Are fuel cells viable for ion engine use?

[Jeb x Valentina]

Title says it all. I'm planning to send a spacecraft to a comet and take a sample for a contract, but when I was climbing up the tech tree towards the rtg... I felt regret inside of me. Researching spec. electrics was already quite expensive, but, spending double the science for a part as expensive as a mun rocket that I'll probably only use once... Felt like a crazy idea. 

However, since I already had fuel cells, I thought, "well, those things also work in the dark, and are much cheaper" but before spending in a rocket and then regretting it, I wanted to reach out to you (who have more exp than me) and take your advice.

The plan is to have a scientist sitting on top of a tank (which tank is also up to you, as long as it's stock, low mass and has a decent amount of Lf/Ox) with a couple cells and batteries attached, powering a single ion engine.

[Bej Kerman]

Fuel cells necessitate bringing lf/Ox, which will significantly reduce dV. It also produces electricity from a limited pool of resources you may run out of before using all your xenon. Either that, or you may end up bringing too much fuel and waste dV. Solar panels and RTGs are a way better choice for ion engines, they're much lighter and they don't run out. It also helps to bring a lot of batteries for longer, darker burns

[Hotel26]

The answer to your question, in general, is yes for fuel cells.  (But probably not for your comet application.)

NB: The fuel cells, btw, have some battery capacity themselves.

Terrapin

 

Terrapin is a useful final or penultimate drive stage for medium-light interplanetary payloads. It is e.g. featured in my Scout mission vehicle.

By itself, 6+ km/sec dV and 0.98 m/s acceleration.  Don’t forget to activate the fuel cell arrays to power it!!

                                                                                                    

In general, peruse my hangar: Mighty Ion

P.S. I see I used Terrapin as the final propulsion stage in an unpublished work named Kontiki, a 7-kerb craft. 

 

Edited November 28, 2023 by Hotel26

[Hotel26]

So I dummied up the following, Kometeer, without knowing exactly which parts you can use.

The main feature is the staggering of the panels.  You can point directly at or away from the sun and they will all receive 100% without shadowing each other.

When the panel axis needs to be pointed at the sun, you simply rotate the craft 180 degrees longitudinally and you are back in business.

That longitudinal rotation will give you 100% solar exposure at any^[tm] attitude.

At Kerbin's altitude, out in deep space, the panels will run the Dawn continuously.  (This is why I, in general, like RTGs or fuel cells: they will work at Eeloo and during the night.)

Looks like this has 17+ km/s dV.  And you can launch it within a fairing.

You may need an antenna for science and some way to get the kerb back to Kerba Firma.

Spoiler

OK, happy to report that I was able to recover the kerb by a) re-entering with 4 empty Xe tanks and b) staging the kerb after slowing to under 1.5 km/s.  O boy, right over the KSC, too!  "Happy days."

 

Part List:

• lawn chair (kerbal lying on its back) facing prograde
• Z-200 battery
• 5x PB-X150
• 6x OX-4L
• small RW
• Dawn ion engine

Edited November 28, 2023 by Hotel26

[king of nowhere]

16 hours ago, Jeb x Valentina said:

Title says it all. I'm planning to send a spacecraft to a comet and take a sample for a contract, but when I was climbing up the tech tree towards the rtg... I felt regret inside of me. Researching spec. electrics was already quite expensive, but, spending double the science for a part as expensive as a mun rocket that I'll probably only use once... Felt like a crazy idea. 

However, since I already had fuel cells, I thought, "well, those things also work in the dark, and are much cheaper" but before spending in a rocket and then regretting it, I wanted to reach out to you (who have more exp than me) and take your advice.

The plan is to have a scientist sitting on top of a tank (which tank is also up to you, as long as it's stock, low mass and has a decent amount of Lf/Ox) with a couple cells and batteries attached, powering a single ion engine.

yes, I read somewhere that the equivalent Isp is roughly 1300 s, so an ion probe with fuel cells and lf/ox tanks will still have more deltaV than a nuclear rocket, albeit at a lower thrust.

as for the right lf/xenon ratio, you will have to run some calculations yourself.

[Lt_Duckweed]

5 hours ago, king of nowhere said:

as for the right lf/xenon ratio, you will have to run some calculations yourself.

Fortunately, this math is pretty easy.  For every 1 unit of xenon consumed, 18 units of electricity are consumed.

For every 1 unit of lf/ox mix consumed, a fuel cell or fuel cell array produces 400 units of electricity.

Thus, each 1 unit of lf/ox mix provides the electricity needed for 400/18 = 22.2222.... units of xenon

put another way, for every 1000 units  of xenon you have, you need 45 units of lf/ox mix

Edited November 28, 2023 by Lt_Duckweed

[Jeb x Valentina]

@king of nowhere and @Lt_Duckweed, tysm 4 the help. Launch will me tomorrow, and I'll update this thread

[Lisias]

On 11/27/2023 at 9:19 PM, Jeb x Valentina said:

The plan is to have a scientist sitting on top of a tank (which tank is also up to you, as long as it's stock, low mass and has a decent amount of Lf/Ox) with a couple cells and batteries attached, powering a single ion engine.

You need to account for the extra mass. Assuming the math above are correct, you will need 45 units of LFO to each 1000 units of Xe. There're fuel tanks more or less with this capacity, the R11 Bagueta, the Round 8 Toroidal and the R-12 Doughnut. Their mass is about 0.33 tons (full).

