Rocket pod warning lights

My understanding of the pylon presence warning lights is that they should remain on even if all rockets are expended. After all the UB-16 or UB-32 pod itself is still present. Currently the store on pylon presence warning light goes out the moment all rockets are expended even if the pod remains on the pylon.

The "rocket zero position" for the UB-32 pod should illuminate when there are 16 and 0 rockets remaining. It goes around twice to fire the "double size" pod as a bit of a hack. The UB-16 illuminates when 0 are remaining.

And I'm not sure but it's possible that rockets are fired up to the amount allowed but will cease firing early if released in reality. Currently the DCS module fires the indicated amount regardless of the release button pressed or not.

In a related question are the wing and ventral drop tanks empty warning lights only supposed to be illuminated when those tanks are empty and the tanks are present? It's an unusual idea but perhaps (or not) when the tanks are absent entirely the warning is not illuminated.

Well, no. Those lights are not for show pods presence, they showing readiness of rocket firing device ( PUS) . After fire sequence device is reseting itself to "0" position which is start position, but for reset is needed rocket presence in pod, no rockets left - no reset signal, lights are off. PUS device for UB-32 is modified, have 32 fire slots, those for UB-16 have 16 slots. Generally no weapon presence signalization on any MiG-21 variant, you have to remmember what is under your's wings.

Pods don't will cease firing because, by pressing fire button you starting fire sequence, PUS is kind of watch like electro-mechanismus and already started fire sequence cannot be stopped.

I'm not sure about drop tanks,but I think "empty" lights should go off when tanks are dropped.

There are two lights with example UB-16 and S-5 loaded station 1

A No.1 RKT ZERO PSN

B INBD STA 1

When switches ready to fire A not illuminated, B illuminated yes? After shooting all rockets A becomes illuminated. Is B illuminated with empty pod or no? I do not know when "B" light is shining and not. Anything present? Only missiles? Only bombs? Only when 1-2 or 3-4 armament switches on?

You say for UB-32 that A light comes on only when all 32 rockets are fired? Old English manual says zero position light comes on for 16 remaining and 0 remaining. But I believe that improvement might be made some time to equipment so light only illuminates for 0 remaining.

If there is latching behavior for rocket salvo why does English manual say (only for AA rocket engagement!) to always hold down launch button for 1-1.5 seconds for 16 rockets to be fired? Is it because of anti-flameout protection?

Empty tank lights I don't know. Are they on when tank was never loaded at any time? Are they on when tank present and empty? Are they on when tank was loaded, later jettisoned?

I think that Frederf is right. It should work the same for SU-25 and L-39ZA, whereas in DCS it does not. The WCS in philosophy is that you have two indicators. One is for indication of weight on a pylon and one as the weapon ready indication. Upper light means that there is weight on the pylon (yellow light in Su-25, upper square light on L-39ZA) and if you spend all rockets from an UB pod, only the weapon ready light should go out, but you should have an indication that there is something hanging on the pylon. Only after jettisoning the pod, the weight on a pylon light should go off. Currently in all aircraft both lights go out when you spend rockets from an UB pod, which is incorrect behavior.

When switches ready to fire A not illuminated, B illuminated yes? After shooting all rockets A becomes illuminated. Is B illuminated with empty pod or no? I do not know when "B" light is shining and not. Anything present? Only missiles? Only bombs? Only when 1-2 or 3-4 armament switches on?

Ok, I digged deeper in manuals about things we talking and seems that true is somewhere in the middle.

PUS-36-86 is able sent 36 electric impulses but in MiG-21 only 16 are active (probably because for many years biggest pod was UB-16 with 16 rockets). Whole impulse sending sequence have 16 steps (time beetwen impulses 20-30 ms, impulse time 5-17 ms). PUS is triggered by weapon release button, on the beginning movable connector is on "0" position (light "0" position in cockpit is on). After pressing fire button, connector is start moving (light "0" position goes off) and closing one by one 16 independent electrical circuits (at end of every circuit is located rocket engine ignitor), when sequence end, movable connector is back to "0" position and light "0" position is on again.

When pilot will choose firing 4 or 8 rockets, movable connector will stop after sending 4 or 8 impulses, light "0" will off during that time and will stay off until whole 16 impulses sequence will be done. So pilot have to press fire button 4 or 2 times to get lights "0" on again.

