diff --git a/GameData/RealismOverhaul/Engine_Configs/XLR99_Config.cfg b/GameData/RealismOverhaul/Engine_Configs/XLR99_Config.cfg
index dfb50272b2..b9d6dd7050 100644
--- a/GameData/RealismOverhaul/Engine_Configs/XLR99_Config.cfg
+++ b/GameData/RealismOverhaul/Engine_Configs/XLR99_Config.cfg
@@ -4,35 +4,36 @@
 //	Manufacturer: Reaction Motors
 //
 //	=================================================================================
-//	XLR99-RM-2
+//	XLR99-RM-1
 //	X-15
 //
-//	Dry Mass: 415 Kg
-//	Thrust (SL): 202 kN
-//	Thrust (Vac): 224 kN
-//	ISP: 220 SL / 244 Vac // slightly inaccurate VAC; Highest ISP was 230 at 18,700ft
-//	Burn Time: 360
-//	Chamber Pressure: 4.13 MPa
+//	Dry Mass: 413 kg	910 lbs dry [C, p.205]
+//	Thrust (SL): ??? kN
+//	Thrust (Vac): 257.3 kN	57850 lbf@100kft [C, p.210] or 57000 lbf@45kft [C, p.205]
+//	ISP: 230 SL / 276 Vac	230@sea level [C, p.205], 265@45kft [C, p.205], 276@100kft [C, p.210]
+//	Burn Time: 180
+//	Chamber Pressure: 4.14 MPa	600 psi [C, p.210]
 //	Propellant: LOX / Ammonia
 //	Prop Ratio: 1.25
-//	Throttle: 100% to 30%
+//	Throttle: 100% to 40%	Capable of 30% thrust, but limited to 50% (early engines) or 40% (later engines) due to combustion stability issues [C, p.210]. Attempted operation at 35% thrust resulted in engine shutdown [C, p.220]
 //	Nozzle Ratio: 9.8
-//	Ignitions: 6
+//	Ignitions: 100	rated for 100 starts [C, p.210]
 //	=================================================================================
-//	XLR99-RM-2A
-//	X-15A-3
+//	XLR99-RM-1A
+//	X-15A-2
+//	Recontoured nozzle plus 22.5:1 radiative extension. No other changes.
 //
-//	Dry Mass: ??? Kg
-//	Thrust (SL): 202 kN
-//	Thrust (Vac): 224 kN
-//	ISP: 200 SL / 276 Vac // slightly inaccurate VAC; Highest ISP was 230 at 18,700ft
-//	Burn Time: 360
-//	Chamber Pressure: 4.13 MPa
+//	Dry Mass: 432 kg	Guessing about 19 kg for nozzle extension (roughly the same as RL10A-4 extension)
+//	Thrust (SL): ??? kN
+//	Thrust (Vac): 281.9 kN	63378 lbf [C, p.226]
+//	ISP: 200 SL / 298 Vac	200@SL (calculated with RPA), 298@cav [C, p.226]
+//	Burn Time: 180
+//	Chamber Pressure: 4.14 MPa
 //	Propellant: LOX / Ammonia
 //	Prop Ratio: 1.25
-//	Throttle: 100% to 30%
-//	Nozzle Ratio: ???
-//	Ignitions: 6
+//	Throttle: 100% to 40%
+//	Nozzle Ratio: 22.5
+//	Ignitions: 100
 //	=================================================================================
 //
 //	==================================================
@@ -47,10 +48,15 @@
 //            X-15 Propulsion System, AIAA paper 97-2682
 //  [B] History of Liquid Propellant Rocket Engines, George P. Sutton, Page 314 Table 7.2-2
 //  http://www.astronautix.com/x/x-15a-3.html
+//
+//	[C] X-15 Extending the Frontiers of Flight, Dennis R. Jenkins
+//  https://ntrs.nasa.gov/api/citations/20080008340/downloads/20080008340.pdf
 
 //	Used by:
 
