US Air Force LRS-B Bomber Thread


TerraN_EmpirE

Tyrant King
What is the reason that B21 is being considered smaller than the B2?

If the engine is based off F135 like F118 off the F110, then dry thrust is likely to be close to 15 tons *2 which would put the power on tap equal to the B2.
I have never seen the engine listed. So assuming it’s payload based off F135 is baseless. The initial requirement was for a regional bomber that moved up to a half B2.
 
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Bltizo

Lieutenant General
Staff member
Super Moderator
What is the reason that B21 is being considered smaller than the B2?

If the engine is based off F135 like F118 off the F110, then dry thrust is likely to be close to 15 tons *2 which would put the power on tap equal to the B2.
It depends specifically on how much thrust the F135 variant for B-21 has.

F118-100 for B-2 had 19,000 lbf each (
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), about 14.4% more thrust than what F110 had for dry thrust.
Four F118-100s provide (19,000 x 4) = 76,000 lbf of thrust total.

F-135's dry thrust is 28,000 lbf of dry thrust. Assuming they uprate it for dry thrust by 14.4% as well (for the sake of discussion) you get 32,032 lbf of thrust.
B-21 by our understanding is meant to be powered by two of these F135 dry thrust engines, giving us some 64,000 lbf of thrust total.


As far as the assumption that B-21 will be a little bit smaller than B-2 goes, the slightly lower total thrust that the B-21 will likely have at its disposal vs what B-2 has is definitely a factor.
The USAF's public announcements of what the LRS-B (which became B-21) and the consistent noises made about how it would be slightly smaller than B-2, are also a factor as well.
 

Bhurki

Junior Member
Registered Member
Could as I if they chose those engine cores as the basis of the power plants.
The reasons I think they are going to chose it -

1) Rep. Wittman, on atleast two separate occasions, has stated that efforts are made to derisk the schedule by using mature tech from other platforms.

2) He also defines a 'hiccup' between the chosen engine supplier P&W and the airflow ducting team due to constraints provided by the former to the latter, while stating "This is a very, very different design as far as airflow". Normally, you'd like to have atleast one of variables to be reliable and predictable if you're taking chances with the other.

3) The only turbofan in production at P&W with adequate military thrust to be able to power the B21 in twin engine configuration is F135, so its logical to use the same core turbine, while augmenting the thrust with a higher rated fan.
 

Bhurki

Junior Member
Registered Member
As far as the assumption that B-21 will be a little bit smaller than B-2 goes, the slightly lower total thrust that the B-21 will likely have at its disposal
An Mtow of 120 tons seems like the ballpark.
Empty weight and fuel capacity being 50-55 tons each and a payload of 15 tons.

Even though the fuel load is 20% lesser than B2, the increase in SFC from F118 to the engine B21 uses should certainly makeup for it, providing equivalent range.
Comparing Dry sfc of engines these are derived from..
F110 - 21 mg/Ns
F119 - 17 mg/Ns
which is about 24% increase, and F135 is considered even more efficient.
 

anzha

Junior Member
Registered Member
which is about 24% increase, and F135 is considered even more efficient.
There are rumors - relatively good ones, but always take with a huge grain of salt even so - that the first platform for the AETP engine program was the B-21. The P&W version was based on a F135 core.
 

Bltizo

Lieutenant General
Staff member
Super Moderator
An Mtow of 120 tons seems like the ballpark.
Empty weight and fuel capacity being 50-55 tons each and a payload of 15 tons.

Even though the fuel load is 20% lesser than B2, the increase in SFC from F118 to the engine B21 uses should certainly makeup for it, providing equivalent range.
Comparing Dry sfc of engines these are derived from..
F110 - 21 mg/Ns
F119 - 17 mg/Ns
which is about 24% increase, and F135 is considered even more efficient.
I'm only giving you an explanation for why B-21 is likely to be smaller than B-2, I'm not talking about the range or capability of one versus the other.
 

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