[DUNA MISSION FAILURE] Ship issues... again.

[Starwhip]

So, I did the calculations for a manned Duna/Ike mission, and designed a ship off of them.

It's three components. The main section (The middle one with the HSCs and the Cupolas) is the Transfer section, dedicated to getting the crew there and back. The front is the Ike Excursion Vehicle, with a CSM and Lander. The back, the Duna EV lander.

That middle section is calculated at nearly 3 km/s^2 of delta-V. Good enough for a Duna trip.

Here's the problem: With the two NERVA's, the thing's TWR is only 0.3 at Kerbin. That gives a 15-minute-ish transfer burn. Not nice.

I have considered two options:

1 - Give it a higher orbit, so that the burn can be started earlier

OR

2 - Redesign the Transfer section.

Hopefully number one, but any other ideas?

Edited May 23, 2014 by Starwhip

[Tex_NL]

Make your orbit highly elliptical with a still low EP in the correct position. Make your AP as high as possible without leaving Kerbins SOI.

When the transfer window 'opens' all you'll have to do is make a short burn at EP.

Just keep in mind such an orbit will take considerable to complete. By the time you get back to your EP the ideal transfer window might have passed already.

[Pursuedtank]

A small thrust to weight on a transfer stage is not a large problem. One thing you could do is to do your burn in multiple orbits. An example is that you seperate your burn out into three 5 minuit burns , therefore taking full advantage of the oberth effect. You would time warp around your orbit to the next burn.

Of course if delta v is not a problem then option 1 is viable. However it is likely to take just as long.

Also don't forget about possibly using physical time warp to shorten the time it takes.

[Pecan]

Send up engine-modules to dock around the 6-way hub. Their sole purpose should be to increase the TWR (for which they will need more fuel, of course). My preferred thrust-not-efficiency engine is the T30, it's mass and ISP aren't too bad. Undock and terminate them after the exit burn, live with the time it'll take to establish orbit at Duna (don't forget aerobraking).

[phoenix_ca]

Definitely split-up your burn into multiple burns. It's an easy-ish way to get it done.

[Sirrobert]

If you want a small engine boost without having to redesign anything, you can activate the Ike Excursion Vehicle's engines.

That should give you a little extra thrust, though it'll cost more fuel because they are less efficient than the NERVAs offcourse

[Tw1]

What you need to do, is the periapse kicks technique. Break your burn up into smaller ones at the same point, over several orbits.

Here I'm flying an ion lander to Gilly, but the principle is the same. Be careful with how long your orbit gets, if you watch your apoapsis and periapsis, you should be able to time things so that last burn happens at the right time to transfer.

Edit: ..and look at all the ninjas. But I brought a picture .:sticktongue:

[michaelhester07]

No picture here but a tip

If your ship is stable enough you can do a physics warp [alt] + [.] to "speed up" the burn. Try to avoid course corrections during the phys warp with that ship. that quad hub is the most unstable point on your ship and could snap at 4x.

[Starwhip]

New & Improved DTV, packing 5km/s^2 of DV on its own.

0.44 TWR. A little better. Hmmph.

And the vessel is strong enough for Physics warp. I've tested it.

So this is a liiitle bit more tricky than I thought.

[phoenix_ca]

Yikes. Try to remember that adding more engines doesn't really help as much as it hinders. It reduces delta-v, usually more than what little extra thrust gained helps.

[Starwhip]

Ugh. Mark Three:

Cleaner, sleeker, MOAR POWER and still has the delta-V.

Yes!

(NOTE: THE GRAY TANKS IN FRONT ARE TO SIMULATE THE PAYLOAD OF THE LANDERS. THEY ARE NOT FOR FUEL.)

With the new version, the second hub near the DEV in the back can be eliminated, saving weight.

[Random Tank]

Live a little. Take the 15 min burn. SHOW THAT SPACE WHO'S BOSS!!

[Captain Sierra]

Make your AP as high as possible without leaving Kerbins SOI.

Starwhip, something to note is that 60% of your dV equates to only about 30% of your orbital change. I find that you can slash your burn times more than in half and still catch an optimal transfer (as orbital period is roughly 1 kerbin day) if you set your apoapsis around Munar altitude. Be careful you don't accidentally get a Munar intercept that screws you over!

Some Munar encounters can help you out by further increasing your apoapsis. If you get one of these encounters, you can get free dV from the Mun. Not all encounters do this and it is rare, though not impossible, that one will occur while you are waiting for your transfer window.

