Duna

[Duxwing]

Dear Forum,

While planning my mission to Duna, I had a thought: what is the most Delta-V efficient flight path for escaping from Kerbin's sphere of influence to a solar orbit? I usually ascend to a high apoapsis and LKO periapsis and then burn prograde to escape velocity along the tangent of my orbit relative to the sun, but I wonder if simply taking off while facing in the direction of my intended solar orbit and burning straight upward--perhaps slingshotting off Mun in my terminal phase--would be more efficient. My present goal is a manned landing on Duna and return with all crew aboard, so conserving Delta-V is of great concern to me. The ship with which I hope to achieve this mission is as follows:

Service Module:

• 1 Mk1-2 Command Pod (Stock 3 man pod)
  
• 1 1600L Main Tank (stacked underneath the Command Pod)
  
• 4 1,200L Drop Tanks (mounted radially to the Support Module's Main Tank)
  
• 4 400L Drop Tanks (stacked on top of the 4 1,200L Drop Tanks)
  
• 1 Rockmax Brand Decoupler
  

Dry Mass (all drop tanks attached): 12 Tons

Dry Mass (all drop tanks jetisonned): 7 Tons

Fuel Mass: 36 Tons

Fuel Volume: 8,000L

Lander:

• 1 Mk2 Lander Can
  
• 1 1,600 L Main Tank by Rockomax (fixed to Lander Can)
  
• 1 Bearcat Trinozzle Engine with a thrust of 1,200kN and an ISP of 350 by Tyberdine Aerospace (fixed to Lander's Main Tank)
  

Dry Mass (all RCS tanks full): 8 Tons

Fuel Mass: 12 Tons

Fuel Volume: 1,600L

Combined Dry Mass (all drop tanks attached): 20 Tons

Combined Dry Mass (all drop tanks jettisonned): 15 Tons

Combined Wet Mass: 70 Tons

Lifter:

• 5 KW Rocketry Titan T-1's in asparagus staging
  
• 5 12,000L KW Rocketry SC-4 LFT's, each fixed to a Titan T-1
  

Miscellaneous:

• ~1 Ton of RCS, Docking Ports, Struts, Legs, Parachutes, ASAS, Maneuvering Thrusters etc.
  

Although, as you may have already divined, the support module of my ship does not have an engine, the powerful Bearcat Trinozzle fits underneath my lander's legs and thereby renders such an engine redundant. My total tonnage may be off by one (I rounded up to account for the miscellania) but the scale of this project makes such an error trivial. As for my navigation: I do not use MechJeb, but I do use ASAS whenever possible and RCS whenever prudent.

Any help is appreciated, guys, so pitch in!

-Duxwing

[KasperVld]

I believe circularizing and then using the tangent as you planned is the most efficient way, but ofcourse you could gain some speed from a Mun slingshot, the question you need to ask yourself with that is if it's really worth the effort?

I cannot back my first claim up (I'm not an astronomer or mathematician) but it's due to the Oberth effect I believe

[Oggula]

1) I assume that going straight up would minimize atmospheric drag and the time spent in the deeper part of Kerbin's gravity well, reducing delta-v needed

2) However, you wouldn't get the 174 m/s boost from Kerbin's rotation going into orbit, and a slingshot around the Mun would be far less effective if approaching from below, perpendicular to its orbit

3) The forum-famous Oberth effect wouldn't make much of a difference

I'd stick to reaching orbit first, simply because its easier.

[Duxwing]

But wouldn't simply orienting your launch site,, pointing straight up, and smashing the gas--Mun slingshot notwithstanding--be simplest?

-Duxwing

[Francesco]

no, going straight up is never the best option.

the only thing you could do would be to go directly to kerbin ejection/duna injection without circularizing your orbit first, but the gains in terms of efficiency are small and such a maneuver requires a high level of accuracy.

[Duxwing]

I plan to go: Kerbin's Surface -> Solar Orbit -> Dunar Orbit -> Solar Orbit -> Kerbin SOI -> Kerbin's Surface. Only the injection into Solar Orbit would be accomplished by burning directly upward. The rest of my journey would proceed normally.

-Duxwing

[itsme86]

I plan to go: Kerbin's Surface -> Solar Orbit -> Dunar Orbit -> Solar Orbit -> Kerbin SOI -> Kerbin's Surface. Only the injection into Solar Orbit would be accomplished by burning directly upward. The rest of my journey would proceed normally.

-Duxwing

You'd want to make sure that the KSC is is currently facing the direction of Kerbin's trajectory (i.e. it's in the "front" of Kerbin in its orbit around Kerbol, kind of like the grill of your car I guess). This way your burn on your way out of Kerbin SOI is raising your Solar apoapsis.

[Anglave]

Personally I launch to a circular LKO (about 80km). Later, I depart from LKO directly to Duna Intercept in one maneuver.

It is theoretically possible to do a "single burn to Duna" from the launch pad to Duna aerocapture. I'm not sure if it's more efficient, but it's certainly more difficult.

You might be interested in this post. That's my craft, which I've flown manually to Duna's surface and back with two crew. It doesn't have a huge dV budget, but it's got plenty to make the round trip with a manual pilot. And the whole thing is only 181 parts, including cosmetic nose cones, and more struts than are strictly necessary.

[RoboRay]

Do not launch straight upwards to escape if efficiency is desirable... you're incurring gravity losses the entire time and you lose the rotational benefit of the planet. Do a gravity turn as normal and simply continue your prograde burn rather than circularizing if you want a direct launch to escape. You'll need to launch just before KSC reaches the proper ejection angle for the transfer orbit, however.

Also do not escape into solar orbit then try to set up a Duna transfer, as that's much less efficient than making your transfer burn in low-Kerbin orbit (or as part of your launch-to-escape burn).

It is theoretically possible to do a "single burn to Duna" from the launch pad to Duna aerocapture. I'm not sure if it's more efficient, but it's certainly more difficult.

Yes, this is the most efficient way to fly the mission, and it's also the most challenging to accomplish.

[Sof]

Always plot interplanetary missions from LKO. If you don't plan to orbit Kerbin at any stage (orbits you've transfered out from before completing one revolution count though) then you've wasted delta v.

The most efficient burns happen on the opposite side of Kerbin to your intended destination.

[RoboRay]

The most efficient burns happen on the opposite side of Kerbin to your intended destination.

To clarify...

The most efficient burns happen on the opposite side of Kerbin to where your intended destination will be when you reach it.