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I've just ran across two mentions in Scott Manley's videos of LiPo batteries (Lithium-polymer) in first stage launch applications. These involve power levels of megawatts.

Many/most large terrestrial transportation applications that I've heard of use lithium ion batteries, often made up of a collection of 18650 cells.

Are LiPo batteries more suitable for 1st stage rockets than Li-ion batteries? If so, why?

Electron rocket engine

above: Cropped from Why The Electron Rocket May Be Cheapest Way To Get To Space below: from Does ARCAspace's Water & Electric Powered Rocket Make Sense?

ARCAspace rocket

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  • $\begingroup$ slightly related and currently unanswered: Are the ejected first stage Electron rocket batteries really incinerated? $\endgroup$
    – uhoh
    May 6, 2019 at 5:36
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    $\begingroup$ The thing we care about is the discharge rate. It is usually given in C a discharge rate of 1 C means we release the stored energy in 1 hour. A lower stage might burn say 2.5 min which means we need a discharge rate of 24 C if we want to completly discharge our batteries which is high but managable for LiPo commonly used in e.g. fpv drones and rc cars (they go up to ~75 and are marketed up to a constant 100). I have no idea about Li-ion batteries therefore just a comment. The internal resistance is the limiting problem here. $\endgroup$
    – Christoph
    May 6, 2019 at 6:43
  • $\begingroup$ @Christoph I see, since mass is such a critical factor in launch vehicles, when comparing two batteries I wonder if the one with higher [specific power]() is really more important than the one with higher C? $\endgroup$
    – uhoh
    May 6, 2019 at 6:50
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    $\begingroup$ Yeah the interesting thing for us is the usable specific energy which is basically min(specific power * burn time, specific energy). But if you throw away your batteries like the electron does you basically only care for specific power so that you can throw away a battery pack as often as possible. You don't want to take the batteries all the way to space with you. $\endgroup$
    – Christoph
    May 6, 2019 at 7:00
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    $\begingroup$ Also, cell voltage drops/cell resistance increases as the battery discharges. There's a tradeoff between discharging batteries in parallel for more power, allowing you to get more useful energy from a given battery mass, and discharging them in sequence so you can jettison some of the battery mass sooner, at the cost of jettisoning some unused capacity in the process. It's also worth noting that you may be throttling down toward the end of a burn to keep accelerations down as the propellant tanks empty, so you may not need as much power at the end of the burn. $\endgroup$
    – Christopher James Huff
    Nov 23, 2020 at 16:15

1 Answer 1

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Lithium-polymer batteries are a subtype of lithium-ion batteries, distinguished by using a solid polymer electrolyte instead of a liquid. The polymer electrolyte performs somewhat worse than the liquid electrolytes, with lower ionic conductivity leading to higher internal resistance and a greater drop in voltage under heavy load ("the physical limitations of solid electrolytes make them inherently less conductive than their liquid counterparts due to the slowed ion diffusion through the solid medium.").

However, since it provides structural support to the cell, a rigid outer casing isn't required, reducing cell mass. This improves specific energy density, and may improve specific power density despite the increased cell resistance.

Pure speculation, but lithium polymer batteries in a thin pouch form factor may also be easier to cool. They could be interspersed with heat sinks or heat exchangers, allowing operation at higher discharge rates. ARCAspace is clearly not doing this, but the fact that they're trying to use battery power for direct heating of propellant says enough about that.

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    $\begingroup$ In the vernacular, lipos have a higher discharge rate and specific energy than li-ion aka LiFe; LiFes are preferred because they're more resistant to thermal runaway (burning down your garage). But if amps per gram is what you want, not 500 recharge cycles (not even SpaceX dreams that big), then the older nickel chemistry is much better. $\endgroup$
    – Camille Goudeseune
    Dec 19, 2020 at 5:13

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