You say for UB-32 that A light comes on only when all 32 rockets are fired? Old English manual says zero position light comes on for 16 remaining and 0 remaining. But I believe that improvement might be made some time to equipment so light only illuminates for 0 remaining.

UB-32 came to service in 1973, when already MiG-21Bis was in production, as a upgrade. What was typical for Soviets, they wanted get new capabilities as cheap and simple as possible. So aircraft's systems stayed as they was, just inside UB-32 was mounted switch box. When PUS finished sequence of 16 impulses , switch box was changing barrels set from 1-16 to 17-32 and after reseting to "0" position PUS was able to ignite next set of 16 rockets. So wasn't possible fire whole 32 rockets in one salvo.Light "0" was illuminated on the beginnig, after rocket number 16 and after fire all rockets. PUS is ready to work after swithing on switch "PUSK SS,RS,RNS" and light "0" will on, whatever pylons are empty or not, pod is loaded or not, because those lights signalizing only PUS sequence start position.

If there is latching behavior for rocket salvo why does English manual say (only for AA rocket engagement!) to always hold down launch button for 1-1.5 seconds for 16 rockets to be fired? Is it because of anti-flameout protection?

1,5 sec delay is only for safety purpose in order to avoid accidental fire. Why only in A-A mode?, probably 1,5 sec is too much time when you attacking ground target in dive.

Now lights B, yep I forgot that they exist. Those light shows that bomb lock inside pylon is closed and secured. In order to hang something under the pylon lock need to be closed, so light on means that something is hanging there (altough is possible close lock without any load and then lights will on as well). Light are just on, when lock is closed, it's not possible show where are bombs or pods. No distinguishing what kind of weapon is hang, and no matter what position have weapon selector.

When fuel tank full and present - one light (lock closed), when empty and present - two lights ( empty and lock closed), when dropped - no lights.

OK, zero position light when 0 16 32 rockets remain and "Missiles - Rockets Launch" switch on. Right now it's only 0 and any armament switch makes no change.

I understand the idea of a movable firing circuit with 17 positions, 16 for firing and 1 safe when full or empty. I also understand UB-32 is a special case that has to run through the circuit twice to fire all. If UB-32 is attempted to fire the quantity 8 then 16 then 8 will the second salvo fire 16 rockets or 8? Must it always pause the sequence at the middle position until the firing button is pressed again?

Also "position zero" light is coming on when AA missiles, bombs, and fuel tanks are released. Lights "A" and "B" are acting simply "there is something useful here" A-off, B-on and then when absent A-on, B-off. Drop fuel tank? A-on, B-off. Launch AA missile? A-on, B-off. Fire all S-5 or S-24? A-on, B-off.

Interesting UPK-23 pods are not showing as present on pylon with "B" type light nor "A" type light even shining related to it's ammo. I think correct is "B" type light unless pod is jettisoned and "A" never.

Is "Zero Position" light ever used for anything but UB-16/UB-32? S-24, AA missile, UPK, bomb, etc?

And fuel tank warning lights are behaving funny. Even DCS with absent=empty if you bring 3x490L tanks and jettison them when still full it is impossible to have "ventral" light come on and "wing" light off or the reverse. Three tanks, jettison wing tank, no lights on. Three tanks, jettison center tank, no lights on. Jettison wing and center only then are both empty lights coming on.

OK, zero position light when 0 16 32 rockets remain and "Missiles - Rockets Launch" switch on. Right now it's only 0 and any armament switch makes no change.

 

I understand the idea of a movable firing circuit with 17 positions, 16 for firing and 1 safe when full or empty. I also understand UB-32 is a special case that has to run through the circuit twice to fire all. If UB-32 is attempted to fire the quantity 8 then 16 then 8 will the second salvo fire 16 rockets or 8? Must it always pause the sequence at the middle position until the firing button is pressed again?

Will fire 8 rockets, because max number of rockets in one sequence is 16, so if first salvo will 8, then only 8 was left before PUS reset to "0", if first salvo was 4 rockets, then 12 impulses in sequence left.

It's seems that PUS is powered always, whatever is position of weapon panel switch or any ASP switch, because all power circuits are powered by one switch on the right cockpit's side ( one switch for PUS, AA missiles fire units, and S-24 fire unit). But PUS can be triggered only in one case: weapon selector on S-5 position and ASP switch on "Rockets launch". So seems that light "A" should be always on, whatever hanging under pylon, "off" only during fire sequence.Yes sequence will always paused somewhere in middle, because signal for reseting to "0" position is only from connector number 16.

