From: tl@none.invalid   
      
   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.   
      
   --- SoupGate-Win32 v1.05   
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