Scouting for Jool 4 Mission delta v estimate

[Nicias]

I am planning a Jool 4 mission (skipping Laythe)

Looking at a delta v map, I get a worse case scenario delta v of 21km/s , without landings. That seems very high. So I wanted to do a mock run with a smaller craft, skipping the landings.  

Is the following a reasonable approach:

Rig up a craft with the same minimum TWR as my actual craft. Set infinite propellant, and navigate the planned trip, keeping track of the required delta v. 

Would that give me a good estimate for how much delta v the real misssion would require?

-Thanks.

[DrLicor]

Had a little inspirating look here: He used 17000m/s of delta-v, with landings.

 

[Aegolius13]

I think that would work, as long as you keep track of everything.  You could also try a near zero payload test craft.

But at some point, a large scale dress rehearsal with the real ship is worthwhile, especially if the final mission is going to be in your "real"game with other stuff going on.  I kept finding little issues with my Jool 5 over a couple run throughs.

[DerekL1963]

6 hours ago, Aegolius13 said:

But at some point, a large scale dress rehearsal with the real ship is worthwhile, especially if the final mission is going to be in your "real"game with other stuff going on.  I kept finding little issues with my Jool 5 over a couple run throughs.

This - in my sandbox dress rehersals I've found and fixed a number of issues.

And I'd skip Tylo before I'd skip Laythe based on my current practice runs for a Jool 5..  Laythe is far easier.

[Empiro]

8 hours ago, Nicias said:

I am planning a Jool 4 mission (skipping Laythe)

Looking at a delta v map, I get a worse case scenario delta v of 21km/s , without landings. That seems very high. So I wanted to do a mock run with a smaller craft, skipping the landings.  

Is the following a reasonable approach:

Rig up a craft with the same minimum TWR as my actual craft. Set infinite propellant, and navigate the planned trip, keeping track of the required delta v. 

Would that give me a good estimate for how much delta v the real misssion would require?

-Thanks.

Without landings, it shouldn't be anywhere close to that.

Make sure you're looking at a map that has direct moon-moon transfers (http://forum.kerbalspaceprogram.com/index.php?/topic/101435-delta-v-from-moon-to-moon/), or use a tool like http://ksp.olex.biz/ . It usually leads to a significantly lower transfer cost because you're doing a single burn (better Oberth effect), and because many maps assume you're starting form low Jool orbit.

Second, plot your course carefully. If you're not doing landings at all, then you don't need to ever get into a circular orbit. An elliptical one with the PE in low orbit and an AP that's very far out will let you get science from both places without using much fuel in your capture burn.

Third, make good use of gravity assists when getting to Jool and around Jool to really lower the cost of the trip (especially by using Tylo). It shouldn't cost more than a 500 m/s to go from an elliptical orbit of any of the moons to an elliptical orbit of any of the other moons, as long as you do your burns from low around a moon (Oberth effect), and you're patient (wait for an encounter instead of directly getting into one).

My math shows that if you plot your course carefully, you should be able to launch, get to all 5 moons and get back for less than 10,000 m/s total.

[Aegolius13]

2 hours ago, DerekL1963 said:

And I'd skip Tylo before I'd skip Laythe based on my current practice runs for a Jool 5..  Laythe is far easier.

Laythe can be done smaller than Tylo, especially if you go the plane route.  But it likely takes a dedicated lander, wings can be harder to accommodate, and it's a little tougher to get a good landing spot.  You can potentially combine Tylo and Vall landers on some fashion - my ascent stage on the former doubled as the Vall lander.

Also worth noting you can potentially save mothership fuel by letting the landers do some of the transfers.  E.g., skip Bop and let one lander do Pol and then backtrack.

[DerekL1963]

1 minute ago, Aegolius13 said:

Laythe can be done smaller than Tylo, especially if you go the plane route.  But it likely takes a dedicated lander, wings can be harder to accommodate, and it's a little tougher to get a good landing spot.  You can potentially combine Tylo and Vall landers on some fashion - my ascent stage on the former doubled as the Vall lander.

In my current Jool-5 workup (see my sig for a link), that's actually exactly how it's working out.  Laythe got a dedicated lander.  The core of my Tylo lander gets refueled and lands on Vall, Bop, and Pol.  It can actually self-ferry from the mothership to Bop or Pol and back.

[Aegolius13]

1 hour ago, DerekL1963 said:

In my current Jool-5 workup (see my sig for a link), that's actually exactly how it's working out.  Laythe got a dedicated lander.  The core of my Tylo lander gets refueled and lands on Vall, Bop, and Pol.  It can actually self-ferry from the mothership to Bop or Pol and back.

I did a separate lander for Bop and Pol - tiny little ion-powered thing with ridiculous delta-v.  In hindsight I probably could have skipped out on even sending the mothership to the outer planets, and gone Laythe-Bop-Pol-Laythe or something like that with the ion lander.  I have honestly not found a great use for ions in any other situation yet, but cannot recommend them enough for Pol/Bop duty.  Maneuvers are cheap enough you can just pack a couple batteries and an RTG and not have to worry about replenishing your electricity in realtime.   And TWR was sufficient for those itty bitty moons. 

My mission report at:

 

[DerekL1963]

38 minutes ago, Aegolius13 said:

I did a separate lander for Bop and Pol - tiny little ion-powered thing with ridiculous delta-v.  In hindsight I probably could have skipped out on even sending the mothership to the outer planets, and gone Laythe-Bop-Pol-Laythe or something like that with the ion lander.  I have honestly not found a great use for ions in any other situation yet, but cannot recommend them enough for Pol/Bop duty.  Maneuvers are cheap enough you can just pack a couple batteries and an RTG and not have to worry about replenishing your electricity in realtime.   And TWR was sufficient for those itty bitty moons. 