Also "position zero" light is coming on when AA missiles, bombs, and fuel tanks are released. Lights "A" and "B" are acting simply "there is something useful here" A-off, B-on and then when absent A-on, B-off. Drop fuel tank? A-on, B-off. Launch AA missile? A-on, B-off. Fire all S-5 or S-24? A-on, B-off.

Light "B" is acting as "something is there", no connection between light "A" and "B", they working totally separatelly.

Interesting UPK-23 pods are not showing as present on pylon with "B" type light nor "A" type light even shining related to it's ammo. I think correct is "B" type light unless pod is jettisoned and "A" never.

It's hard to comment UPK-23-250, because RL aircraft never used them ( except some trials ).

Is "Zero Position" light ever used for anything but UB-16/UB-32? S-24, AA missile, UPK, bomb, etc?

PUS is used only with UB-16/32 launchers, all others kinds of weapon have own fire circuits.

And fuel tank warning lights are behaving funny. Even DCS with absent=empty if you bring 3x490L tanks and jettison them when still full it is impossible to have "ventral" light come on and "wing" light off or the reverse. Three tanks, jettison wing tank, no lights on. Three tanks, jettison center tank, no lights on. Jettison wing and center only then are both empty lights coming on.

In my opinion most systems in DCS MiG-21 works funny. Generally signalization "Empty tank" shouldy be on only when empty tank is present, and......"on" when full/or half full fuel tank was dropped. Why?, because lamp is triggered by pressure sensor in fuel pomp, fuel pressure dropped - light "on", fuel tank empty - no pressure, fuel tank dropped - no pressure.

Page from manual:

Please forgive the never ending questions.

I see manual gives instruction for ground check "after external power source has been connected, turn on the MSL, RGM, RKT LCH and PWR TO RKT PODS, M-SH, RACKS 1-3 circuit breakers and make sure that the No. 1 RKT ZERO PSN, No. 2 RKT ZERO PSN, No. 3 RKT ZERO PSN, and No. 4 RKT ZERO PSN lights have come in (in compliance with the variant of the armament suspension and loading), then cut out the MSL, RGM, RKT LCH and PWR TO RKT PODS, M-SH, RACKS 1-2 circuit breakers."

• ZERO PSN lights are not being shown if MSL, RGM, RKT LCH switch is off because this controls PUS power, correct?
  
• Why is only pylon 1-2 switch set on? What if rockets are carried on 3-4 as well? Is it because it is assumed that rockets are never carried on 3-4? Is pylon 1-2 switch necessary to check ZERO PSN signal?
  

Theory, ZERO PSN light on only when:

• UB pod is present
  
• PUS power on
  
• UB/PUS firing position before first or after last rocket
  

Theory:

• Flights without S-5 rocket light is never on
  
• If S-5 rocket pod is jettisoned ZERO PSN light turns off
  

Also if inboard rockets are less than the amount selected to fire and there are more rockets on outboard station the weapon button must be pressed again to continue firing outboard rockets. If inboard holds 4 rockets and 8 or 16 rockets are commanded by weapon select switch only 4 inboard rockets will be fired on the next button press. "automatic switchover will take place after all rockets have been fired from the inboard pods and once the firing button is depressed another time." I was going to ask but I see answer in manual. I didn't check DCS.

DCS behavior that firing AA guided missile or S-24 or bomb having change of "zero position" light is incorrect.

I don't understand exactly your meaning. What do you estimate signal is on or off?

1. Fuel tank carried not empty
   
2. Fuel tank never installed
   
3. Fuel tank carried empty
   
4. Fuel tank discarded not empty
   
5. Fuel tank discarded empty
   

Of course signal is off for case 1. Do you think signal is on for cases 2-5? Only case 3-5? I understand pressure when path for fuel is suddenly allowing air when present and empty but I don't know how this behaves in other cases (tank is jettisoned, tank never installed).

Please forgive the never ending questions.

