From: snidely.too@gmail.com   
      
   Torbjorn Lindgren suggested that ...   
   > Snidely   wrote:   
   >> It seems Rocket Lab will not be doing this battery swap on Neutron.   
   >   
   > Not really surprising, one of the reasons that batteries makes sense   
   > on Electron is because it's a small rocket and turbo-pumps doesn't   
   > really scale down that well while electric motors and batteries do. It   
   > also let them avoid having to design the turbo pump which is not a   
   > trivial exercise which likely helped with time-to-market.   
   >   
   > While there's other methods that can be used for small engines but   
   > they all come with downside so it comes down to weighing (pun   
   > intended) the downsides of the various approaches.   
   >   
   > I believe it was the right call for both weight and complexity for a   
   > rocket of Electron's size (and likely got them running earlier) but   
   > note that the only other electric pump engine so far is only very   
   > marginally larger (Astra's Delphin engine at 29kN sealevel thrust vs   
   > Rutherford's 25kN).   
   >   
   > Rocketlab's Archimedes engine is supposed to have 730 kN sealevel   
   > thrust and 890 kN vacuum thrust (vs 26kN), IE 29-34 times more   
   > powerful - as I understand it the battery pack required would likely   
   > weight many times more than the turbo pumps at this scale which could   
   > easily eat into the payload.   
   >   
   > As mentioned designing a turbo-pump definitely isn't trivial but... in   
   > this size class it's probably not avoidable if you want a rocket that   
   > can compete! And it seems Rocketlab came to a similar conclusion given   
   > the likely time-to-market advantage of electric pumps.   
   >   
   > Exactly how big a rocket engine can get before battery weight becomes   
   > a BIG problem is hard to say for an outsider and will vary depending   
   > on specific requirements (including development time) but I think   
   > there's good reason to believe it currently is well below the size of   
   > Neutron/Archimedes and will stay that way for the foreseable future.   
   >   
   > I am a bit surprised we haven't seen more electric pump based upper   
   > stage engines yet, I think they could make more sense there than for   
   > big lower stage engines (higher Isp than open cycle which is the usual   
   > other alternative). They may well be coming, it takes a long time to   
   > design new engines.   
   >   
   >   
   >> The  Archimedes engine is a staged combustion methalox engine, used on   
   >> both stages.   
   >   
   > If you're designing a new reusable rocket and new rocket engine it   
   > probably makes sense to go for methalox even if SpaceX has shown that   
   > it is in fact possible to do with kerolox though that is a   
   > gas-generator engine which likely helps with that.   
      
   Good summary.   
      
   /dps   
      
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   You could try being nicer and politer   
   > instead, and see how that works out.   
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