So your probe will carry such extra mass, what will impact your D/V - but at least this mass will lower as you consume the fuel.

Then there's the mass of the Fuel Cell itself, the Fuel Cell Array will cost you 0.24 tons, and this mass is constant the whole mission. This will give you 18 E/sec, what's enough to feed two Ion Engines.

On the other hand, a PB-NUK RTG costs you 0.08 tons in mass each, but gives you only 0.8 E/sec - so you will need 12 of them for each Ion Engine (or you will need to have batteries to accumulate the energy produced by the RTGs). Or 0.96 tons only for the RTGs - and this mass is constant for the whole mission, but you will not run out of fuel, as it will happen with the Fuel Cells.

Going back to our Fuel Cells, you will be better served by them as long you plan to consume 2.000 units of Xe or less per Ion Engine (as the tanks and fuel needed will cost about 0.66 tons of mass, and you will need a Fuel Cell Array with additional 0.24 tons). Otherwise, the Equation of the Rocket will start to bite you and you will be probably better served by using the 12 RTGs per ion engine.

Of course, I'm conveniently ignoring the cost in Funds of the whole ordeal. RTGs are pretty expensive, 23.300F each!

[DennisB]

My mass efficient solution would be 1 RTG + 2x1000EC batteries. This would give you a burn time of 4 minutes with a recharging period of 45 minutes on infinite occasions at a mass of 0,18t, or with 4 batteries 8 minutes burn time with 90 minutes recharging for 0,28t. If you put on 2 OX-4 solar panels too with a mass of 0,035t, you can get even better. If you can split the burns, I don't think, there's a better solution, if you have the money for it.

[Kryxal]

If you're not going to be deep in a fairly substantial gravity well, the recharge time shouldn't be much of a concern.  It might be worth having some light solar panels for the initial boost phase, or even use a different propellant in another stage.

[Jeb x Valentina]

1 hour ago, Kryxal said:

If you're not going to be deep in a fairly substantial gravity well, the recharge time shouldn't be much of a concern.  It might be worth having some light solar panels for the initial boost phase, or even use a different propellant in another stage.

That's the idea

 

1 hour ago, DennisB said:

My mass efficient solution would be 1 RTG + 2x1000EC batteries. This would give you a burn time of 4 minutes with a recharging period of 45 minutes on infinite occasions at a mass of 0,18t, or with 4 batteries 8 minutes burn time with 90 minutes recharging for 0,28t. If you put on 2 OX-4 solar panels too with a mass of 0,035t, you can get even better. If you can split the burns, I don't think, there's a better solution, if you have the money for it.

Haven't rtgs

[Jeb x Valentina]

Bad news. Turns out it's impossible to get the ratio right while still having over 10 km/s... 

So I'll have to grind for the rtg... if that's even possible and still get to Maubrett on time...

Should I cancel? I mean, I already have 10M funds and 45% ish reputation...

[18Watt]

It is possible to fly Ion engine missions to Eeloo using only solar power- although I don't recommend that.

One of the few cases where I found it useful to use Fuel Cells with Ion engines is for vessels which have both LFO rocket propulsion and Ion engines.  One example is a SSTO mission to the surface of Moho and back.  To get the required DV the vessel used Ion engines for everything except initial LKO circularization.  Since I had some leftover LFO anyway, may as well throw a Fuel Cell on there.  Obviously solar works fantastic at Moho, but sometimes you want to burn in the dark...

Although it is certainly possible to power Ion engines with Fuel Cells, I don't personally consider that the ideal way to go about it.  First choice is solar, if you are outside of Duna's orbit you'll have to suffer with extremely long burns though.  Next I'd go with RTGs, spamming them as needed.  My last choice would be LFO Fuel Cells.  But like I mentioned, there are cases where Fuel Cells is a solution worth considering.  Just like RTGs, they will at least allow you to continue burning in the dark.

[Spricigo]

On 11/27/2023 at 9:19 PM, Jeb x Valentina said:

The plan is to have a scientist sitting on top of a tank (which tank is also up to you, as long as it's stock, low mass and has a decent amount of Lf/Ox) with a couple cells and batteries attached, powering a single ion engine.

 

4 hours ago, Jeb x Valentina said:

Bad news. Turns out it's impossible to get the ratio right while still having over 10 km/s... 

It is very possible. In fact you have so little payload (1 kerbal) that it could be done with chemical rockets. Example given:

Spoiler

(no fuel cell there because i don't need to power a ion engine. 2 batteries and 2 solar panel are more than i need already)

With the dawn you can get even more deltaV or just have the required 10km/s in a much lighter craft. maybe you just need to drop some dead weight or better use the staging. Pst a screenshort of your craft and we can see what can be improved.

[king of nowhere]

23 hours ago, Jeb x Valentina said:

Bad news. Turns out it's impossible to get the ratio right while still having over 10 km/s... 

So I'll have to grind for the rtg... if that's even possible and still get to Maubrett on time...

Should I cancel? I mean, I already have 10M funds and 45% ish reputation...

it's possible. you can get 10 km/s even with a nuclear engine, and the right ratio of fuel cells + on should give almost twice the Isp.

 

furthermore, you can get nearly as much deltaV as you want with drop tanks. simply put your tanks on decouplers and discard them when they are empty.

[Jeb x Valentina]

thanks. I'll try