 

I see manual gives instruction for ground check "after external power source has been connected, turn on the MSL, RGM, RKT LCH and PWR TO RKT PODS, M-SH, RACKS 1-3 circuit breakers and make sure that the No. 1 RKT ZERO PSN, No. 2 RKT ZERO PSN, No. 3 RKT ZERO PSN, and No. 4 RKT ZERO PSN lights have come in (in compliance with the variant of the armament suspension and loading), then cut out the MSL, RGM, RKT LCH and PWR TO RKT PODS, M-SH, RACKS 1-2 circuit breakers."

 

 

Why is only pylon 1-2 switch set on? What if rockets are carried on 3-4 as well? Is it because it is assumed that rockets are never carried on 3-4? Is pylon 1-2 switch necessary to check ZERO PSN signal?

Because this is procedure for connections check, UB-32 needs extra power for sequence switch box, external pylons are only for UB-16 and those pods don't need extra power.

ZERO PSN lights are not being shown if MSL, RGM, RKT LCH switch is off because this controls PUS power, correct?

Yes "MSL,RGM,RKT LCH" switch is for power PUS device.

Theory, ZERO PSN light on only when:

 

• UB pod is present
  
• PUS power on
  
• UB/PUS firing position before first or after last rocket
  

ZERO PSN light is on when power swith is on and PUS is in "0" position. Presence of any kind of weapon under pylon have nothing to do here.

Manual is stating: ZERO PSN light not shows UB pod presence, and don't shows S-5 presence inside pod.

Theory:

 

 

• Flights without S-5 rocket light is never on
  
• If S-5 rocket pod is jettisoned ZERO PSN light turns off
  

Light is always on, if PUS is in "0" position, whatever UB launcher is hang or not, rockets are in launcher or not. PUS device circuit, and bomb lock circuit are two separate circuits and no connections between them. Don't complicate simple things: ZERO PSN light shows ONLY PUS readiness.

Also if inboard rockets are less than the amount selected to fire and there are more rockets on outboard station the weapon button must be pressed again to continue firing outboard rockets. If inboard holds 4 rockets and 8 or 16 rockets are commanded by weapon select switch only 4 inboard rockets will be fired on the next button press. "automatic switchover will take place after all rockets have been fired from the inboard pods and once the firing button is depressed another time." I was going to ask but I see answer in manual. I didn't check DCS.

Yes, aircraft have special relay box for switching between internal pods and external, and seems that is imposible to shot salvo from all four pylons. First rockets from internals pylons and then from externals. So, if you have full S-5 load (96 rockets) and you will set weapon switch for position 16, sequence will look: 32 rockets from internal pylons, then 32 again from internal pylons and then 32 from external pylons.

DCS behavior that firing AA guided missile or S-24 or bomb having change of "zero position" light is incorrect.

Yes, because PUS are only for S-5.

I don't understand exactly your meaning. What do you estimate signal is on or off?

 

1. Fuel tank carried not empty
   
2. Fuel tank never installed
   
3. Fuel tank carried empty
   
4. Fuel tank discarded not empty
   
5. Fuel tank discarded empty
   

Of course signal is off for case 1. Do you think signal is on for cases 2-5? Only case 3-5? I understand pressure when path for fuel is suddenly allowing air when present and empty but I don't know how this behaves in other cases (tank is jettisoned, tank never installed).

Every group of tanks have own fuel flow pressure sensor, according manuals in case of fuel pressure drop, sensor is closing cockpit's lights circuits. Sensor measuring only fuel flow pressure, don't taking into account any others factors. So in my opinion is not important what is reason of fuel pressure drop ( fuel exhaustion, tanks drop ). No fuel flow - light are on.

OK "1-2 pylon power" because this includes control box for UB-32 which is connected to 1-2 pylon power. Is procedure to turn 1-2 pylon power on for check without mentioning if UB-16 or UB-32 is carried because this power is always needed even without UB-32 or simply to make a single procedure for all cases?

I believe you about PUS-0 position regardless of pod situation but I wanted to exhaust possibility that engineer made some circuit which avoids this light for missions without rocket pod. For me this makes sense as a perfect design but I am convinced now that PUS-0 lights will simply shine for missions without rocket pods. For such missions when PUS power is on the zero position is just a constant signal.

OK, fuel tank empty lights are on except when there are present and have fuel. What I find troubling is that if 3 tanks are carried and center or wing is dropped that single light does not come on. Only if all tanks are dropped does both lights come on together. If fuel is consumed normally lights come on at slightly different times.

