bigger lifter..

And I think that that's the reason why I always end up building a Mun space station quite early on.
After all, the return from the Mun is very minor. Only 300 m/s or so needed. Getting to the Mun isn't a huge problem: 860+300 m/s from LKO to LMO. It's relatively trivial to get a load of fuel into LMO using 2.5m parts. It's the 580m/s x2 for the landing that is the pain, especially if you need to go home using the same ship: it needs to include a heatshield, parachutes, decouplers and whatnot, all of which are sapping away at your fuel economy.
So, therefore, the Mun is just a touch too much to do, reasonably, on a single mission without exotic parts. A permanent orbiting station with fuel makes it so very much easier.

So, if you want to get multiple Kerbals down to the surface of the Mun, the easiest way is to use 2.5m parts to get a 2.5m fuel tank + docking facilities into low orbit around the Mun, then use a dedicated lightweight lander to get to the surface, and use whatever supply ships you send out there (with heatshields, parachutes etc.) to bring them back home. It's certainly much more complicated than doing a single round trip, but even the simplest round trip requires a degree of over-engineering, so why not...

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9 hours ago, Plusck said:

 

And I think that that's the reason why I always end up building a Mun space station quite early on.
After all, the return from the Mun is very minor. Only 300 m/s or so needed. Getting to the Mun isn't a huge problem: 860+300 m/s from LKO to LMO. It's relatively trivial to get a load of fuel into LMO using 2.5m parts. It's the 580m/s x2 for the landing that is the pain, especially if you need to go home using the same ship: it needs to include a heatshield, parachutes, decouplers and whatnot, all of which are sapping away at your fuel economy.
 So, therefore, the Mun is just a touch too much to do, reasonably, on a single mission without exotic parts. A permanent orbiting station with fuel makes it so very much easier.

So, if you want to get multiple Kerbals down to the surface of the Mun, the easiest way is to use 2.5m parts to get a 2.5m fuel tank + docking facilities into low orbit around the Mun, then use a dedicated lightweight lander to get to the surface, and use whatever supply ships you send out there (with heatshields, parachutes etc.) to bring them back home. It's certainly much more complicated than doing a single round trip, but even the simplest round trip requires a degree of over-engineering, so why not...

This is great info, thanks!  I have started to train for this mission by doing the docking mission from the training menu.  I hope matching orbits gets easier like it seems the earlier parts did?

 

it seems like the training scenario expects you to NOT make the orbit match (roughly circular) but just get an interception point close?  I'll keep working that training missing but at this point I haven't been able to get the interception to less than 10km which doesn't satisfy that step with the instructor.

 

i am eager to learn this maneuver because one of my big goals is as you describe to put refueling stations in orbit around kerbal and the mun.

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15 minutes ago, Elroy Jetson said:

This is great info, thanks!  I have started to train for this mission by doing the docking mission from the training menu.  I hope matching orbits gets easier like it seems the earlier parts did?

 

it seems like the training scenario expects you to NOT make the orbit match (roughly circular) but just get an interception point close?  I'll keep working that training missing but at this point I haven't been able to get the interception to less than 10km which doesn't satisfy that step with the instructor.

 

i am eager to learn this maneuver because one of my big goals is as you describe to put refueling stations in orbit around kerbal and the mun.

Meeting something in orbit is actually a lot easier than it looks, especially if you simply use the NavBall and the map.

Definitely use the tutorial to get the hang of intercepts.

Then, when coming from far outside the orbit of your target craft, just remember that tiny changes at a distance can make a huge change in the time of arrival in orbit. Therefore you can always combine a little prograde/retrograde and a little radial in/out to make sure that you meet the target craft exactly at your Pe as it touches (or, ideally, very very slightly crosses) the target's orbit. Always. If you're having trouble with the tutorial, keep playing with the maneuvre node until it twigs for you.

And once you have that intercept set up (to less than 1km, touching or crossing orbits), you can do all of the rest simply with the NavBall and map view without any planning required.
You need to know your approximate acceleration. A small craft with a single Terrier probably has an acceleration of around 15-20 m/s2. In the map, click on the intercept chevron to see your "time to". Click on the NavBall to get target mode. Line up with the retrograde vector to target. Check velocity to target, and divide by your acceleration to get maximum burn time to match orbits. Start burning when you're about 2/3 of that time away from intercept (absolute minimum is 1/2 your velocity divided by acceleration, but you want a little room for error and therefore the ability to throttle).
While burning, notice how if you aim off to one side of the retrograde vector, it will move away from where you are aiming. Use that "pushing the marble" effect to push the retrograde vector exactly over the anti-target (pink cross) marker. If your intercept was relatively close to start with, it will get closer and closer as you do this. Throttle down if the "time to" countdown stops counting down or slows too much. Stop burning altogether when the intercept chevron starts getting further away from your current location.
Just by doing the above, you will perfectly match orbits at maximum efficiency in a minimum of time and effort. You can easily end up dozens of metres from your target at nearly zero difference in velocity with a single burn. Then you can switch to outside view for the docking procedure, which is way harder to start with :D

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