 //	Notes:
+//	C p. 565 SERJ
+//	C p. 575 delta-wing X-15
 
 //	==================================================
 @PART[*]:HAS[#engineType[XLR99]]:FOR[RealismOverhaulEngines]
@@ -80,14 +86,15 @@
 		type = ModuleEngines
 		modded = false
 		configuration = XLR99-RM-2
-		origMass = 0.412
+		origMass = 0.413
 		CONFIG
 		{
 			name = XLR99-RM-2
 			specLevel = operational
-			minThrust = 67.2
-			maxThrust = 224
+			minThrust = 102.9
+			maxThrust = 257.3
 			heatProduction = 100
+			varyFlow = 0.0263	//[C, p.415] +-1500 lbf(!) throttle error at 57000 lbf nominal thrust
 			PROPELLANT
 			{
 				name = LqdAmmonia
@@ -108,22 +115,24 @@
 			}
 			atmosphereCurve
 			{
-				key = 0 244
-				key = 1 220
+				key = 0 276
+				key = 1 230
 			}
 			ullage = False
 			pressureFed = False
-			ignitions = 6
+			ignitions = 100
 			IGNITOR_RESOURCE
 			{
 				name = ElectricCharge
 				amount = 1.00
 			}
 
+			//X-15: 169 flight attempts, 10 failures (4 cycle, 6 ignition)
+			//X-15 could actually usually restart after an ignition failure. Since this is not a feature in TF, halve ignition failures.
 			TESTFLIGHT:NEEDS[TestLite|TestFlight]
 			{
 				name = XLR99-RM-2
-				ratedBurnTime = 3600		// "required overhaul after 1 hour of burn time"
+				ratedBurnTime = 3600		// "required overhaul after 1 hour of burn time".
 				ratedContinuousBurnTime = 180 // 180s rated burntime at full thrust
 				safeOverburn = true
 				overburnPenalty = 1		//No penalty
@@ -135,23 +144,23 @@
 					key = 1.00 1.00 3 3
 				}
 
-				// static-fired for long periods between refurb.
-				ignitionReliabilityStart = 0.94
-				ignitionReliabilityEnd = 0.996 // pretty much all flights were successful, but ground tests failed
+				ignitionReliabilityStart = 0.977941
+				ignitionReliabilityEnd = 0.995588
 				ignitionDynPresFailMultiplier = 10.0
-				cycleReliabilityStart = 0.94
-				cycleReliabilityEnd = 0.99 // very low chance early on, but 1% chance of failing after 2700s
+				cycleReliabilityStart = 0.971557
+				cycleReliabilityEnd = 0.994311
 				reliabilityDataRateMultiplier = 20
 			}
 		}
 		CONFIG
 		{
 			name = XLR99-RM-2A
-			specLevel = concept
-			minThrust = 76.0
-			maxThrust = 253.4	//correct for increased expansion ratio
+			specLevel = prototype //engine was tested.
+			minThrust = 112.8
+			maxThrust = 281.9
 			heatProduction = 100
-			massMult = 1.1	//idk, a little more for the nozzle extension
+			massMult = 1.046
+			varyFlow = 0.0263
 			PROPELLANT
 			{
 				name = LqdAmmonia
@@ -172,12 +181,12 @@
 			}
 			atmosphereCurve
 			{
-				key = 0 276
+				key = 0 298
 				key = 1 200
 			}
 			ullage = False
 			pressureFed = False
-			ignitions = 6
+			ignitions = 100
 			IGNITOR_RESOURCE
 			{
 				name = ElectricCharge
@@ -186,8 +195,8 @@
 
 			TESTFLIGHT:NEEDS[TestLite|TestFlight]
 			{
-				name = XLR99-RM-2A
-				ratedBurnTime = 3600		// "required overhaul after 1 hour of burn time"
+				name = XLR99-RM-2
+				ratedBurnTime = 3600		// "required overhaul after 1 hour of burn time".
 				ratedContinuousBurnTime = 180 // 180s rated burntime at full thrust
 				safeOverburn = true
 				overburnPenalty = 1		//No penalty
@@ -199,12 +208,11 @@
 					key = 1.00 1.00 3 3
 				}
 
-				// static-fired for long periods between refurb.
-				ignitionReliabilityStart = 0.94
-				ignitionReliabilityEnd = 0.996 // pretty much all flights were successful, but ground tests failed
+				ignitionReliabilityStart = 0.977941
+				ignitionReliabilityEnd = 0.995588
 				ignitionDynPresFailMultiplier = 10.0
-				cycleReliabilityStart = 0.94
-				cycleReliabilityEnd = 0.99 // very low chance early on, but 1% chance of failing after 2700s
+				cycleReliabilityStart = 0.971557
+				cycleReliabilityEnd = 0.994311
 				reliabilityDataRateMultiplier = 20
 				techTransfer = XLR99-RM-2:50
 			}