dv map, is it wrong?

[fommil]

I've known about the classic delta v map for a while, and I looked at it to plan a mission to Duna.

It took me about 1000dv to go from Kerbin to a Duna flyby, whereas this claims it is 130dv. Luckily I discovered the error and did it all with waypoints instead. It was also a lot more than 130dv to go from flyby to (wide) orbit.

Are the numbers wrong or am I reading it incorrectly?

[Gaarst]

You need 130 m/s of dV once you've escaped Kerbin. You need 950 m/s to escape Kerbin. To go to Duna you need to add the two: 950+130.

[fommil]

27 minutes ago, Gaarst said:

You need 130 m/s of dV once you've escaped Kerbin. You need 950 m/s to escape Kerbin. To go to Duna you need to add the two: 950+130.

It takes more like 1000dv to go to Duna once you've escaped Kerbin. It takes another 600dv more to turn a flyby into an orbit. What I see is more like what I see at https://alexmoon.github.io/ksp/#/Kerbin/1000/Duna/150/true/ballistic/false/8/143

[Spricigo]

 

Let alone the fact that it need to be approximated values (to account  for gravity losses, steering losses, bad timing...), yes you are reading it wrong as @Gaarst pointed out.

[fommil]

18 minutes ago, Spricigo said:

 

Let alone the fact that it need to be approximated values (to account  for gravity losses, steering losses, bad timing...), yes you are reading it wrong as @Gaarst pointed out.

I think I need somebody to show me a flight that takes 130dv from Kerbin escape to Duna flyby before I'm willing to believe that.

[Gaarst]

5 minutes ago, fommil said:

I think I need somebody to show me a flight that takes 130dv from Kerbin escape to Duna flyby before I'm willing to believe that.

There you go:

http://alexmoon.github.io/ksp/#/Kerbin/100/Duna/100/true/ballistic/false/1/1

1038 m/s ejection dV. Count 950 dV for escaping Kerbin, that makes 88 dV to Duna from Kerbin escape. You even have room for inclination changes.

Edited July 7, 2017 by Gaarst

[fommil]

1 minute ago, Gaarst said:

There you go:

http://alexmoon.github.io/ksp/#/Kerbin/100/Duna/100/true/ballistic/false/1/1

1038 m/s ejection dV. Count 950 dV for escaping Kerbin, that makes 88 dV to Duna. You even have room for inclination changes.

Hmm, I was breaking out of Kerbin and then plotting a rendezvous with Duna. I guess you have to use one of the cheating mods to be able to exploit the physics this much... is this the orbit multiplier one gets by being near a body?

I still don't see how it's possible to turn that flyby into an orbit in such a small dv.

[Kerbal101]

Bah, I just launch the exploration satellite with KER and "current stage dV" value written down before and after a lazy imprecise burn, exactly how much deltaV is needed. So next time the "real thing" should have at least that.

The deltav table has approach problems, there were several versions around, and even then no burn is "perfect". And there is not much exploration with the table.

[Gaarst]

14 minutes ago, fommil said:

Hmm, I was breaking out of Kerbin and then plotting a rendezvous with Duna. I guess you have to use one of the cheating mods to be able to exploit the physics this much... is this the orbit multiplier one gets by being near a body?

I still don't see how it's possible to turn that flyby into an orbit in such a small dv.

Launching for a flyby directly from orbit is a lot more efficient than waiting to escape Kerbin and getting a flyby once you're in solar orbit.
For this example from another thread, you save about 2000m/s of dV by directing transferring to a Venus flyby:

On 01/07/2017 at 1:02 AM, Gaarst said:

Thanks to the Earth's gravity well giving you tons of gravitational energy, a 3462 m/s burn in Earth orbit is equivalent to a ejection burn of about 3200 m/s in orbit plus a 2300 m/s burn once you've escaped Earth.

It's not exploiting physics, and you don't need cheat mods to do it: it's the standard way to do interplanetary transfers, the calculator to find launch windows uses this method as well as the dV map you linked.

 

Here's how to break down the transfer in detail:

• In Kerbin orbit:
  • In a single burn, burn 950 m/s to escape Kerbin, and
  • burn 130 m/s to get a Duna flyby
• At this point, you've expanded about 1100 m/s of dV and have a flyby of Duna (it may be a bit more expensive depending on how accurate your manoeuvre is).
• In Duna's SOI, at flyby periapsis:
  • In a single burn, burn 250 m/s to go from a Kerbin-Duna transfer orbit, to a simple Duna escape orbit, and
  • burn 360 m/s to lower your apoapsis to a circular LDO.
• At this point, you're in low Duna orbit and you've expanded about 1700 m/s of dV total from Kerbin orbit.

