Calculating Electric Charge Needed from Delta-V

[Duxwing]

How can I calculate how much electrical charge I will need for each delta-V?

I want to build a reusable interplanetary tug that can bring ten tons (1) from Low Kerbin Orbit to any other low orbit except Low Kerbolar Orbit. Its technical specifications are 1,200m/s dV without recharging, 0.15 TWR, and indefinite aerobraking anywhere with and without payload. It will have a reserve of electrical charge because operating without one around Jool and Eeloo would require too many panels (2). I want a standard design to simplify infrastructure and would rather not use modified parts.

Footnotes

1 - I may need more for my Interplanetary Shuttle, which does with crew what the Interplanetary Tug does with cargo.

2 - I use the Stock Rebalance Mod, wherefore as far from Kerbol, so the inverse square of power received.

-Duxwing

Edited June 1, 2014 by Duxwing

[SpenSpaceCorp]

Try using those thermoelectric generator gizmos. they produce small amounts of power and are heavy, but useful when you are too far for solar power to do much. Also, consider using 1/2 of the largest batteries to store electric charge after it is recharged slowly with those electric generators. You only really need power for reaction wheels, so the principle would be to have a battery to use stored power when rotating, and 1/2 thermoelectric generators to slowly recharge the battery later.

Also, delta-v is independent from electric charge. Electric charge is limited by its source(s) of power.

Hope that helps!

[UmbralRaptor]

Just use 3x the panels you would at Kerbin, and you'll have enough for Eeloo's apoapsis plus a small margin. Barring massless part shenanigans, this is likely to be lighter than the equivalent amount of batteries. If you already have the craft layed out, and are using the massless radial batteries, just multiply xenon by 18 (as the PB-ION engine consumes 18 units of electricity for each unit of xenon).

[Duxwing]

Try using those thermoelectric generator gizmos. they produce small amounts of power and are heavy, but useful when you are too far for solar power to do much. Also, consider using 1/2 of the largest batteries to store electric charge after it is recharged slowly with those electric generators. You only really need power for reaction wheels, so the principle would be to have a battery to use stored power when rotating, and 1/2 thermoelectric generators to slowly recharge the battery later.

I will need to carry more electrical charge because I will also return from Eeloo without recharging.

Also, delta-v is independent from electric charge. Electric charge is limited by its source(s) of power.

For my purpose, as delta-v, so electric charge because I want this craft to change velocity by 3,400 m/s without recharging.

-Duxwing

[cantab]

So basically it's an ion craft using battery banks to sustain it through the burns? I think you'd do well to add an RTG to "trickle charge" the batteries on the coasts, so the batteries need only run one burn at a time.

Anyway, as mentioned the PB-ION uses 18 units of electricity for every 1 unit of xenon. So if you know how much xenon you need to do a such-and-such m/s burn (bear in mind it'll be less once the craft is lightened by burning some xenon off), take 18 times the units of electricity.

Of course, if you use stack batteries that'll make the ship heavier, meaning you need to use more xenon to get the same delta-V, meaning you need more batteries, and so on. You may find your idea impractical if the batteries just weigh too much. If you use radial batteries you'll circumvent the problem since they're massless in .23.5, but that feels a bit exploity for me.

For realistic (ie not massless) batteries, I don't think it's a simple thing to calculate except by trial and improvement. Unlike the propellant, the battery charge doesn't work properly in the rocket equation because the "tankage" has mass but the "fuel" is massless.

[Duxwing]

So basically it's an ion craft using battery banks to sustain it through the burns? I think you'd do well to add an RTG to "trickle charge" the batteries on the coasts, so the batteries need only run one burn at a time.

I brought two to charge them between transfer windows of the Joolian moons. And, just for the extra TWR, I put six more engines on.

Anyway, as mentioned the PB-ION uses 18 units of electricity for every 1 unit of xenon. So if you know how much xenon you need to do a such-and-such m/s burn (bear in mind it'll be less once the craft is lightened by burning some xenon off), take 18 times the units of electricity.

Thanks. I therefore will need 1.62e3 * 18 = 2.91e5 units of electrical charge.