For me I wish MiG-21bis would consume first wing tanks then center tank instead of all three together but I guess 3 tank is only for transit mission without combat.

OK "1-2 pylon power" because this includes control box for UB-32 which is connected to 1-2 pylon power. Is procedure to turn 1-2 pylon power on for check without mentioning if UB-16 or UB-32 is carried because this power is always needed even without UB-32 or simply to make a single procedure for all cases?

I forgot write about one thing more: PUS devices from external pylons are connected parallel with internals ones, so maybe that's why during check switching on circuits of pylons 1-2 is enough.

I believe you about PUS-0 position regardless of pod situation but I wanted to exhaust possibility that engineer made some circuit which avoids this light for missions without rocket pod. For me this makes sense as a perfect design but I am convinced now that PUS-0 lights will simply shine for missions without rocket pods. For such missions when PUS power is on the zero position is just a constant signal.

I know this looks bit weird those lights always on, but.....well, when you flight with bombs only or with fuel tanks only, RKT PSN lights can be off, because you don't need "MSL,RGM,RKT LCH" switch on.:thumbup:

OK, fuel tank empty lights are on except when there are present and have fuel. What I find troubling is that if 3 tanks are carried and center or wing is dropped that single light does not come on. Only if all tanks are dropped does both lights come on together. If fuel is consumed normally lights come on at slightly different times.

 

For me I wish MiG-21bis would consume first wing tanks then center tank instead of all three together but I guess 3 tank is only for transit mission without combat.

I think, this depends from fuel consumption sequence. According manual: first 100 liters from tank 7, then all fuel from wing drop tanks, then 100 l from tank 7, then all from under fuselage drop tank, then 100 l again from tank 7 and so on.

Yet of course bombs will not release from DCS airplane if LCH switch is off.

Fuel tank is notably different from DCS. Current behavior for 490Lx3 tank is:

start 4425L

wing tank light blinking 3250L

wing tank light solid 3100L

fuselage tank blinking 2950L

fuselage tank solid 2925L

It is not feeding first from wing and then center but almost together. Difference between wing tanks empty and fuselage tank empty should be consumption of 100+480L. As seen above it is 175.

Initial fuel gauge to set (by real manual)

Internal 2750

Center 3250

Wing 3750

Triple 4200

Fuel at lights (by real manual)

Center, center empty 2700-2500 (personal estimate 2550)

Wing, wing empty 2700-2500 (personal estimate 2650)

Triple, wing empty 3200-3000 (personal estimate 2650)

When wings and fuselage tank carried to wing empty signal 490L should remain in fuselage tank and about 2650 internal.

This suggests that at wing or fuselage or wing & fuselage exhaustion there should be 50-250L missing from internal fuel. Because DCS module is using 0L internal fuel until zero fuel remains in any external tank then this amount is missing from the fuselage tank instead.

If from triple tanks the center is immediately jettisoned ~820L is used before wing tank light comes on. ~200L more is used until fuselage light comes on (which was jettisoned long ago). Perhaps it is impossible for light to come on until internal fuel has decreased enough until that external wing (-100L) or external fuselage (-200L) is used. Filling from external wing tanks is keeping level in #2,#7 internal tank full enough that fuselage empty pressure is not tested?

Normal drop order is then tested, immediately wing tanks are removed and check until lights come on. Wing light on solid at 4075L so 350L is used (too much) before absence of wing test pressure circuit is tested. This is almost exactly 2920 (initial fuel) -2600 (average 2700,2500). Fuselage tank is coming on 3975 or 100L later.

If center tank only is carried and dropped immediately then light comes on immediately before internal fuel has reduced for pressure test. Theory is that if tank empty is only tested when internal fuel has dropped suitably then this light should delay signaling until internal fuel drops slightly (100L or so).

Normal tank order not being followed strictly. Lights are coming on based on mixture of ideas from manual and simple consumption order. Idea of reduced internal fuel before wing stage and centerline stage external transfer is absent. Separation of wing stage and centerline stage external transfer is poor, for much of the time both are being taken from at once.