Edited July 7, 2017 by Gaarst

[MaverickSawyer]

Seeing as I use that exact chart extensively... you read it wrong.

The 130 m/s is the dV required to go from an elliptical orbit that stretches out to the edge of Duna's SoI to a Hohmann transfer to Kerbin at the ideal time. Even the most basic capture requires at LEAST 250 m/s, and usually ~20% more due to non-perfect approaches.

My rule of thumb has been to add up the entire dV required for a given phase of a mission and add 10% to it if there's an optimal window available and I'm sure I can swing that mission. For more complex missions, I'll typically add ~30-40% more dV to provide extra insurance against Jeb.

[Spricigo]

41 minutes ago, Kerbal101 said:

The deltav table has approach problems, there were several versions around, and even then no burn is "perfect". And there is not much exploration with the table.

IMHO moot points, getting a ballpark based on the table is as good as getting a ballpark based in an exploratory probe. Personally, I use a reference value found in some table for the exploratory probe.

[Weywot8]

5 hours ago, Kerbal101 said:

Bah, I just launch the exploration satellite with KER and "current stage dV" value written down before and after a lazy imprecise burn, exactly how much deltaV is needed. So next time the "real thing" should have at least that.

Alternatively, just let KSP do the calculations for you - map out your entire trajectory to Duna/target body using one of your relay sats around Kerbin.  Or better yet, have a tiny sat orbiting around Kerbin at your preferred typical orbit. Then just add launch and LKO insertion cost (2950-3400dV) and that's what you design the ship with + whatever % dV buffer you are comfortable with.

This only works because time pauses in the VAB & SPH. Also, planning the transfer orbit a day or two early doesn't change your dV requirements by much.

The chart works for most of the part but with highly eccentric target orbits, it can fail badly on both transfer and capture dV. 

Edit: An added advantage is that you can just copy the entire setup on your own ship after it launches by making your planning sat the target so it's nodes are visible for replication. So you can plan your trajectory to Duna launching directly from LKO. The numbers in the map for the transfer only work if you launch from LKO direct to Duna.  

Quote

Hmm, I was breaking out of Kerbin and then plotting a rendezvous with Duna. I guess you have to use one of the cheating mods to be able to exploit the physics this much... is this the orbit multiplier one gets by being near a body?

I still don't see how it's possible to turn that flyby into an orbit in such a small dv.

It's not cheating, it's the Oberth Effect -  the map is calculating how much extra dV you have to put in launching from LKO to achieve the correct hyperbolic excess velocity for the transfer, taking into account the added relative orbital velocity your ship gains when exiting Kerbin's SoI. 

It's an extra 250dV to convert a flyby into the minimum capture orbit with Pe just above atmosphere and Ap just below SoI. You do this by ensuring your flyby passes as close as possible to Duna and then burn retrograde when at this closest point. Again, Oberth effect.

 

Edited July 8, 2017 by Weywot8

[HebaruSan]

6 hours ago, fommil said:

It took me about 1000dv to go from Kerbin to a Duna flyby, whereas this claims it is 130dv. Luckily I discovered the error and did it all with waypoints instead. It was also a lot more than 130dv to go from flyby to (wide) orbit.

Did you launch during a Duna transfer window, or at some other random time?

[fommil]

5 hours ago, HebaruSan said:

Did you launch during a Duna transfer window, or at some other random time?

This was during the transfer window. I think my problem is that I broke free of Kerbin THEN transferred. By doing it in a oner, it seems more efficient... same principle as trying to get into a high orbit perhaps. I'll believe the capture dv when I see it, I just can't see how it could be so low.

Thanks all, I'll try again for my manned landing.

[Lelitu]

10 hours ago, fommil said:

This was during the transfer window. I think my problem is that I broke free of Kerbin THEN transferred. By doing it in a oner, it seems more efficient... same principle as trying to get into a high orbit perhaps. I'll believe the capture dv when I see it, I just can't see how it could be so low.

Thanks all, I'll try again for my manned landing.

Part of getting the very cheap capture burn is that the chart expects you to do the capture burn at the lowest safe periapsis to exploit the oberth effect. The chart also considers the delta v to capture, as just enough to go from a hyperbolic escape orbit to an eccentric orbit with apoapsis just inside the SOI, and periapsis just over the atmosphere. 

At Duna, you'll need to burn much more than this at the capture burn (most of the circularization burn), unless you're going to Ike, because Ike's massive SOI gets in the way, and you'll either slingshot, or collide with Ike in a very few orbits unless you get your apoapsis low enough

To do this cheaply, you'll want to make a *very* small adjustment burn shortly after leaving kerbin, to set your periapsis. Done early enough, you can move your periapsis at duna from just grazing the SOI to just missing the atmosphere for about 2 m/s.