Ship to Duna

[BlazerTheAmazer]

Does anyone know how to create a fuel efficient rocket to get to Duna, as i want to send a satellite to orbit the planet. Any suggestions? 

[monstah]

Thread moved to Gameplay Questions and Tutorials

[Stef Morojna]

18 minutes ago, BlazerTheAmazer said:

Does anyone know how to create a fuel efficient rocket to get to Duna, as i want to send a satellite to orbit the planet. Any suggestions? 

Search for tutorials on youtube, there are plenty

[DD_bwest]

you really dont need that much more than a mun shot.   i would read up on delta V and engine ISP.

over budget and build your  vessel to have 2500m/s delta v.  then all you have to do is get it in orbit.   the spark or the terrier would work from there,  dont forget solar panels and some batteries.

 

[Snark]

3 hours ago, DD_bwest said:

you really dont need that much more than a mun shot.   i would read up on delta V and engine ISP.

^ This.  If you pick a good launch window, ejecting from LKO to Duna only takes around 200 m/s more dV than going to the Mun.  And capturing at Duna is practically free, if you use aerobraking (whereas you'd need a couple of hundred m/s to capture to Mun orbit).  In other words... with a good transfer window and judicious use of aerobraking, the dV from LKO to low Duna orbit is practically identical to the dV to get from LKO to low Mun orbit.

And if you're landing, you can actually land on Duna for less dV than landing on the Mun.

(Coming back from Duna is considerably more dV than coming back from the Mun, though, so it matters whether you're talking about a one-way or a round trip.)

There are various useful transfer planners out there.  My personal favorite is http://ksp.olex.biz, but there are others.  I like it because it's simple to use, and provides nice graphical representation of how/when to burn and by how much.

[Aegolius13]

To get the most range (delta-v) out of a craft, it's usually best to start from the final stage and work your way back.  You generally get more efficiency by using the lightest engine you can put up for each stage, which means lower thrust-to-weight ratios, but in space you don't need that much TWR.  (Note the big exception here is nuclear engines, which are heavy but VERY efficient.  But nukes are usually overkill for sending a light payload to Duna.  Ions also play by some different rules). 

For a satellite, if the payload is light enough, you may be able to get a decent top package of delta-v from a couple Oscar-B tanks and an Ant.  However, the Ant is only very good for teeny tiny stages.  So if your satellite is on the larger size, you may want to skip the Ant and go with a top stage of a Spark and a few hundred units of fuel.   If you do have the Ant, make the Spark your next stage.  The next stage back could be a Terrier, and roughly twice as much fuel as on the Spark stage.  Depending on the delta-v requirements of a particular mission, you can keep building backwards all the way to enormous lower stages. 

But as other folks have mentioned, Duna is a pretty easy delta-v destination if you hit your transfer window right.  So all you probably need is a launch stage to get this thing high enough for the Terrier to be effective.  For the rocket described above, I might go with a Swivel and a couple radially mounted Thumpers with their thrust turned down to keep your starting TWR reasonable.  The Swivel is not that great a launch rocket, but options are fairly limited if you have something too small for a 2.5 core stage.  

Here's a quick rocket along the lines of the above, with a fairing to protect the satellite until it reaches space.  Update after putting it together -- I would add a bit more fuel over the Swivel, and bump the thrust on the Thumpers back up a bit -- since my Terrier stage does not have that much TWR it would be good to get it fairly close to orbit.  It might be worth it to add a reaction wheel to one of the top stages - forgot about it at the time.  

This thing can go WAY further than Duna, so you could trim it down further, but since this is pretty cheap to begin with, it's worthwhile to leave a decent range buffer in there. 

 

 

[Magzimum]

Work your way down this checklist:

1. Check the cheat sheet to see how much dV you need to get to Duna (depends if you want a flyby, orbit, landing, or even a return flight).
2. Install KER. It will give you the dV of your rocket.
3. Design the satellite (the payload).
4. Add a tank and an engine to that satellite.
5. Add a decoupler.
6. Add aq bigger tank and a bigger engine.
7. Keep adding more and bigger engines until your dV is more than you need according to the cheat sheet.

So... I basically completely agree with the construction guide of @Aegolius13 in the post above.

Edited January 3, 2017 by Magzimum

[Palaceviking]

One minor thing, solar panels will be slightly less efficient out at duna, but not much.

[BlazerTheAmazer]

20 hours ago, DD_bwest said:

you really dont need that much more than a mun shot.   i would read up on delta V and engine ISP.

over budget and build your  vessel to have 2500m/s delta v.  then all you have to do is get it in orbit.   the spark or the terrier would work from there,  dont forget solar panels and some batteries.

 

thank you very much! 

[BlazerTheAmazer]

19 hours ago, Aegolius13 said:

To get the most range (delta-v) out of a craft, it's usually best to start from the final stage and work your way back.  You generally get more efficiency by using the lightest engine you can put up for each stage, which means lower thrust-to-weight ratios, but in space you don't need that much TWR.  (Note the big exception here is nuclear engines, which are heavy but VERY efficient.  But nukes are usually overkill for sending a light payload to Duna.  Ions also play by some different rules). 