My guess of correct order and figures (manual gauge setting):

Initial

Total: 2920+490+490+490 = 4400

Usable: 2810+480+480+480 = 4250 (4200)

Beginning internal consumption

Total: 2820+490+490+490 = 4300

Usable: 2710+480+480+480=4150

Consume wing tanks

Total: 2820+490+10+10 = 3330

Usable: 2650+480+0+0=3130 (3200-3000)

Continue internal consumption

Total: 2720+490+10+10 = 3230

Usable: 2550+480+0+0=3030

Consume fuselage tank

Total: 2720+10+10+10 = 2780 (2500-2700)

Usable: 2550+0+0+0=2550

the air is discharged into the tank 7 through the drain valve 4, which is open, since the command pressure supplied to it is discharged through the open valve in the float valve 4. The air pressure in the wing tanks during this period is equal to the pressure in the tank 7.

Once out of the tank produced about IOO liters of fuel, closes valve For float valve 4. Stops fuel drain teams - tion of pressure applied to the special valve 36, the pressure below the membrane tion of the valve increases and opens it. Fuel from the wing pendant

tanks enters the tank 2.

If less fuel is supplied to the engine than is supplied from the wing tanks to the tank 2, the fuel level in the tank 7 increases, the float - new valve I opens the drain again through the valve 3A, a special valve 36 closes and the fuel production from the wing hanging tanks stops until the next fuel level in the tank 7 decreases.

 

At the end of the production of fuel from the wing outboard tanks, the pressure in the pipeline of pressurization of these tanks drops (air passes through tank 2 to tank 7), the pressure indicator connected to the pipelines is triggered, and a green lamp lights up in the cockpit (DEVELOPMENT KR. BAKOBV).

 

From tank 7, another 100 L of fuel is produced. The fuel drain through valve 2b of the float valve 1 is blocked, as a result of which the pressure in the cavity under the diaphragm of special valve 64 will increase, the valve opens and the fuel from the fuselage outboard tank starts to flow into tank 2. After the fuel is produced from the fuselage outboard tank and a pressure alarm connected to the pipeline, gives a signal for the green lamp to light up (EXT. TANK) in the cabin.

Also later

1.2.2.

System health check

To test the performance of the system to produce power when the tanks are filled with fuel and the connected ground source of electricity in the following order:

(I) turn on the switch on the starboard side "ACCUM. BOARD. AEROD.".

2) Turn on the right side of the gas station "ADM HYDR. The ALARM. TANKS" will light warning light "CONSUMPTION. TANK" and the lamp "is produced. PODV. TANK", "DEVELOPMENT OF CU. TANKS" (see figure 1.13).

3) If the overhead bins are installed to enable gas stations "RELIEF TANKS" at the right side under the glass, with the middle panel of the dashboard should light up the lamp "tank SUSPENSION", for the wing tanks-over-pisi "SUSPENDED 3 EXTERNAL, "" SUSPENDED 4 EXTERNAL,".

4) Turn on the right side of the gas station "RASHOMON. HACOC", with the signal inscription " FLOW. TANK" on the display of the instrument cluster should go out.

5) Turn on the right side of the gas station "HACOC I GR. TANKS", at the same time correspond-

the signal sign on the dashboard display should light up and go out .

6) Turn on the right side of the gas station "HACOC 3 GR. TANKS", at the same time correspond-

current signal inscription on the scoreboard, the dashboard needs to tan Xia and off.

At the end of the test, turn off all the gas stations and the battery.

After starting the engine with suspended and filled with drop tanks (which creates pressure) warning light "is produced. PODV. TANK" after a while, necessary to fill the tank with air, should go out. The lamp goes out at the speed of n, = 45+60%.

When the engine is running without external fuel tanks lamp "is produced by, the MOB. TANK" and "DEVELOPMENT of CU. TANKS" should be lit.

Most tank lights are pressure sensing. This is why 1st and 3rd tank group lights should normally be off when starting since signal is related to combination of system being powered on and pressure not yet enough. Proper result is that on switching on signal lamp comes on because power is applied but pressure is low. As pump runs for a time the pressure comes up and the signal goes out. For external tanks the same to show that external tank pressure shows empty until engine is up to speed to provide bleed air.

Great Frederf, I wish that developers had yours attitude to details. Maybe you should put this on the bug tracker?. Only one thing: you overestimated difference between theoretical and usable amount of fuel. For fuselage tanks theoretical capacity is 2885 liters, usable 2805 liters, for PTB-490 usable 482 liters and for PTB-800 usable 795 liters. So from maximum fuel capacity (internal+2x490+1x800) fuel system lose 101 liters.