Nice mission!  I had originally penciled in a dedicated Bop/Pol lander, but I deleted it once I determined that the core of the Tylo lander could do the trick.  I'm already using orbital refueling and a tanker, so why not go for broke?  Not only does it save on parts, hitting all five moons with two landers just seems to be cool since most everyone else uses three or more.

I tried like you wouldn't believe to do it with one since my Laythe lander actually overperforms...  But I just couldn't master a Tylo landing that didn't require that crasher stage.

You did give me a good idea though, I haven't looked at Mk3 parts for the mothership - and I really should.

[Aegolius13]

49 minutes ago, DerekL1963 said:

Nice mission!  I had originally penciled in a dedicated Bop/Pol lander, but I deleted it once I determined that the core of the Tylo lander could do the trick.  I'm already using orbital refueling and a tanker, so why not go for broke?  Not only does it save on parts, hitting all five moons with two landers just seems to be cool since most everyone else uses three or more.

I tried like you wouldn't believe to do it with one since my Laythe lander actually overperforms...  But I just couldn't master a Tylo landing that didn't require that crasher stage.

You did give me a good idea though, I haven't looked at Mk3 parts for the mothership - and I really should.

Thanks!  At the end of the day, the ion lander probably did not save me all that much weight.  The Vall lander had enough delta-v to do both Pol and Bop in one go, so the marginal mass cost of using it instead would have been just another 800 units of fuel (~4 tons - I had some empty oxidizer capacity in one of my adapters).  That would also save duplicating the lander cans and so forth.  But... the ion lander was just so perfectly suited to those planets, I couldn't help myself.  

I feel you on Laythe uni-taskers.   Unfortunately, you really have to go Rapier on Laythe, and it's just an awful engine for vacuum flight.  Putting wings on decouplers is one option to trim down the weight if you plan to reuse in vacuum only.  I'm working on that concept with a ridiculous 4-kerbal, ISRU-capable rocket-plane hybrid that would land on each moon, with wings and jets rigged to detach after Lathe.  Development thus far is... troubled though.  

The Mk 3 cargo bay arrangement was pretty much the only way I could come up with to hold everything together in a way that kept the ship from being floppy AND the mass aligned with the center so I could fly straight.  That seems less problematic with only two landers, since for example, you could do one in front and one behind your ship.  But that cargo bay was HEAVY (6 tons for the long version, and that was with some draconian cuts to the Tylo/Vall lander to keep it short enough to fit).  But Mk 3 in general was great for keeping part count (and ensuing floppiness) low while still packing lots of LF to feed the nukes.  

[DerekL1963]

3 minutes ago, Aegolius13 said:

The Mk 3 cargo bay arrangement was pretty much the only way I could come up with to hold everything together in a way that kept the ship from being floppy AND the mass aligned with the center so I could fly straight.  That seems less problematic with only two landers, since for example, you could do one in front and one behind your ship.

If you look at the pic in my thread, I carry one on each side of the mothership on top of radial fuel tanks.  (Another reason why two landers is attractive.)  With the huge size of the mother, the CG isn't pulled that far off center.  Some preliminary testing shows she flies just fine.

[Aegolius13]

59 minutes ago, DerekL1963 said:

If you look at the pic in my thread, I carry one on each side of the mothership on top of radial fuel tanks.  (Another reason why two landers is attractive.)  With the huge size of the mother, the CG isn't pulled that far off center.  Some preliminary testing shows she flies just fine.

I'd be most worried for the period when you had deployed one lander but not the other.  But it it's tolerable then, cool!  One bright side of the low TWR of nukes is your reaction wheels don't have as much work to do to stay centered. Unbalanced mass/thrust is one of my irrational pet peeves (which is actually a good idea for a discussion topic), so I may have gone a tad overboard in combating it.  Plus, the giant bay adds to the "mothership" ambiance, I guess.

[DerekL1963]

23 minutes ago, Aegolius13 said:

I'd be most worried for the period when you had deployed one lander but not the other.

That's the beauty of my scheme...  Once the (extremely heavy) mothership settles in Jool orbit, it's done.  It never maneuvers again, all the orbital transfers are done by the landers and the refueling/crew transfer vehicle.  The refueling/crew transfer vehicle also doubles as the Kerbin return vehicle.

[Nicias]

I ended up using the c

On September 30, 2016 at 10:04 PM, DerekL1963 said:

Nice mission!  I had originally penciled in a dedicated Bop/Pol lander, but I deleted it once I determined that the core of the Tylo lander could do the trick.  I'm already using orbital refueling and a tanker, so why not go for broke?  Not only does it save on parts, hitting all five moons with two landers just seems to be cool since most everyone else uses three or more.

You did give me a good idea though, I haven't looked at Mk3 parts for the mothership - and I really should.

That is what I did. 2.5 stage lander. Dumps first stage on Tylo, dumps extra fuel tanks after Vall, does Bop and Poll. Mothership is also return vessel. Currently landed on Poll.

I did have redo the launch, rendezvous, LKO burn, and Jool transfer since I had the wrong kind of solar panels and not enough control authority. My solar panels kept burning off and then I would tumble (two unrelated problems).   

[DerekL1963]

1 hour ago, Nicias said:

That is what I did. 2.5 stage lander. Dumps first stage on Tylo, dumps extra fuel tanks after Vall, does Bop and Poll. Mothership is also return vessel. Currently landed on Poll.

Asparagus for me...  The radials are dumped shortly after liftoff on Tylo, then the core does the other three.  The refueling tug also serves as the return vessel.