Of course, if you use stack batteries that'll make the ship heavier, meaning you need to use more xenon to get the same delta-V, meaning you need more batteries, and so on. You may find your idea impractical if the batteries just weigh too much. If you use radial batteries you'll circumvent the problem since they're massless in .23.5, but that feels a bit exploity for me.

2.91e5 EC / 4e3 EC per battery = 73 batteries

73 batteries * 2e-1 tons per battery = 14.6 tons

My battery mass therefore would exceed not only my propellant mass but my payload mass.

Whereas if I transferred from Eeloo to Kerbin via two burns spread over less than a day, I could use fewer batteries. This multi-burn plan could be used elsewhere, reducing necessary electrical charge that of the longest necessary burn; e.g., Jool to Pol.

On the WAC dV map of 2013-06-23, Jool -> Pol requires 2,500m/s dV

Kerbal Engineer says my rocket needs 1.18e4 Xenon for that dV

1.18e4 Xenon * 18 EC per Xenon = 2.16e5 units EC

2.16e5 EC / 4e3 EC per battery = 54 batteries

54 batteries * 0.2 tons per battery = 10.8 tons

This tug would have too many batteries to be structurally stable. Solar panels might reduce my effective battery usage rate.

My recent Laythian lander--hooray!--has given me data on the Joolian system. On the Laythian surface, its best-performing OX-STAT produces 0.03 EC/s at noon, 4% of its rated output. A Gigantor XL is rated for 18 EC/s and therefore would produce 0.72 EC/s. My craft would consume 14*18 = 252 EC/s--three orders of magnitude more. So many solar panels would be needed that using them would be mass and cost-ineffective.

Might RTGs do better?

Each masses more than two men and produces almost no electrical charge: no.

I feel like a quarian in a bramble patch. >_< Wherever I go, I'm dangerously stuck.

Might orbital gerrymandering help?

Orbital gerrymandering would involve transferring from one satellite to another, minimizing my burn time. The least burn would be Jool-Laythe, which would require 1,600m/s dV.

1,600m/s dV requires 7.43e3 Xenon

7.43e3 Xenon * 18 EC per Xenon = 1.44e5 EC

1.44e5 EC / 4e3 EC per battery = 36 batteries

36 batteries * .2 tons per battery = 7.2 tons

I must further reduce my EC requirement because thirty-six batteries would destabilize my craft and 7.2 tons would be too much dry mass for acceptable propellant use and TWR.

Could fancier gerrymandering solve the problem?

Fancy Joolian aerobraking could keep my apoapsis Laythian, necessitating only a maximum 1,200 m/s dV for Tyloian capture from Vall.

1,200 m/s dV necessitates 5.67e3 Xenon

5.67e3 Xenon * 18 EC per Xenon = 1.02e5 EC

1.02e5 EC / 4e3 EC per battery = 25.5 batteries

25.5 batteries * .2 tons per battery = 5.1 tons

That mass and number of batteries is manageable. Can I lower it?

I speculate that my Tyloian capture window would be so short as to prevent my splitting the burn and recharging between the mini-burns.

I cannot aerobrake at Tylo.

I cannot interrupt my voyage.

Any ideas?

For realistic (ie not massless) batteries, I don't think it's a simple thing to calculate except by trial and improvement. Unlike the propellant, the battery charge doesn't work properly in the rocket equation because the "tankage" has mass but the "fuel" is massless.

Hence Kerbal Engineer.

-Duxwing

Edited June 1, 2014 by Duxwing

[UmbralRaptor]

On the Laythian surface, its best-performing OX-STAT produces 0.03 EC/s at noon, 4% of its rated output.

That's not the stock solar panel performance curve. Please give a list of all mods you are using.

[Duxwing]

That's not the stock solar panel performance curve. Please give a list of all mods you are using.

I did: see the OP.

I use the Stock Rebalance Mod, wherefore as far from Kerbol, so the inverse square of power received.

My solar panel performance greatly decreases with distance.