Fuel consuption sequence graphic:

I see 80L unusable in manual. My 2920 number comes from 2280kg at 0.781g/mL or 2919.3L and looking at gauge needle position. Maybe that's wrong...

Aha! I have an idea. 0-6000 for animation gives 0-1 output. All I must do is set 0.480833 for animation output (min and max) then 0.48655 for animation output. Then I can compare by three screenshot. This will be pixel perfect.

OK screenshot of forced 2920 and 2885 output is reading more than default gauge. I will try 0.4675 which is 2805/6000 and compare to original. Original is half way between forced 2805 and 2885... 2850? You know I bet it is (2750+80)/6000. Yeah it's really really close to 2830/6000. OK default gauge shows ~2830 (not 2920) it just looks like >2900 because of parallax or something. This means fuel density ~0.795.

You know I can fudge that offset of 80L just make input 0080-6080 equal to 0 to 1 output >.>. I'm sure that's not a proper adjustment but I'll see how it works out. Not bad. It will be nice not to always have to scrub off 100L every spawn.

2.7% Dispenser, 78L

8-9% Group 3 247L

15-16% 450L 452L

26-27% Group 1 734L

I think manual and signal would be gauge setting so usable amount of more so DCS lights are coming on too late. At 450L usable remaining there should be 530L in the system. Exception that dispenser tank should come on at near zero usable so that's fine.

2750 amount is used for manual procedure for some reason, 135L not 80L less than total. Is fuel rate sensor mass rate or volume rate so guard against low density? Extra flight safety? Sensor margin of error? Different tank for this nation's equipment to fit? Thermal expansion? Leave 50L not filled? Who knows.

Interesting Grp I. warning light depends on engine on or off but Grp III not. I think by pumps or bleed pressure both should work the same. I or III light only goes on when I or III switch is on and pressure is insufficient. In contrast dispenser tank signal is on regardless of its switch if pressure is insufficient.

Interesting stuff. While searching around for more info, I ran across this page: http://www.topedge.com/panels/aircraft/sites/kraft/fuel.htm, which describes the fuel system of a Mig-21US. A few minor differences from what foxbat posted, like tank order and lack of wing drop tanks. I wish the system diagram was easier to read.

I want to know which fuel tanks are in what group. Is my image correct? And where is tank no.1?

Tank no. 1 is missing in some MiG-21 (including 75B for some equipment) but normal tank numbering is preserved.

By hand written note Group I (2,7, wings?), Group II (3upper, 3lower), Group III (4,5,6). I and III supply to II where production to engine occurs. By manual there are 13 tanks in total.

Five rubber cell in fuselage, tanks no. 2-6 between frame 13-28

Four compartments front and rear

Fuselage tank no. 7 between frame 13-25 and wings

Drop tanks, three

Hey check this one out, more negative G fuel buffer info

Negative g compartment ensures the engine fuel supply under the action of a negative overload in the non-afterburning regimes over the 15s and in the afterburning mode during 5s.

https://www.dropbox.com/s/ensfroc6dimhalo/MiG-21bis%2075B%20Section%201%20Fuel%20System.pdf?dl=0

Some quick colorization fun with tanks groups:

Summary of changes to make

Armament and signalling

• Zero position light only with Rocket/Missile firing system power
  
• Pylon presence light shows presence of any suspended item
  
• Zero position light on if pod is steady at 32, 16, 0 or no S-5 carried
  
• 32 rocket pod will pause firing sequence at half load until fired again
  
• Zero position light is not involved in other weapons than S-5
  
• Rocket/Missile firing system not required for releasing bombs or UBK
  

Fuel and fuel tanks

• Set gauge to show usable fuel amount ~2805L, not total
  
• Fuel signal lights coming on at e.g. 250-350, 400-550, 700-1000L usable, not total
  
• Group I or Group III signal never on if Group I or Group III switch off
  
• Group I or Group III signal on briefly until pressure builds
  
• Internal fuel depleted 100L before wing tanks transfer
  
• Internal fuel depleted 200L before center tank transfer
  
• ~20L negative G buffer (or more): 3s A2, 5s A1, 15s mil., 90s idle
  
• External tanks signal empty without sufficient (N1 45-60%) pressure
  
• Jettison or empty of center tank produces immediate signal "ventral"
  
• Jettison or empty of wing tanks produce immediate signal "wings"