DeltaV

[BgDestroy]

Where i find normal Hohmann delta V map ?

 

Example this map is bad: 

Example travel to Duna. This map say i need from Sun Orbit to eliptic orbit Duna only 480 dV m/s (130 intercept, 240 eliptic orbit). I need 840 dV only intercept !!! 

Where i find Hofmann delta V map ? 

[Superfluous J]

That map assumes you burn all the way from LKO to Duna intercept in a single burn. You are likely burning out of Kerbin's SOI in one burn, then to Duna in a second burn. That is much more expensive.

[GoSlash27]

^ This. If you do the Duna intercept burn in low kerbin orbit, the map is correct.

Best,
-Slashy

[Talos]

In all of my attempts to make an interplanetary transfer, I almost always have to do a second burn to correct for distance, since my initial burn in LKO will usually put me out way too far. I've learned with bigger ships using nuke engines that you can space out your burn once you pass prograde, rotate around back to your maneuver, then burn again, repeating until your burn is complete.

[pincushionman]

The map also assumes you've got a favorable transfer window and an efficient intercept.

[pandaman]

Stock mission planning tools like Dv, transfer planner and alarm clock anyone?  Maybe we are in with a fighting chance now @TriggerAu is on staff.

[Wjolcz]

What @5thHorseman said. It's better to perform one burn instead of few. Simply add all the numbers appearing from your body of origin to your target. It's simple. Also mind that you might want to do a few correction burns on your way so always keep some more fuel just in case.

BTW it would be nice to have this in KSPedia. With a simple note or a tutorial showing how to use the map and how a proper burn should look like.

Edited August 14, 2016 by Veeltch

[mhoram]

You can find the map you are looking for here. It is based on the assumption that you first leave Kerbins SOI before starting the Hohmann Transfer to the other planets.

[Brainlord Mesomorph]

I use that chart only for a rough idea of fuel requirements during ship design. 

for actual mission planning I go to this webpage https://alexmoon.github.io/ksp/ and get a departure date, ejection angle and exact dV. 

(look at the tutorial in my sig)

[soulsource]

Maybe one should also mention the scientific term for the fact that it's (quite a bit) more efficient to directly burn for the transfer from low orbit: Oberth Effect.

[Godot]

21 hours ago, Brainlord Mesomorph said:

I use that chart only for a rough idea of fuel requirements during ship design. 

for actual mission planning I go to this webpage https://alexmoon.github.io/ksp/ and get a departure date, ejection angle and exact dV. 

(look at the tutorial in my sig)

 

This.

dV requirements to other planets, especially to the outer planets, can show such a large variation (example: Jool, where it can vary between 5km and 18km, depending on when you start) that a single dV value doesn't make sense. Instead the mission should be planned with the dV requirements for the actual starting time (and possible return windows, if manned) in mind

[Sereneti]

16 hours ago, soulsource said:

Maybe one should also mention the scientific term for the fact that it's (quite a bit) more efficient to directly burn for the transfer from low orbit: Oberth Effect.

Not alway.

its more expensiv to go to the inner planets from Lko then from a higher Sun orbit....

 

[Gaarst]

17 hours ago, soulsource said:

Maybe one should also mention the scientific term for the fact that it's (quite a bit) more efficient to directly burn for the transfer from low orbit: Oberth Effect.

Bi-elliptic transfers would like to have a word with you. In many situations when going from a low orbit to a higher one they are more efficient. Usually not by much, but a small bit of nitpicking never hurt anyone.

[foamyesque]

On 2016-08-18 at 3:43 AM, Gaarst said:

Bi-elliptic transfers would like to have a word with you. In many situations when going from a low orbit to a higher one they are more efficient. Usually not by much, but a small bit of nitpicking never hurt anyone.

 

It's still more efficient to burn directly into the first part of the bi-elliptic transfer

[Red Iron Crown]

On 8/18/2016 at 7:13 AM, Gaarst said:

Bi-elliptic transfers would like to have a word with you. In many situations when going from a low orbit to a higher one they are more efficient. Usually not by much, but a small bit of nitpicking never hurt anyone.

Well, if you want to get nitpicky....

Bielliptic transfers can improve efficiency in both directions, low to high and high to low.

Further, they are only more efficient than Hohmann when the ratio of SMAs between origin and destination is greater than ~11.9 (assuming coplanar orbits). No stock destination body meets this requirement when transferring from Kerbin, so it is not a good idea to use bielliptics for transfers from there.

 

[foamyesque]

6 hours ago, Red Iron Crown said:

Further, they are only more efficient than Hohmann when the ratio of SMAs between origin and destination is greater than ~11.9 (assuming coplanar orbits). No stock destination body meets this requirement when transferring from Kerbin, so it is not a good idea to use bielliptics for transfers from there.

 

I believe low Kerbol orbit does

[Red Iron Crown]

5 hours ago, foamyesque said:

I believe low Kerbol orbit does

Haha, fair enough. Since we're being nitpicky.  

[GoSlash27]

As long as we're all going nit- picky on what should've been a simple question, do you have a few minutes to talk about our Lord and Savior the Gate orbit?

Trips to Duna and Eve are much cheaper if you refuel in low Munar orbit and depart directly from there.

LKO isn't the ideal starting point for most interplanetary trips.

 

Best,
-Slashy

 

[Red Iron Crown]

2 minutes ago, GoSlash27 said:

As long as we're all going nit- picky on what should've been a simple question, do you have a few minutes to talk about our Lord and Savior the Gate orbit?

