3x 500 Gallon & 2x 1,000 Gallon fuel tanks for the XR2 to ascend above 20 km and accelerate to at least Mach 3 for scram engine cruise up to mach 20. Then back over to the main liquid fueled rocket engines for orbit. These Fuel tanks are part of Sputnik's Velcro Rockets v1.11
A Config override of the XR2 is included for a more realistic flight experience. So be sure to set AF control to pitch for takeoff. And yes the scram fuel tanks are Payload only.
However if you want to use the same fuel type for all engines: rstarkov's Refuel MFD is really nice for cross feeding. But it crashes the simulator when the drop tanks auto delete after being jettisoned as they register as internal tanks. Simply activate that MFD after they jettison to avoid the crash.
Another option would be to feed some of the fuel from the drop tanks into the scram engines, but in the real world; they would melt off of the orbiter and explode!
To drop the tanks manually: Set focus (F3) to any tank you want to jettison and press J. Or you can preset them in the scenario file to jettison automatically; either on MECO or when empty. JETT_ON_MECO 1
JETT_ON_EMPTY 1
The setting in this scenario is to jettison on MECO so mind your throttle below 20 km at under mach 3
Fixes:
Scenario file replaced with the correct one
Config file updated to allow normal refueling
Main fuel payload replaced with 2nd scram payload
External tank feeds rebalanced for even flow rates