[Redshift OTF]

I like the new design. Some of the components are a bit heavy though. The cupola cockpit thingy is quite heavy, over 4 tons and you have two of them. Also do you need RCS on the transport part of the ship? I think a reaction wheel or two might be lighter and more efficient to turn it. The 6 way connector thing is also pretty heavy for what it does, there are alternatives, although I can understand that asthetics are important for ship design.

[xcorps]

I have got to quit trying to rotate the VAB when I'm looking at screenshots.

Edit: Great mod for simulating weight is http://forum.kerbalspaceprogram.com/threads/76231-0-23-x-Kerbal-NRAP-adjustable-test-weight-for-rocket-building%21-v1-4-11-04-14

Single part with adjustable height and radius. Let's you adjust mass to a specific amount.

Edited May 21, 2014 by xcorps

[Starwhip]

Thank you, SLS Block II!!!

Going to get there. Going to get there... soonish.

[Red Iron Crown]

I have got to quit trying to rotate the VAB when I'm looking at screenshots.

Get out of my head!

Another option for the OP would be to use a KR-2L. A little less efficient than the aerospikes for about the same mass, but it will solve your TWR problems nicely. Of course, you won't be able to mount it radially.

[Starwhip]

Friggin'... Absolute... BEHEMOTH!!!

Weighs 70 tons right now, but was clocking in over 100 at launch (Just the bit you see here)

Need to an entire orange tank up there to refuel it; in fact, a little bit more. Probably 1.5 OTs.

Also need RCS, plus all of the modules on the other bit. CREW TRANSFER TIME!

Oh. Wait. It's in a 500 km orbit. That's going to be annoying.

MOAR THAN ONE REFUELER!!!

[Dkmdlb]

Getting to Duna only requires about 200 m/s dv more than getting to Mun. Why not just put a more powerful engine on there? It's not like you need to wring 4,000 m/s out of your ship. You only need 1,200 or less to get into a stable Duna orbit. Sacrifice efficiency for convenience.

[Starwhip]

No, but the getting back is the tricky bit. No Kethane here, sir. Totally contained in ONE ship!

On a side note:

The IEV and DEV are now connected. Just need to refuel and... uh... rescue the guys stranded on the previous DTV. Heh.

[Starstrider42]

Live a little. Take the 15 min burn. SHOW THAT SPACE WHO'S BOSS!!

This is actually not a bad idea. I made a very large Mun exploration ship with the LVN, and the transfer burn also took something like 15 minutes. But starting the burn 7 minutes early worked well enough (though I ended up editing the maneuver node a couple of times mid-burn to account for deviations between my half-burn trajectory and the maneuver node position; that's a bit harder to do for an interplanetary transfer).

[KerbMav]

The IEV and DEV are now connected. Just need to refuel

That would have been my first suggestion had I come here earlier.

Lessen the payload by using (part of) the Duna lander for the Ike excursion too.

Second: Is your vessel capable of dropping empty fuel tanks e.g. used for getting into Duna orbit?

[Starwhip]

Empty fuel tanks? You mean, like, an Asparagus lander? Nay, sir. It's a single stage. And it's using a "Poodle". So no drop-tanks there. Based off of the hand calculations I did, it's got something like 1660 m/s^2 of Delta V.

The Ike lander is also one stage. It has 1502 m/s^2. Note that it only really needs around 1400. But I like a safety margin. Same with the Duna lander: it only really needs about... oh, wait. I calculated 1675. Oh, well, close enough. I implemented the safety margin before anyway.

The main reason the Transfer stage has to be so large is that it has to carry the crew and landers to Duna as well as back to Kerbin. In one stage. No Kethane refueling. At best, I could send over a refueler next time a window opens.

Anyways, the mission seems to be on the right track. Just got to refuel the darn thing. Any ideas on what to name it? The Duna Transfer Vehicle is... not very creative, to say the least.

I think there's still one guy stuck on the original Transfer section. I'll send up a Koyuz and dock with it. It's got room for two, if I kick one guy out on launch.

[Padishar]

You really should update KER to version 0.6.2.4. It is much more stable than 0.6.2.3 and fixes many issues with the deltaV calculations.

[Skorpychan]

This is actually not a bad idea. I made a very large Mun exploration ship with the LVN, and the transfer burn also took something like 15 minutes. But starting the burn 7 minutes early worked well enough (though I ended up editing the maneuver node a couple of times mid-burn to account for deviations between my half-burn trajectory and the maneuver node position; that's a bit harder to do for an interplanetary transfer).

I second this. 'Man up and just sit through the burn on physwarp'.