dV efficient building?

[Taiber]

Hi everyone!

Im new here and this is officialy my first post slowly getting grip on KSP absorbing info as much as i can. I have a few qestions for you as a intermediate player.

What im trying to do is build the most efficient rockets and stuff because thats the way i like to play KSP. So Recently i downloaded KSP dV map and started building

my new project. The project is Ike rover (Leonardo 1). According to dV map i need 3400 m/s vac. to LKO (80km), etc. 1600 m/s vac. to Duna "hoffman style" and around

500 m/s vac. to catch, orbit, and land on Ike. I know these are harch numbers, but just for reference. Using both KER and Mechjeb to calculate my dV i built something like this:

Everything is ok until i get to lunch pad. Mechjeb is showing different numbers and KER is showing different numbers:

My idea was to make orbit (80km) with first 3 stages (asparagus staging) and using 200 m/s dV from stage 4 to finish circularization (to leave no orbital trash) and then leave to Duna transfer with 1600 m/s left in that stage.

I can get this design to get the job done with little more fuel but im trying to be efficient as possible. I know theres a lot of factors i need to look at (eng. atmo. eff., twr, ...) but my qestion is why

is Mechjeb and KER showing different vacuum dV numbers in VAB and on the lunch pad? And how do you build your efficient vessels? Sorry for my grammar im from Slovakia. Thanks!

Edited August 20, 2015 by Taiber

[Belphegor]

... how do you build your efficient vessels?

welcome

i don't use mechjeb or KER, i just put something together and do a test flight. although the many rescue missions for stranded crafts that -nearly- made it back to kerbin are getting expensive so i might install KER soon.. unfortunatly that means i don't know why the data in VAB and on the launchpad are different.

anyways.. if you are only a bit short on fuel i think the easiest way would be to add boosters so you don't have to redesign all the stages of your rocket. however, you can still adjust the thrust limiter of those boosters to get a better ascend speed, and in case the boosters give you excessive Dv, adjust/balance the amount of fuel in both the boosters and fuel tanks which in turn will make the boosters more efficient due to the decreased weight. let them do the heavy lifting for the first kilometres

also, i'm impressed at how small your duna rocket is.. i remember when i first tried to get into orbit and everything i built turned out to be seriously oversized

Edited August 20, 2015 by Belphegor

[Streetwind]

As an intermediate player, you should really start calling it "Hohmann transfer", not "hoffmann style"

As for why KER and MechJeb display what they display, I can't tell you. Each mod likely uses its own algorithm and gets minimally different results. I'm also not sure whether you having a stageable element (the fairing base) in between the fuel tanks of the 4th stage somehow confuses the calculation - the time of when the fairing mass is staged away definitely has an impact on the dV you can expect to get. If you want details, you should probably ask in the mods' development threads.

I'd probably do away with the entire asparagus shebang, too. 2.5m engines have amazing TWRs and generally stomp on 1.25m engines. Try something like:

Payload

Decoupler

FL-T200 tank

48-7S engine

Decoupler

Inline reaction wheel

2.5m aeroshell base

x200-16 tank

Poodle engine

Decoupler

x200-16 tank

x200-32 tank

Skipper engine

Napkin math suggests that with good piloting, the poodle will take you all the way to Duna - make your transfer so that you encounter Ike directly. Shouldn't be hard, considering you can't swing a cat around Duna without getting an Ike encounter. Use the small third stage to finish the ejection burn if necessary, and to capture at your destination. If I'm reading your design right, the rover can land on its own.

Edited August 20, 2015 by Streetwind

[Superfluous J]

I can't really tell based on the images, but that ship looks okay just eyeballing it. Maybe it's your ascent path and gravity turn?

If you supply a .craft file I'll take it for a spin. If I can get it into orbit I'll film it and put it on YouTube so you can see how I got it done. If I can't, then I would be willing to say it needs more fuel or a rework of staging, engines, etc.

Edited August 20, 2015 by 5thHorseman

[DCWarHound]

You're wasting allot of Delta/V with that payload,you need to make sure that the payload and payload shield is not wider then the rocket itself.

[Superfluous J]

You're wasting allot of Delta/V with that payload,you need to make sure that the payload and payload shield is not wider then the rocket itself.

While I technically agree, sometimes you just gotta do it, especially early in career. It's far easier to get a wide lander into LKO than it is to land a tall skinny lander on sloped ground on Mun.

[Godot]

Of course KER shows different stats.

On the launchpad you are in an atmosphere (KER in flight values are always based on the current surrounding air pressure),

whereas the dV values you have displayed in the VAB are the values for a vacuum.