For a satellite, if the payload is light enough, you may be able to get a decent top package of delta-v from a couple Oscar-B tanks and an Ant.  However, the Ant is only very good for teeny tiny stages.  So if your satellite is on the larger size, you may want to skip the Ant and go with a top stage of a Spark and a few hundred units of fuel.   If you do have the Ant, make the Spark your next stage.  The next stage back could be a Terrier, and roughly twice as much fuel as on the Spark stage.  Depending on the delta-v requirements of a particular mission, you can keep building backwards all the way to enormous lower stages. 

But as other folks have mentioned, Duna is a pretty easy delta-v destination if you hit your transfer window right.  So all you probably need is a launch stage to get this thing high enough for the Terrier to be effective.  For the rocket described above, I might go with a Swivel and a couple radially mounted Thumpers with their thrust turned down to keep your starting TWR reasonable.  The Swivel is not that great a launch rocket, but options are fairly limited if you have something too small for a 2.5 core stage.  

Here's a quick rocket along the lines of the above, with a fairing to protect the satellite until it reaches space.  Update after putting it together -- I would add a bit more fuel over the Swivel, and bump the thrust on the Thumpers back up a bit -- since my Terrier stage does not have that much TWR it would be good to get it fairly close to orbit.  It might be worth it to add a reaction wheel to one of the top stages - forgot about it at the time.  

This thing can go WAY further than Duna, so you could trim it down further, but since this is pretty cheap to begin with, it's worthwhile to leave a decent range buffer in there. 

 

 

Thank you so much. I have successfully landed on Duna with your method and many others! Now I will head back to Kerbin.

[Aegolius13]

3 hours ago, BlazerTheAmazer said:

Thank you so much. I have successfully landed on Duna with your method and many others! Now I will head back to Kerbin.

Sweet!  That first interplanetary trip, like the first Mun landing, was one of the big highlights of KSP for me.

One thing I did not really clarify, though, is specific impulse (fuel efficiency) of an engine.  You don't always want the lightest engine a stage can take - sometimes a higher specific impulse can more than make up for a higher engine weight.  Engine mass tends to matter more when the mass of a stage is small, and specific impulse tends to matter more when a stage is really big.  (As an extreme example, if you're pushing around a 3000 ton asteroid, a 10% increase in specific impulse is more than worth a couple extra tons of engine mass).  As an example at the other end, I mentioned the Ant is very light but relatively inefficient, so it's only worth it if your stage is very small.  The Spark has better specific impulse, so if your top stage gets relatively massive, the Spark's better efficiency can compensate for the Ant's lower weight and give the stage more range (plus way more thrust, which is always nice).

Also, specific impulse is generally most important at the top of your rocket and least important at launch.  This is because you have to haul the fuel for those final stages a long way before they're used, which translates to an exponential increase in launch weight.   Hence, SRBs (which are very low in efficiency, but are cheap and offer pretty good thrust) are a good choice for a launch stage but not worth carrying with you.

Ultimately it becomes a sometimes-complicated balancing act between mass, thrust/TWR, ISP and other factors (cost, ground clearance for landers, etc.), but to me that's a lot of the fun of designing ships.  

 

[Sharpy]

Note: Duna's thin atmosphere and low gravity make LV-N (the very efficient nuclear engine) entirely viable for surface landings.

[BlazerTheAmazer]

On 1/4/2017 at 1:31 AM, Aegolius13 said:

Sweet!  That first interplanetary trip, like the first Mun landing, was one of the big highlights of KSP for me.

One thing I did not really clarify, though, is specific impulse (fuel efficiency) of an engine.  You don't always want the lightest engine a stage can take - sometimes a higher specific impulse can more than make up for a higher engine weight.  Engine mass tends to matter more when the mass of a stage is small, and specific impulse tends to matter more when a stage is really big.  (As an extreme example, if you're pushing around a 3000 ton asteroid, a 10% increase in specific impulse is more than worth a couple extra tons of engine mass).  As an example at the other end, I mentioned the Ant is very light but relatively inefficient, so it's only worth it if your stage is very small.  The Spark has better specific impulse, so if your top stage gets relatively massive, the Spark's better efficiency can compensate for the Ant's lower weight and give the stage more range (plus way more thrust, which is always nice).

Also, specific impulse is generally most important at the top of your rocket and least important at launch.  This is because you have to haul the fuel for those final stages a long way before they're used, which translates to an exponential increase in launch weight.   Hence, SRBs (which are very low in efficiency, but are cheap and offer pretty good thrust) are a good choice for a launch stage but not worth carrying with you.

Ultimately it becomes a sometimes-complicated balancing act between mass, thrust/TWR, ISP and other factors (cost, ground clearance for landers, etc.), but to me that's a lot of the fun of designing ships.  

 

Cool! I'll keep that noted for my future exhibitions to other planets, thank you for the help!