Spending way too much fuel to get to Duna - help

[jfjohnny5]

It's my first crack at sending anything on an interplanetary mission. I decided to send a little unmanned probe - something light to make as efficient a trip as I could muster. I used this Delta V Map to plan the trip. It indicates I'm looking at around 6100 m/s total delta v for Kerbin to Duna orbit. I built and designed the rocket with the help of Kerbal Engineer Redux. I gave the rocket about 500 extra delta v as a buffer. I then used this orbital transfer calculator to plan the departure.

I used Kerbal Alarm Clock to make sure I didn't miss the launch window. I put the rocket into a 100 km parking orbit and burned at the ejection angle. I got a close approach, but no encounter. Halfway there I did a correction burn to adjust for orbital eccentricity. The closest encounter I could muster was something like 800,000 m. I started to burn retrograde upon entering Duna's influence at my periapsis to establish a capture orbit. At which point I ran out of fuel. I was short by almost 1000 dv.

I built another rocket with even more dv in the final stage and waited for my next launch window. It went almost exactly like the last time. I managed to get a stable Duna orbit this time, but it was only because I used that extra fuel I had packed. So two attempts both ended up costing me around 7500 delta v instead of the 6100 predicted.

Any ideas on where I could look to improve? What errors might I be making?

Also I just want to say, this experience, like so many others in this game, really make me appreciate how challenging space travel is. I love it.

[ComradeGoat]

Assuming you are at an ascending or descending node, you should be able to adjust all three axes to get the encounter distance down to zero. Do this as early as possible.

Once you are in system, don't burn retrograde to slow down; burn *sideways* to move your periapsis to about 12,000-14,000 metres. Any fuel you use to *slow down* on entering Duna's SOI is wasted; the atmosphere will do that for you. It's usually much cheaper to make sure you hit the atmosphere, as long as you get a decent intercept.

Good luck!

[Tw1]

Are you using the LV-N as your engine?

Also, aerobreaking will change the way you fly forever!

Edited May 13, 2013 by Tw1

[jfjohnny5]

Yeah I was thinking of trying to aerobrake the next time I try it. But the two delta v maps here and here I've seen do not account for aerobraking in either one. So, at least mathematically, it's possible to get a much more efficient trip than I'm getting (even without aerobraking). I'm trying to figure out what I'm missing.

[Tw1]

Use what you've made, and aerobrake with it.

It you're going for minimalism, take out most of the DV you used for the capture burn, leaving enough for maneuvering, and you'll get there. Aerobreaking is basically free, except for the maneuvers used to set it up.

With a parachute or three, you should even be able to land something, should you so wish.

[Guest]

Duna's orbit is not a circle so phase angles for launch can vary by as much as 36-54 degrees according to the time of the year. Getting your launch window right has a huge effect on how much fuel is needed. Read this tutorial and use this table to find accurate launch times for Duna. Hope this helps.

[Quorthon]

Duna's orbit is not a circle so phase angles for launch can vary by as much as 36-54 degrees according to the time of the year. Getting your launch window right has a huge effect on how much fuel is needed. Read this tutorial and use this table to find accurate launch times for Duna. Hope this helps.

Kerbal Alarm Clock already uses this table in its latest version to predict the transfer windows.

It's more acurate than the circular, no inclination model.

I like this mod a lot because it changed how I play KSP immensely.

[sadron]

Assuming you are at an ascending or descending node, you should be able to adjust all three axes to get the encounter distance down to zero. Do this as early as possible.

Once you are in system, don't burn retrograde to slow down; burn *sideways* to move your periapsis to about 12,000-14,000 metres. Any fuel you use to *slow down* on entering Duna's SOI is wasted; the atmosphere will do that for you. It's usually much cheaper to make sure you hit the atmosphere, as long as you get a decent intercept.

Good luck!

Also you should use maneuver nodes to help show you where to burn rather than blindly choosing an area sideways to Retrograde, so to speak.

[jfjohnny5]

Duna's orbit is not a circle so phase angles for launch can vary by as much as 36-54 degrees according to the time of the year. Getting your launch window right has a huge effect on how much fuel is needed. Read this tutorial and use this table to find accurate launch times for Duna. Hope this helps.

