Gigantors for a distant ion-powered flight

[Nich]

Just re read the post how the hell did you get retrograde between Jool and Eloo?

I guess it depends on how long your maximum burn is and where. I am guessing burn down to Moh, slingshot into a highly essentric orbit but your still going to need a 5000-ish dV burn to go retrograde. I would bring some solar panels so you can burn in kerbin and moh for the full 4600 then stage it off and array it from there.

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And nuke only has enough power to run 5/8ths of an ion engine. Need 2 nukes per full ion and then 98% of thrust comes from nukes anyways.

Edited July 30, 2015 by Nich

[ajburges]

What would be a more fuel efficent route to a Jool altitude retrograde orbit?

> Retrograde eject from Kerbin into elliptical solar orbit with low Pe. Hohman transfer to target.

> Low dV transfer to Jool (K->E->K->J). Gravity turn/capture and retrograde eject to target orbit.

> As the first, but with a bi-elliptical transfer (only if the second is lower cost than the first. The time hit of a solar bi-elliptical transfer is daunting).

[DancesWithSquirrels]

Sharpy - From Kerbin orbit do you need 20 km/s delta-V? My estimate(s) runs considerably lower.

Burn from Kerbin to Jool orbital distance ~2 km/s

Speed at elliptical apoapsis ~2.3 km/s, circular orbit speed ~4 km/s, for a 6.3 km/s to reverse orbit at Jool distance and match your target. Total ~8.3 km/s

Add 1 km/s to intercept (assuming sloppy maneuvers like mine). Total ~9.3 km/s

Reverse for another 8.3 km/s assuming no aerobraking (reverse orbit with low periapsis, retrofire at Kerbin), only 6.3 km/s with reasonable Kerbin aerobraking (reverse orbit to low peripasis), or as little as 1.7 km/s (just lower the periapsis) if you don't mind screaming into Kerbin on a retrograde path (hot hot hot!). A gravity assist at Jool either coming or going will help considerably.

Total around 17.6 km/s, 15.6 km/s, or only 10 km/s depending on how toasty you like things on return. Of course, if you come in retrograde to Kerbin you better stack heat shields and send a copious supply of iced drinks...

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Edit: I'm guessing you're including Kerbin launch at ~3.5 km/s for this. I would assemble/fuel the ship in LKO, since you don't need anything like that kind of TWR for an orbital rendezvous.

Edited July 30, 2015 by DancesWithSquirrels
Added LKO deltaV

[Skorpychan]

Fuel cells. Solar panels are useless all the way out. HOWEVER, that doesn't mean you can't get creative!

- High-thrust chemical rockets. Heavy and expensive, right? Right. However, they don't need to leave the Kerbin SoI. Just use them to knock the probe out of Kerbin orbit, then have them burn back to orbit Kerbin. Either refill the tanks with ISRU for the next boost, or recover them with chutes.

- Solar panels. Useless way out, better all the way in. Use gigantors to escape Kerbin, jettison them and any empty xenon tanks/batteries/LFO tanks on the way out. Maybe even recover them like the prior method.

- Gravity assists. Use Jool and the muns to tweak your orbit, and maybe go prograde all the way out before using them to flip to retrograde.

[AG5BPilot]

Sharpy, the originator of the thread, hasn't unlocked RTGs yet, so that's not an option.

That's true but possibly not relevant. This 'beyond-Jool' rescue will take quite a few years. In far less time than it takes to fly the mission he could obtain the RTG technology. One RTG plus about 5K worth of battery as a buffer works quite well as a trans-Jool high ISP propulsion system. The most effective way to do this mission, rather than attempting to do it with limited technology, is probably to first go and get the RTG node and then fly the rescue mission. Trying to do is without an RTG is possible, but in my opinion falls into the "appendectomy through the nostril" category. If you're efficient in your science gathering operations, it's not all that hard to open up the entire tech tree. A huge amount of science is available within the Kerbin system, and if that's not enough, a single trip to either Eve or Duna should provide the rest.

The Dawn Ion engine is also pretty deep in the tech tree, so he's probably not that far from getting the RTG.

Ion + big Xenon tank + 1 RTG + 5K battery can go a long long ways, and IIRC can do about 5 or 10 minute burns before running out of juice and having to recharge.

[Laie]

If DancesWithSquirrels' calculations work out, this vessel should suffice. Arrives in LKO with ~2000m/s left in the last KR-2L. Then 34 parts for a good 16.5km/s from nukes. The radiators may even be unnecessary.

You only want to pick up the kerbal, not the debris, right?

[Reactordrone]

I've got a small probe out near Jool at the moment and the three gigantors on it can recharge a 1k battery in just under 6 minutes so solar power is viable but you have to plan your burns to allow recharge.

[Sharpy]

Experimentally established by burning on the launchpad:

400 units of xenon (small tank) take 6.72 units LF and 8.22 Oxidizer.

700 units of xenon (medium tank) take 11.74 units of LF and 14.34 units of oxidizer

That gives about 1.68 LF and 2 Ox units per 100 units of xenon respectively.

(following these figures, 5250 units (the large tank) would need 88,2 units of LF and 102 units of oxidizer (and I really don't want to wait through burning that.)

Also, wet:dry mass ratios (calculations following wiki data)

small 0.571

medium 0.539

large 0.563

What that means, it pays more to pack a number of small tanks than one of the larger ones.

There may be a small inaccuracy coming from the fuel cell charging the 400 units of own capacity.

test rig:

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Just re read the post how the hell did you get retrograde between Jool and Eloo?

