Dual Alternator for a Rocket?

[aviatoreb]

Hi Folks, I am interested to hear what the Mooney brain trust has to say.  I have a new to me M20K Rocket - a 1981 former 231.  So it has one alternator.  My normal reaction is to think about getting a backup alternator for dual alternator reliability.  But rocket has dual 35amp batteries which is a lot of backup juice right there if there were an alternator failure.  So now I am leaning to just go with that battery situation as enough backup right there by Darwin's design.  BTW, how long do you folks think those batteries would last in a pinch - obviously the exact load and load shed - but go ahead and guess.

Thanks!

[David Mazer]

Interesting you should ask.  I recently had what I thought was an alternator failure on my Rocket immediately after takeoff on a short 40 min flight.  It turned out that it was an annunciator problem that kept popping the circuit breaker.  However, I can tell you that the load, all lights and radios on since it was such a short flight and I wanted to see how fast things would deteriorate in severe VFR if it might ever happen in IMC, never dropped the voltage a single volt.  I discussed adding a backup alternator and, at the end of the discussion, all felt the battery capacity was sufficient and there wasn't any room for a standby alternator anyway.

[74657]

After 2.5 hours of flying this past week in my Missile with the field breaker pulled the voltage readout was 24.6 volts.  We were running VFR in the daytime.  All avionics were on.  I don't think you'll have any issues........

[aviatoreb]

Quote: 74657

After 2.5 hours of flying this past week in my Missile with the field breaker pulled the voltage readout was 24.6 volts.  We were running VFR in the daytime.  All avionics were on.  I don't think you'll have any issues........

[aviatoreb]

Quote: Mazerbase

Interesting you should ask.  I recently had what I thought was an alternator failure on my Rocket immediately after takeoff on a short 40 min flight.  It turned out that it was an annunciator problem that kept popping the circuit breaker.  However, I can tell you that the load, all lights and radios on since it was such a short flight and I wanted to see how fast things would deteriorate in severe VFR if it might ever happen in IMC, never dropped the voltage a single volt.  I discussed adding a backup alternator and, at the end of the discussion, all felt the battery capacity was sufficient and there wasn't any room for a standby alternator anyway.

[Piloto]

With two batteries I wouldn't bother putting a second alternator unless you want redundancy for dispatchability. I had an alternator failure over the North Atlantic and after I shut down non-essential equipment the battery had plenty of juice to lower the gear after 7 hours of flying with no alternator. The benefit of the second alternator is being able to dispatch with a failed alternator. Alternators are highly reliable and is rare when they fail. They have proven themself in millions of cars.

José

[aviatoreb]

Quote: Piloto

With two batteries I wouldn't bother putting a second alternator unless you want redundancy for dispatchability. I had an alternator failure over the North Atlantic and after I shut down non-essential equipment the battery had plenty of juice to lower the gear after 7 hours of flying with no alternator. The benefit of the second alternator is being able to dispatch with a failed alternator. Alternators are highly reliable and is rare when they fail. They have proven themself in millions of cars.

José

[David Mazer]

You aren't filthy rich?  I have to get a new crowd to run in!

[RJBrown]

I had an alternator failure in a Rocket IFR out of Jackson Hole heading to Spokane. The ring gear that drives the alternator is secured by 6 bolts that are fixed with bend tabs to keep them from backing out. The factory (Continental) forgot to bend the tabs and the bolts backed out at 80 hours. Chewed up the alternator and sent garbage into the engine. I continued to Lewiston Id where Rocket brought me a mechanic and an alternator. Once the culprit was found we removed the bad alternator and ferried it up to Felts Field. Batteries were strong as can be the whole way. Including the restart in Idaho. Quite a bit of backup in those two batteries.

[Piloto]

BTW the dual battery configuration found on the Rocket and the long body Mooney is not that much for backup but for ballast. The addition of the heavier Continental 520 and Lyc 540 required the added ballast to keep the CG within range. Instead of adding dead weight Mooney and Rocket decided on some useful ballast.

José 

[Barry]

Is there an STC to put dual batteries in "lesser" Mooney's ?? I had a failure in IMC on my trip to Boulder, Co the other week. A 2nd battery would be worth the weight in any aircraft. If so, who would hold the STC ??

[aviatoreb]

Quote: Mazerbase

You aren't filthy rich?  I have to get a new crowd to run in!

[aviatoreb]

Quote: Piloto

BTW the dual battery configuration found on the Rocket and the long body Mooney is not that much for backup but for ballast. The addition of the heavier Continental 520 and Lyc 540 required the added ballast to keep the CG within range. Instead of adding dead weight Mooney and Rocket decided on some useful ballast.

José 

[aviatoreb]

Quote: Barry

Is there an STC to put dual batteries in "lesser" Mooney's ?? I had a failure in IMC on my trip to Boulder, Co the other week. A 2nd battery would be worth the weight in any aircraft. If so, who would hold the STC ??