Does it use the table? I just downloaded the latest version to make sure. From game start, the first window it gave me was for a phase angle of 44 degrees. But the table shows the first window is at 36 degrees. I'm not sure Kerbal Alarm Clock is using the table...

[kookoo_gr]

the way i go to the other planets is the following.

1. i make gradual burns of 30secs in each Kerbin orbit in order to increase the apoapis to leave the kerbal system. This method is time consuming but it uses less than a quarter of a Rockomax X200-32 Fuel Tank. If i want to go to the inner planets i plan for my apoapis to be at the light side of the planet, for the outer planets the apoapis is at the dark side of kerbin. If during a planning a burn i get an encounter with Mun or Minmus i wait for one orbit.

2. Once outside the Kerbin system at the ascending or descending nodes i match the plane with the target parent planet

3. After that i try to get an encounter with the planet with the least amount of burning time at various points at the ship's orbit

4. Once i have an encounter i fiddle with the periapisi/apoapsis (blue circles), inclination (triangle icons) at the maneuvering node in order to get a planet periapsis within a planets atmosphere or a height of roughly 10-20 km if there is no atmosphere

5. I execute the burn at half the time before i encounter the node (i do this for every burn i make), eg for a 10 min burn i start at T -5 mins

6. Once the burn is complete with the use of RCS i try to correct my periapsis, the further away you use it the more distance you shave off.

7. Once at a planet's influence i save (hit F5) and check my periapsis to see if it's correct, if not i plan another node and correct the periapsis/inclination using RCS or the main engine if the burn is over 100 m/s (depending from your ship's design). At this step, and when correcting the periapsis at solar orbit, i don't wait for the ship to reach the node i execute the correction, this is very helpful especially when you enter a planet's influence and you don't have enough time inside its influence, create the node at say 5-10 mis away and once you have the altitude/inclination you want burn, you can get a polar orbit with very little fuel consumption at this step. At solar orbits i create a node 1 hour away from my ship so i can have all the time i need to make my corrections.

8. If at a planet with an atmosphere i aerobrake in order to create an orbit, if you don't succeed at first reload and try another altitude

9. I circularize my orbit with burns at the apoapsis/periapsis

This is my basic interplanetary design, it contains a Rockomax X200-32 Fuel Tank, a FL-R1 RCS Fuel Tank and a LV-N Atomic Rocket Engine, if i need to get to further planets i add extra tanks at the Rockomax X200-32 Fuel Tank but i get to Duna with almost half a tank of Rockomax X200-32 Fuel Tank (depending the lander weight of course). This is a design that reached Tylo i aerobraked at Jool created an encounter with Tylo and as you can see i just had enough fuel to deorbit and reach a speed of 1500-ish m/s and still had enough fuel when i separated in order to land that sucker on top of that stage.

Best of luck at your interplanetary journeys

P.S there are some youtube videos about fuel efficiency, look them up

[Jason Patterson]

Yeah I was thinking of trying to aerobrake the next time I try it. But the two delta v maps here and here I've seen do not account for aerobraking in either one. So, at least mathematically, it's possible to get a much more efficient trip than I'm getting (even without aerobraking). I'm trying to figure out what I'm missing.

You really should practice aerobraking; it's a vital skill for interplanetary returns. That said, if you do it carefully, you should only have a few hundred m/s of excess velocity upon reaching Duna. It's hard to say where you're losing delta-v though, without monitoring your flight.

1. When you launch, how much delta-v are you using to get to LKO? If it's more than 4700 m/s, you need to work on your ascent. A skilled pilot can make it to orbit in a well designed ship for about 4400 m/s, but between 4500 and 4600 m/s is a much more realistic goal for most vessels. (~4600 m/s used)

2. Once you're in your parking orbit (which should be as low as possible), set up a maneuver node to plan your ejection burn to Duna. This will allow you to correct for any inclination differences and fix any timing mistakes in advance. It also gives you a good idea for how far in advance you'll need to start your burn to get there when you want to.

