What don't I understand about dV?

[Loren Pechtel]

21 hours ago, Nich said:

Depend on tech level and spread sheeting skill.  Minimus with safety margins is around 6000 dv.  3800 Kerbin orbit, 250 inclination change, 1000 transfer, 150 capture, 300 landing, 150 taking off, 150 return. For a total of 5800.  Yes it can be done for a lot less but margins are always nice.

With Mechjeb at least I get that inclination change for far less.  I've had basically zero luck trying to get Mechjeb to launch into the right plane so these days I just launch to an equatorial orbit and then do a Hohmann transfer to Minmus.  With the planes wrong this of course will not actually work but then I do a refine closest approach--it usually schedules something like halfway to Minmus and costs far less than 250 m/s.

[Nich]

5 minutes ago, Loren Pechtel said:

With Mechjeb at least I get that inclination change for far less.  I've had basically zero luck trying to get Mechjeb to launch into the right plane so these days I just launch to an equatorial orbit and then do a Hohmann transfer to Minmus.  With the planes wrong this of course will not actually work but then I do a refine closest approach--it usually schedules something like halfway to Minmus and costs far less than 250 m/s.

Good for you I can use hyperedit to get there for 0 dv

Sorry I didn't post this sooner but I have another thread where I am trying to mathematically derive max dv for a configuration of stages if I ever get time to work on it it will be interesting.  But a really important thing to look at is DV per ton ( except first stage) for most engines you DV per ton goes down after about 2000 DV. Do not include payload as that is fixed. You want to find the max DV for the added mass of engines fuel and decouplers 

[wumpus]

Are the delta-vs of each stage even vaguely similar?  Typically, the most efficient sizes for you stages should be fairly close to one another (in delta-v, as mentioned already this will be exponentially increasing in mass), and that is one of the best places to start (another starting point is to double the mass of each stage).  These are pretty basic tricks I've seen elsewhere that rarely get mentioned in kerbal sites.

If your "final stage" is too massive, it shouldn't be that hard to break it into more stages.  Note that adding another stage need not require an additional engine, simply using drop tanks can let you leave orbit on an intercept course, do a capture burn, de-orbit to land, and then drop tanks so you land with minimal mass.  Adding drop tanks does fight with the 1.0+ (or FAR) aero model a bit (fairing might help, but can often be ignored, especially until that bug is fixed) but the overall gains are likely worth it.

[OhioBob]

1 hour ago, wumpus said:

Are the delta-vs of each stage even vaguely similar?  Typically, the most efficient sizes for you stages should be fairly close to one another (in delta-v, as mentioned already this will be exponentially increasing in mass), and that is one of the best places to start (another starting point is to double the mass of each stage).  These are pretty basic tricks I've seen elsewhere that rarely get mentioned in kerbal sites.

The general rules of thumb for optimizing staging are,

1. Stages with higher I_sp should be above stages with lower I_sp.
2. More Δv should be provided by the stages with the higher I_sp.
3. Each succeeding stage should be smaller than its predecessor.
4. Similar stages should provide the same Δv.

[Plusck]

Using Kerbal Engineer, I have in the past found that it doesn't quite calculate dv correctly with drop tanks, especially if you have a complicated staging process. This is probably because its stage list seems to be dependent on the amount of time that each stage will last, and it gets confused when it doesn't have any burn time that it can apply to a stage. Adding tiny rocket engines like the Ant to drop tanks just to prod KER into recalculating might help.

[KocLobster]

The mobile software on these forums is glitchy as all hell

On 3/21/2016 at 2:16 PM, OhioBob said:

Landing on Minmus requires the least Δv of any destination in the game.  I one way trip to the surface of Minmus shouldn't take more than 5,000 m/s.  That should be well within the reach of chemical rockets.

Very true. Not with the rocket I'm talking about though.

 

On 3/22/2016 at 10:34 PM, 5thHorseman said:

The accepted numbers for Minmus are 180 to low orbit from the flats and 160 from low orbit to to a trajectory that will intercept Kerbin's atmosphere. That mans it should take about 340m/s. In my experience this is pretty right on. I'd be surprised to see someone do it for 300, surprised enough that I'd want proof

It could (and probably is) a simple case that Nich rounded down while you round up. I mean, in the whole vast scheme of things 300 m/s isn't that much.

I did it the other day for about 210dV somehow

[OhioBob]

33 minutes ago, KocLobster said:

Very true. Not with the rocket I'm talking about though.

You mentioned a payload of ~60 tons.  If that's what you've got, then it's possible to get 5000+ m/s using chemical rockets.  A couple days ago I did a test where I slapped together a launcher using three Mammoths (central core + 2 strap-ons) and a Rhino second stage.  This could deliver 5300 m/s with a 62t dummy payload.  Total launch mass was 606 t.  That's one of the biggest rocket that I've ever built and launched.  Of course, if the payload is much bigger than that, then I too would consider multiple launches and/or in orbit refueling.

The problem with using nukes is that they have a low thrust.  If you have a really big payload then you'll either have a very low acceleration, or you'll need a lot of nukes.
 

Quote

I did it the other day for about 210dV somehow

That's a pretty neat trick considering Minmus' escape velocity is 243 m/s.

[KocLobster]

Just now, OhioBob said:

You mentioned a payload of ~60 tons.  If that's what you've got, then it's possible to get 5000+ m/s using chemical rockets.  A couple days ago I did a test where I slapped together a launcher using three Mammoths (central core + 2 strap-ons) and a Rhino second stage.  This could deliver 5300 m/s with a 62t dummy payload.  Total launch mass was 606 t.  That's one of the biggest rocket that I've ever built and launched.  Of course, if the payload is much bigger than that, then I too would consider multiple launches and/or in orbit refueling.

The problem with using nukes is that they have a low thrust.  If you have a really big payload then you'll either have a very low acceleration, or you'll need a lot of nukes.
 

That's a pretty neat trick considering Minmus' escape velocity is 243 m/s.

That was definitely a typo sorry, I'm on my phone.

I'm playing career and haven't unlocked that tier of rockets yet.