Northrop Grumman B-21 Raider (LRS-B)

Jacky said:
If this is the B-21's APU exhaust duct, why isn't it designed to be a closable door like the B-2's? Will the exhaust duct shown in the picture affect the overall stealth performance?
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The F-35’s Integrated Power Pack functions as an APU as well as the compressor for the environmental control system (ECS), it has a permanent opening on the F-35’s tail boom because even if it is not working in APU mode, the ECS mode is powered by the engine bleed air and needs to dump the bleed air overboard once it is used. If the B-21 uses a similar architecture then it is quite possible for it to have permeant openings for the APU/IPP.

https://www.lockheedmartin.com/cont...webt/F-35_Air_Vehicle_Technology_Overview.pdf
 
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BDF said:
Any consensus on what those apertures are on along the bottom of the nose and lower fuselage? I'm guessing part of the air data system but seems odd that they have so many for that. I noticed there are 4 more on the trailing edge where the wing joins with the fuselage. They all look identical or nearly so. Really curious as overall, assuming they're all the same thing, that would seem excessive for ADS or IR sensors.
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If you are talking about those black diamond-shaped apertures, they are on similar positions and of similar arrangement as the B-2’s air data sensors… They may or may not be the same sensors as the ones on the trailing edge.
 
JoshuaH said:
The F-35’s Integrated Power Pack functions as an APU as well as the compressor for the environmental control system (ECS), it has a permanent opening on the F-35’s tail boom because even if it is not working in APU mode, the ECS mode is powered by the engine bleed air and needs to dump the bleed air overboard once it is used. If the B-21 uses a similar architecture then it is quite possible for it to have permeant openings for the APU/IPP.

https://www.lockheedmartin.com/cont...webt/F-35_Air_Vehicle_Technology_Overview.pdf
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In the traditional design, APU and ECS are two independent systems. APU needs a hot exhaust duct and ECS heat exchanger needs an exhaust duct. The PTMS of the F-35 integrates the functions of APU and ECS, with only one exhaust duct. These exhaust will involve infrared signal control. In the infrared picture of F-22, the infrared signal of ECS exhaust duct on the top side of the fuselage is obvious, second only to the engine nozzle. In the infrared video of B-2, no other hot signals were found except the hot signals of the engine nozzle. I guess the ECS exhaust duct of B-2 is integrated into the engine exhaust system. The question is, why didn't the B-21 adopt a similar design to control the infrared signal?
 
Jacky said:
In the traditional design, APU and ECS are two independent systems. APU needs a hot exhaust duct and ECS heat exchanger needs an exhaust duct. The PTMS of the F-35 integrates the functions of APU and ECS, with only one exhaust duct. These exhaust will involve infrared signal control. In the infrared picture of F-22, the infrared signal of ECS exhaust duct on the top side of the fuselage is obvious, second only to the engine nozzle. In the infrared video of B-2, no other hot signals were found except the hot signals of the engine nozzle. I guess the ECS exhaust duct of B-2 is integrated into the engine exhaust system. The question is, why didn't the B-21 adopt a similar design to control the infrared signal?
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The F-35 ECS dumps its heat in the fan duct heat exchanger (circled in red) of the F135 engine. Its IPP exhaust duct is used to dump the power turbine exhaust from engine bleed air (circled in blue), so it may or may not be as hot as the heat exchanger exhaust… IMG_2228.jpeg
 
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JoshuaH said:
The F-35 ECS dumps its exhaust in the fan duct heat exchanger (circled in red) of the F135 engine. Its IPP exhaust is used as the power turbine exhaust from engine bleed air (circled in blue), so it may or may not be as hot as the heat exchanger exhaust…
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The exhaust temperature of CM, of course, is lower than the the exhaust temperature of Self-Start Mode(SSM). Because no fuel is burned in the combustor.
 
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Jacky said:
No, I think you are wrong, please read paper "Next Generation Power and Thermal Management System" that written by Simon Yu and Evgeni Ganev( from Honeywell International).

In the Cooling Mode(CM), PTMS is powered by bleed air from F135. The bleed air drives the power turbine(PT) and is then exhausted from the fuselage. The compressor(C) inlet is closed from ambient air and compressed air is cooled by the fan duct heat exchanger #2. It is further cooled by expansion through the cooling turbine(CT).

The exhaust temperature of CM, of course, is lower than the the exhaust temperature of Self-Start Mode(SSM). Because no fuel is burned in the combustor.
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I think we are describing the same thing...? I don't see a deviation between our descriptions.
 
TomcatViP said:
@Jacky : it's probably just for the test aircraft that have a greater elecrical load on their system from the test equipment.
The radar coating seems also to have been pulled out to prevent damage with the hot flux of exhaust and the doors that seems to be designed to go there are removed to ease cooling (it could also be a guillotine door that remain half open).
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From the photo, the exhaust duct is side direction rather than the vertical direction. During the operation of the APU, During the operation of APU, I think the nearby fuselage will still be affected by hot exhaust.
 
JoshuaH said:
If you are talking about those black diamond-shaped apertures, they are on similar positions and of similar arrangement as the B-2’s air data sensors… They may or may not be the same sensors as the ones on the trailing edge.
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Yeah. They are probably air data sensors. They throw me off because they are much more spread out (the B-2s are all grouped together) and outwardly don't look anything like the B-2's ADS' which isn't surprising given the generational difference. Its' interesting that there are 3 more near the trailing edge as well.
 
Probably a bit off topic, is there already a good model kit or preferably even a high quality resin/diecast model of the B-21?
 