3. Make your burn in map view as much as possible. Sometimes you get to the target for less delta-v than expected, some times it takes a bit more. I usually delete the maneuver node 10-15 seconds before it finishes so that I can see what my orbit is actually doing without the node's interference. As you get close, throttle back and watch the close approach indicators. Stop burning when the indicator tells you you're starting to move farther from the target instead of closer (shocking, huh?) (~1100 m/s)

4. Timewarp until you're out of Kerbin's SOI, then set up a maneuver node to fix your encounter, getting your periapsis as low as possible (preferably under 250km at this point, but anything under 2Mm will do.) (~50 m/s)

5. Timewarp again until you're ~10 days out from Duna, then finally get your Duna encounter just right (use a maneuver node to plan it if you're uncertain in which direction to burn based on the map view and try to get your periapsis to ~70km if you're not aerobraking.) (~10 m/s)

6. Once you enter Duna's SOI, burn at the retrograde direction's heading, but with zero pitch, until your periapsis is where you want it. It should already be very close, but it will shift slightly upon changing SOI. (~10 m/s)

7. Timewarp until shortly before periapsis, then burn retrograde to capture. (~300 m/s) (Alternatively, set your periapsis to ~12km and enjoy the show as you coast through the atmosphere. Be sure to burn prograde at your apoapsis so that you don't make a disastrous second trip through the atmosphere.)

Total delta-v - 6200 m/s, give or take

[jfjohnny5]

My launches are pretty regularly around 4500-4600 and I setup for a 100 km parking orbit.

It's step #7 where I see the deviation. My delta v for capture the last two trips has been closer to 1000-1500 m/s instead of 300. So I'm moving waaaaaay too fast. A lot of information linked in the above tutorial looks really good and makes a lot of sense. I'm going to try and dig in to that later.

[rhoark]

Flight Engineer gives you the vessel's current remaining dV potential, so verify you're in sync with the dV map at parking orbit. (dV to orbit is highly variable)

[jfjohnny5]

Flight Engineer gives you the vessel's current remaining dV potential, so verify you're in sync with the dV map at parking orbit. (dV to orbit is highly variable)

Yeah Flight Engineer what I use to gauge everything. My current lifter is an asparagus staged design. I use it alone to achieve LKO at 100 km. Then the probe is a nuclear powered stage that starts with a nice even 2500 m/s of dv. That makes it nice and easy to see "what's left in the tank" when I get to a Duna orbit.

My lifter has a potential dv of 5000 m/s in atmosphere and 5800 m/s in vacuum. So it's actually overkill and I always have fuel left. But I don't use it for my Duna injection burn. I only use the nuclear engine with it's 2500 m/s dv between Kerbin orbit and Duna orbit.

Just thought of one thing that might make an effect. Should I approach Duna from it's Prograde or Retrograde side?

Edited May 13, 2013 by jfjohnny5

[Francesco]

Duna is the easiest planet to reach, people say it's Eve but that doesn't account for orbital inclination change: for Duna it's only 0.1°.

with some careful piloting, you can get there for something like 5500-5600 m/s: 1050-1100 for the ejection burn, plus pennies for a correction burn.

Jason already gave you thorough advice, I would add this:

you can get a nice equtorial orbit when arriving on Duna with a single, stress-free correction burn, by doing that at the AN or DN node (shown by setting Duna as target): 99% of the time, that would be outside Kerbin's sphere of influence, as you're orbiting the Sun.

at this time, you can also roughly set your desired periapsis for aerobraking: usually in the 10-12km range.

lower than that, and you'll probably go straight to landing

I say roughly because if you're still very far from Duna, even a little puff of RCS can change your periapsis height noticeably, so you may not be able to be sufficiently precise right away.

therefore, another very small correction burn may be necessary a little before you encounter Duna's SOI or just after you've entered it: this is to fine-tune your periapsis as mentioned above, and set it to the desired value.

in general, use the maneuver nodes: take your time to play around with them once you're in LKO, move the handle slightly forward and back and see how your "closest approach" value changes.

Edited May 13, 2013 by Francesco