A contract.

Interestingly, the contract doesn't exist - at one point Kraken ate all my active contract (no regrets, got better ones and got to keep all the advances on the old ones). But the poor Kerbal is still out there.

I checked possible maneuvers. Jool assist helps, but not all that much. Eve assist is hopeless. I got to some 14,000m/s in about 30 years, but I don't want it to take that long. The direct approach - retrograde encounter and orbit matching burn - give the best results time-wise and will take some 20km/s.

The vessel definitely will be staged - I'm planning an asparagus-style staging for the ion engines (of course fuel will need to be transferred manually, not through pipes). The escape burn will be on something like Rhino; then the encounter and orbit matching on nukes, and the ions will be for the return. Yep, the part count will be pretty high and money-wise I'd be far better off just hiring a kerbal, but well... no kerbal left behind!

[cephalo]

I agree with the guy who said that the gigantor is not useless on this mission, as you will make some burns close to the sun. That should save some fuel for the cells.

[DancesWithSquirrels]

Took a look at the numbers for LV-N's: I can get a delta-V of >20 km/s, min/max Kerbin TWR of 0.18/0.7, with 73 parts and 229 tons in orbit (175 in fuel, 54 dry). Seems manageable if you launch empty, then go out and refuel at Minmas or by sucking a couple of Type E asteroids dry.

Base structure I ran the numbers on is a Mk1 with small chute and heat shield, on a 2+2+2+1 asparagus arrangement with one LV-N, one Mk3 medium tank, 3 small active radiators, SAS and batteries on each unit. Don't forget a few medium solar panels to keep the batteries topped off and the probe core running. You will probably want a docking port on a separator for refueling in orbit, and you could likely return the Mk3 pod!

Drop that to a small Mk3 liquid tank on each stage and the delta-V is 16.7 km/s, min/max TWR of about 0.3/0.9, 158/45 tons in orbit, which should be just fine if you aerobrake on return.

Edited July 31, 2015 by DancesWithSquirrels

[Sharpy]

Okay, got at least 20000m/s. Two nuke stages - 1st based on 2 LF fuselages long, 2nd on some 3 MK1 LF fuselages, for the first 10k. The rest is ion in asparagus staging in 4 stages. The initial 7 engines run on 3 fuel cell arrays. 2nd stage drops along with 2 fuel cell arrays, so what's left is 3 engines on 1 array, will run at 2/3 throttle. Since you need to move the xenon manually for ion asparagus staging, and KER can't account for that, it displays some 10,000 of delta-V, but I'm certain the staging will improve upon that quite a bit.

Yesterday I captured a Class E asteroid so the first stage will be refuelled in orbit; the mass drops from some 230 tons to below 100 when fuel is removed.

[PB666]

Interesting delimna.

Theoretically if one uses an efficient ION engine one should be able to collapse an orbit at apogee near kerbin-Kerbol altitude, and progressively bring the periapsis closer to Kerbol, and at closest Kerbol periapsis bring up the apogee to past the targets orbit, then once at apogee the dV to flip the orbit should be small, then once at perigee reduce the apogee as much as possible (say Eve) and circularize at Eve, then match planes, and hohman transfer from eve orbit. You might even be able to transfer from a perigee close to moho if you plan your orbit right, but as you aproach you will definitely have to start adjusting relative velocity well in advance with an ION engine.

I would say just get the target in any orbit and you can pick it up from there. If you could get the target in very high kerbol orbit that is prograde kerbin and you have solved 80% of your problem.

What you really need is an EM drive ( :^) )with a low weight XL-MGS (scale mod small and chained) covered with OX_STATs. Gigantors are not the most weight efficient, there are fixed panel mods that have are more power/weight ratio. The MGS and XL-MGS are overkill on strength, so you need to find a way to get the structural extention without all the weight and then you can cover them with OX-STAT panels.

[panzer1b]

if all you want is one kerbal rescued, make something akin to my older fighters, a MK1 command pod, FLT-100, ant engine, probe core, and 1 RTG (for that long a trip batteries make no sense). When your vessel weighs like 2t, youd be surprised how far even a puny little engine like the ants can take you, i originally made these fighters for close range operations, but apparently they had better range then the carrier they were launched from.

All in all, the smaller the better, you can even settle for a fuel tank+RTG+probe core+external seat+engine setup. If you bring the smallest parts around, you can get it incredibly far out, and itll cost basically nothing, although you would need to transfer said kerbal to a reentry vessel afterwards once you return to kerbin or whatever area you want to get to.

[Sharpy]

Interesting delimna.

Theoretically if one uses an efficient ION engine one should be able to collapse an orbit at apogee near kerbin-Kerbol altitude, and progressively bring the periapsis closer to Kerbol, and at closest Kerbol periapsis bring up the apogee to past the targets orbit, then once at apogee the dV to flip the orbit should be small, then once at perigee reduce the apogee as much as possible (say Eve) and circularize at Eve, then match planes, and hohman transfer from eve orbit. You might even be able to transfer from a perigee close to moho if you plan your orbit right, but as you aproach you will definitely have to start adjusting relative velocity well in advance with an ION engine.

I tried Eve gravity assist variants. Just getting down to Eve cost more fuel (and orbital speed!) than I could get back from the gravity assist. Low Kerbol Orbit (1Mm) is more costly than Eloo.

And bringing the orbit higher... the delta-V gains are there, but the time grows so excessively it's scary. I tried it with 1.5 Jool orbit and it would take some 30 years.