Boeing demonstrated a conformal load bearing antenna in a 737 over 15 years ago as part of a ARFL project. If such tech is not in B-21 Incr 1 it’s bc it wasn’t mature enough for the Raider Programs rough TRL requirements.

CLAS would be great for F-47, theoretically, especially if you didn’t need wideband capability and were happy with LPI/D X band for example. Maybe S or L band goes into another aircraft or a discrete array. Who knows!
 
@donnage99 That could also be seams in the leading edge radar absorbing structure though...I would consider the trapezoidal panels underneath more likely to be RF apertures.
 
4decaa said:
EMRIS-shaped panels are visible on both test articles. I would be surprised if they did not have EMRIS installed on the second article.
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Actually this is very confusing to me... One of the points of having multifunctional ultra wide band arrays is to reduce the number of apertures (the other points include better LPI capabilities due to the opponent's ESM system having to look through huge bandwidths to fish for the signal and better resolution due to the common aperture being larger than the individual apertures before), but the B-21 seems to have at least four 2D arrays and three 1D arrays on each side...? The ASQ-239 EW system on the F-35 only has the primary radar multifunctional array (APG-81), two arrays on each side for band 3 and 4 (presumably for long baseline interferometry), and one array on each side for band 2 (probably shared with the ASQ-240 CNI system), I was fully expecting the B-21 to have one giant array on each side for everything RF...
d4su190pirof1 copy.jpeg f35_sensors_wide.png
 
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JoshuaH said:
Actually this is very confusing to me... One of the points of having multifunctional ultra wide band arrays is to reduce the number of apertures (the other points include better LPI capabilities due to the opponent's ESM system having to look through huge bandwidths to fish for the signal and better resolution due to the common aperture being larger than the individual apertures before), but the B-21 seems to have at least four 2D arrays and three 1D arrays on each side...? The ASQ-239 EW system on the F-35 only has the primary radar multifunctional array (APG-81), two arrays on each side for band 3 and 4 (presumably for long baseline interferometry), and one array on each side for band 2 (probably shared with the ASQ-240 CNI system), I was fully expecting the B-21 to have one giant array on each side for everything RF...
View attachment 784913View attachment 784914
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Agreed - that is a lot of extra paneling and avionics that needs to be dealt with in the case of this amount of apetures in the front. I wonder what the reasons for this choice may be. Space is clearly not an issue, and there would have been no issue fitting 2 (huge) arrays in the front. Attached is what I guess may be EMRIS, installed in a spot that gives the B-21 an additional aspect of coverage for EW/communication.

also... note how the T-4's aftmost exhaust panel has a different LE serration from the one in the photo below.
1757995975164.png
 
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4decaa said:
Agreed - that is a lot of extra paneling and avionics that needs to be dealt with in the case of this amount of apetures in the front. I wonder what the reasons for this choice may be. Space is clearly not an issue, and there would have been no issue fitting 2 (huge) arrays in the front. Attached is what I guess may be EMRIS, installed in a spot that gives the B-21 an additional aspect of coverage for EW/communication.

also... note how the T-4's aftmost exhaust panel has a different paneling from the one in the photo below.
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I would expect at least the Ku-band communication capability to be fully spherical, as it probably has to at least be compatible with the MADL datalink on the F-35. Satcom probably uses Ku-band as well.
 
By the way the large (and different) trapezoidal aperture on the side seems to have the same length and distribution of letters with the (presumed) air data sensors nearby... The warning message on the B-2's air data sensors were "WARNING HOT", which also matches the approximate distribution. I think someone in this thread mentioned the possibility of the IR aperture being covered by a mesh like the F-117 (I could not find the original quote, maybe I misremembered). It might be possible that this trapezoidal aperture is mesh-covered and also requires heating to avoid possible ice buildup...? Screenshot 2025-09-15 at 10.58.27 PM.png E0ED32264075E0604370950FA14C5A50.jpg
 
JoshuaH said:
Actually this is very confusing to me... One of the points of having multifunctional ultra wide band arrays is to reduce the number of apertures (the other points include better LPI capabilities due to the opponent's ESM system having to look through huge bandwidths to fish for the signal and better resolution due to the common aperture being larger than the individual apertures before), but the B-21 seems to have at least four 2D arrays and three 1D arrays on each side...? The ASQ-239 EW system on the F-35 only has the primary radar multifunctional array (APG-81), two arrays on each side for band 3 and 4 (presumably for long baseline interferometry), and one array on each side for band 2 (probably shared with the ASQ-240 CNI system), I was fully expecting the B-21 to have one giant array on each side for everything RF...
View attachment 784913View attachment 784914
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Splitting up the arrays to narrow down the frequencies range allowed by FSS. Narrowing down the frequency window helps make practical for the use of other techniques to control RCS (possibly metamaterials, RAS, passive cancelling etc). At least that was true with the F-22.
 
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You have plenty of photos from other angles to understand that there's no any meshed aperture there. And why do you think it's WARNING HOT and not DO NOT PLUG?
 
flateric said:
You have plenty of photos from other angles to understand that there's no any meshed aperture there. And why do you think it's WARNING HOT and not DO NOT PLUG?
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Right, my bad then... As of the latter part, because it is hard to imagine plugging up a huge trapezoid array...?
 
It's just too low res to judge what is written there, that's the point. You can presume that heated areas are bare metal.
 
NMaude said:
Now that the second B-21 is flying no doubt that will greatly accelerate the test-schedule.
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They said in the press release that the second test aircraft begins the mission avionics and weapons testing, so approximately the job description for the third and fourth B-2 prototypes. Also explains why the second B-21 test aircraft took so long to fly…
 

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