-Duxwing

[cantab]

Though included in your spec, bear in mind that if you're going to or from any of Jool's moons it's very wasteful to go via a low Jool orbit. Not that it makes much difference for your task since 2500 m/s or even more is plausible for a Moho capture burn. (Then again, if you use panels that's not a problem).

RTGs might be viable if you compromise the overall TWR of your craft. You need 12 per ion engine, so you're effectively making the engine weigh 1.21 tons, giving it a Kerbin TWR of 0.16. So if half the mass of the craft was engines and RTGs you'd have a 0.08 TWR.

This little design is something along those lines:

A 1-ton payload (the pod with a chute and two mid-gain antennae), just over 10,000 m/s of total delta-V, but a Kerbin TWR of a measly 0.072.

The real killer for this idea though may be the part count. Scale it up ten times and you're looking at nearly 500 parts just for the tug.

[Duxwing]

Though included in your spec, bear in mind that if you're going to or from any of Jool's moons it's very wasteful to go via a low Jool orbit. Not that it makes much difference for your task since 2500 m/s or even more is plausible for a Moho capture burn. (Then again, if you use panels that's not a problem).

RTGs might be viable if you compromise the overall TWR of your craft. You need 12 per ion engine, so you're effectively making the engine weigh 1.21 tons, giving it a Kerbin TWR of 0.16. So if half the mass of the craft was engines and RTGs you'd have a 0.08 TWR.

This little design is something along those lines:

https://farm6.staticflickr.com/5278/14138228388_02146497d5_o.png

A 1-ton payload (the pod with a chute and two mid-gain antennae), just over 10,000 m/s of total delta-V, but a Kerbin TWR of a measly 0.072.

The real killer for this idea though may be the part count. Scale it up ten times and you're looking at nearly 500 parts just for the tug.

Wow, thanks for building that craft and sharing it: you're nice.

The RTGs' output need not meet the ion engines' input to reduce my battery mass; e.g., meeting it halfway halves my effective input, vastly reducing my battery requirements. Ion engines seem to be the thinking Kerbal's engine, and I enjoy this intellectual challenge.*

-Duxwing

*Arrogance duly noted.

[LethalDose]

Due to your use of the stock re-balance mod (leading to the need for huge battery banks) maybe you should reconsider choosing ion thrusters for your tug. Using NERVAs would probably be more mass efficient.

[Duxwing]

Due to your use of the stock re-balance mod (leading to the need for huge battery banks) maybe you should reconsider choosing ion thrusters for your tug. Using NERVAs would probably be more mass efficient.

What effect does Stock Rebalance have here?

My interplanetary fleet uses NERVAs and therefore so much fuel as to necessitate extensive, heavy-duty ISRU (Kethane) and enormous launch vehicles. I want this ion-engine tug to reduce my propellant requirements and thereby my fleet size. I hope to replace my ISRU system with a fuel tanker carrying Xenon that will refuel my likewise-fueled Interplanetary Shuttle and two Interplanetary Tugs, one with payload and the other with supplies.

Perhaps some further explanation is necessary. For the past few months, I have been developing a fleet that can explore, exploit, and return from any world. This fleet will have one kind of each craft--tug, shuttle, station, and rover--with extra tugs hauling mission-specific payloads. My current-generation plan is:

1 - Gravimetric Probe for testing dV requirements

2 - Sensory Lander

3 - Sensory Rover

4 - Kethane Scanner

5 - Station, preliminary ISRU, vehicles, and supplies

6 - Interplanetary Shuttle

7 - Finish ISRU, do science.

8 - Fleet returns.

I want such a low-propellant tug as an ion-powered one could be because it would need much less fuel from Kerbin after the fleet returned. By my mental estimate, this reduction would be 6-12 fold.

-Duxwing

Edited June 2, 2014 by Duxwing

[cantab]

Without the Stock Rebalance mod, you'd be able to use far fewer solar panels to run your ion engines. One Gigantor per ion would probably do it out by Jool in stock.

[Duxwing]

Without the Stock Rebalance mod, you'd be able to use far fewer solar panels to run your ion engines. One Gigantor per ion would probably do it out by Jool in stock.

I have considered getting Near Future Electrical to solve that problem.

-Duxwing