Trips to Duna and Eve are much cheaper if you refuel in low Munar orbit and depart directly from there.

LKO isn't the ideal starting point for most interplanetary trips.

Last statement is debateable. I would imagine most IP trips are single launch, in which case the costlier launch to the gate orbit outweighs the savings from the IP burn. Cheaper to burn directly from LKO.

Agree though that if the craft is being fueled in orbit then a gate orbit departure is better delta-v-wise for the IP craft. 

[GoSlash27]

11 minutes ago, Red Iron Crown said:

Last statement is debateable. I would imagine most IP trips are single launch, in which case the costlier launch to the gate orbit outweighs the savings from the IP burn. Cheaper to burn directly from LKO.

Agree though that if the craft is being fueled in orbit then a gate orbit departure is better delta-v-wise for the IP craft. 

RIC,

 If you set up an infrastructure so that you have a refueling station in LMO, trips to every destination in the system are cheaper than direct burns from LKO.
 Closer planets get a single burn from LMO, while farther destinations get 2 burns; first to gate periapsis and second to transfer.

 This makes the IP vehicle much smaller and more efficient, reducing costs (both in DV and dollars).

 I just pointed this out to counter the notion that direct transfers from LKO are always cheapest because Oberth. They are not. Oberth doesn't always win out over the gravity well and it's useful to know when it does and when it doesn't.

You could be right though; most IP trips probably are single launch. My mileage varies because I never do single launch IP trips.

Best,
-Slashy

Edited August 20, 2016 by GoSlash27

[Red Iron Crown]

1 minute ago, GoSlash27 said:

If you set up an infrastructure so that you have a refueling station in LMO, trips to every destination in the system are cheaper than direct burns from LKO.
 Closer planets get a single burn from LMO, while farther destinations get 2 burns; first to gate periapsis and second to transfer.

 This makes the IP vehicle much smaller and more efficient, reducing costs (both in DV and dollars).

Cheaper fund-wise to refuel the IP vessel in LKO with a tanker from Minmus (I'm assuming the M in LMO is Minmus and not Mun as that's more efficient). Even if it uses more fuel, the fuel from Minmus is free while the fuel for ascent (and a subsequent transfer to Minmus or a gate orbit, if used) costs. The best strategy I found when I used Kerbin-system ISRU was to use the IP craft's fuel to help with insertion into LKO, reducing the lifter cost. Then bring a tanker/tug from Minmus to LKO, dock, burn to Minmus Ap, top off fuel and undock, then complete the IP burn. The tanker/tug would then wait for a Minmus encounter for reuse. This strategy also has the benefit of not needing Minmus to be in a good position for the transfer. 

Though, thinking on it a bit further, it might have been worthwhile to raise Ap from LKO to gate orbit altitude, then burn the rest of the transfer there. I'd have to run some numbers, I guess.

1 minute ago, GoSlash27 said:

 I just pointed this out to counter the notion that direct transfers from LKO are always cheapest because Oberth. They are not. Oberth doesn't always win out over the gravity well and it's useful to know when it does and when it doesn't.

Agreed, Oberth is useful but it isn't a magical end-all be-all.  

[foamyesque]

They're the cheapest deltaV-wise. If we're going to start bringing in ISRU to this, or gravity assists, or other assorted shenanigans, it becomes a lot more complicated.

 

I prefer Minmus as a staging area. It's much easier to land and depart from, and you can fix the inclination problem by ejecting to a Kerbin orbit at Minmus's altitude at the node. At that point you're already 99% of the way to being ejected and can perform a straightforward pair of burns to, at the correct ejection angle, drop your periapsis back down to LKO and then, at that periapsis, boot yourself the rest of the way out. Or you can even transfer directly, if you're headed somewhere nearby like Eve or Duna.

 

Edited August 20, 2016 by foamyesque

[magnemoe]

9 hours ago, GoSlash27 said:

RIC,

 If you set up an infrastructure so that you have a refueling station in LMO, trips to every destination in the system are cheaper than direct burns from LKO.
 Closer planets get a single burn from LMO, while farther destinations get 2 burns; first to gate periapsis and second to transfer.

 This makes the IP vehicle much smaller and more efficient, reducing costs (both in DV and dollars).

 I just pointed this out to counter the notion that direct transfers from LKO are always cheapest because Oberth. They are not. Oberth doesn't always win out over the gravity well and it's useful to know when it does and when it doesn't.

You could be right though; most IP trips probably are single launch. My mileage varies because I never do single launch IP trips.

Best,
-Slashy

You also have the advanced option to drop your Pe down to 100 at the place and time of the burn, this have you moving at over 3 km/s at the start of burn, this has two benefits, first you save 7-900 m/s, second you get more out of Oberth and higher starting speed, downside is that this take time to set up, however its very nice then launching heavy stuff towards Moho or Eeloo. 
 

[Superfluous J]

21 minutes ago, magnemoe said:

You also have the advanced option to drop your Pe down to 100 at the place and time of the burn, this have you moving at over 3 km/s at the start of burn, this has two benefits, first you save 7-900 m/s, second you get more out of Oberth and higher starting spe

Actually those two things are one and the same. You save the dV because of the Oberth Effect.

[Red Iron Crown]

14 minutes ago, 5thHorseman said:

Actually those two things are one and the same. You save the dV because of the Oberth Effect.

I took that to mean 7-900 m/s over starting in LKO (i.e. your speed at Pe will be about that much more than LKO speed); those savings are not from Oberth but from the initial orbit having greater energy.