As to why values are different for MJ and KER ... dunno. Maybe their algorithms are slightly different with regards to precision of datatypes and rounding.

As for the needed dV values for Duna (and other planets):

Always be aware that the values in the dV tree diagram/s (which you probably use) don´t have to reflect the reality at the time of your launch.

depending on the time the launch windows (and therefore the constellation of the planets on their orbital path) the dV needed can differ to an extreme.

Which is why I recommend the Launch Window Planner in order to get precise values for the time of launch

(as well as detailed data for the angle and dV for your burn)

[FancyMouse]

On launchpad I believe both mods show current, i.e. atmospheric, dV because it has access to current atm pressure.

[Streetwind]

Of course KER shows different stats.

On the launchpad you are in an atmosphere (KER in flight values are always based on the current surrounding air pressure),

whereas the dV values you have displayed in the VAB are the values for a vacuum.

On launchpad I believe both mods show current, i.e. atmospheric, dV because it has access to current atm pressure.

Except that's not what he's asking about.

If you actually look at the screenshots you'll find that for example MechJeb shows both atmospheric and vacuum dV in the VAB, and it shows both atmospheric and vacuum dV on the launchpad. But they are not the same values between the situations. KER's vacuum values are again different from the two different MechJeb vacuum values, and KER's atmospheric values are also different from the two different MechJeb atmospheric values. That's the thing that confuses him.

[DCWarHound]

Well looking at your craft,here are a few improvements you can make.

1.Add fuel ducks to go from the radial tanks to the center tank.This allows you to carry more fuel on the radial tanks and your center tank will be full when you reach the upper atmosphere (you only need 1.2 surface TWR to get your rocket moving,just add a few small boosters to get it off the landing pad.

2.make the rover a 4 wheeler and ditch the default body so it can fit in the service bay,your current rover should be able to fit in the larger service bay by raising the wheels a bit.

3.You don't need an antenna and solar panels on the stage below the payload,you can disengage the shield once you're above 40km or so.

4.get retractable solar panels so you can retract them when entering duna's atmosphere

5.You don't need payload shielding if you properly aerobreak on duna's upper atmosphere (parts might get hot though but anything that can survive 1000 temp should be save,anything lower or that can snap off should go in the service bay.

6.Not sure if it's my imagination but i don't see any parachutes on the payload.

7.You should switch the "Reliants" out for "Swivels",the may be heavier but they have a better ISP in a vacuum.

[Godot]

Well looking at your craft,here are a few improvements you can make.

1.Add fuel ducks to go from the radial tanks to the center tank.This allows you to carry more fuel on the radial tanks and your center tank will be full when you reach the upper atmosphere (you only need 1.2 surface TWR to get your rocket moving,just add a few small boosters to get it off the landing pad.

...

Which he actually does according to the staging sequence and dV stats.

Asparagus staging ... he has the 4 boosters in 2 pairs ... the first pair (in staging sequence) connects its fuel line to the second pair (which is the fuel line that can be seen)

and the second pair connects to the middle rocket (for which the fuel ducts obviously are not seen due to perspective).

The dV stats for Stage 7 and 6 fit with this assumption

[DCWarHound]

Which he actually does according to the staging sequence and dV stats.

Asparagus staging ... he has the 4 boosters in 2 pairs ... the first pair (in staging sequence) connects its fuel line to the second pair (which is the fuel line that can be seen)

and the second pair connects to the middle rocket (for which the fuel ducts obviously are not seen due to perspective).

The dV stats for Stage 7 and 6 fit with this assumption

Sorry,didn't notice.

[GoSlash27]

Taiber,

I follow the same philosophy that you do, except I don't use KER.

Instead, I mathematically model each engine in a scenario simultaneously, exactly how much fuel and how many engines it needs, and then determine which configuration is the most efficient. This eliminates all trial- and- error in the VAB.

I have put together a spreadsheet for this purpose, so I can design an entire vehicle wherever I happen to be.

Best,

-Slashy

[Taiber]

Thanx guys for sharing your thoughts. I think that the difference between the dV values is twisting some force on lunchpad which i cannot describe. Something like gravitational force which is not in VAB pulling the rocket down from lunch stability enhancers and twisting the calculations or some atmo pressure ..However i managed to land on Ike ! Needed little bit more fuel and changed the spider engines to twictches and done it with 3550 "lunchpad" vacuum dV.

[wumpus]

One thing that seems to be forgotten by the kerbal community is that in general it is best to have roughly equal delta-v for each stage. This isn't always exactly optimal, but it is almost always a better place to start. If you still need "more boosters", I look to adding to either the final stage or doubling the tanks on the first asparagus stage.