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J-X4 "Whiplash" Turbo Ramjet Engine

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J-X4 "Whiplash" Turbo Ramjet Engine
Part image
Jet engine by
C7 Aerospace Division
Radial size Small
Cost (total) 2 250.00 Fund
Mass (total) 1.80 t
Drag 0.2
Max. Temp. 2000 K
Impact Tolerance 7 m/s
Research Hypersonic flight.png Hypersonic Flight
Unlock cost 18000 Fund
Since version 0.15
Part configuration jetEngineTurbo
Maximum thrust 130 kN
Isp (max) 4000 s
Fuel con­sumption 0.04 units of fuel/s
Intake air consumption 0.62 Air unit/s
Thrust vectoring 1 °
Testing environments
On the surface ✓ Yes
In the ocean ✓ Yes
On the launchpad ✓ Yes
In the atmosphere × No
Sub orbital × No
In an orbit × No
On an escape ✓ Yes
Docked × No
Test by staging ✓ Yes
Manually testable ✓ Yes

The J-X4 "Whiplash" Turbo Ramjet Engine is an air-breathing engine. These require liquid fuel from tanks and constant supply of intake air using air intakes inside an oxygenated atmosphere. Note, intakes collect air and send it to jet engines regardless of where the intakes are on the craft; fuel, however, flows automatically only from the same stage as the engines. It is similar in appearance and function to the Pratt & Whitney J58, used on the famous SR-71 Blackbird.


Visualization of the thrust multiplier curve for velocity
Visualization of the thrust multiplier curve for atmospheric pressure

Being the fourth air breathing engine to unlock in the tech tree, it marks a great step towards space-usability. As on all engines, thrust varies with altitude. Contrary to all rocket engines, air breathing engines' thrust diminishes with increasing height. Furthermore differing from most rocket engines, this has a real impact on usage, as the effect is far more distinct.

Contrary to pre KSP 1.2, its maximum stationary thrust is only slightly higher than that of the Basic Jet Engine, rated with 130 kN at sea level. Rising up to 6000 m the thrust drops to about 75 kN, more or less staying at this performance up to a height of about 12,500 m (63 kN). From there on, thrust output drops somewhat linear until the engine shuts down at about 26000 m, which is 4000 m higher than in KSP 1.0.

As with the other jet engines, thrust output also varies with speed. Like the other two high-speed engines, this engine draws a huge boost out of the airspeed, as the name already implies. Up to mach 0.2 (e.g.: still on the runway) it stays somewhat constant, being boosted by further acceleration to a maximum of about 580% from mach 2.8 to 3.5, which would be about 750 kN at sea level! Getting even faster, thrust starts dropping, reaching zero output at mach 5.5.

Since KSP 1.0 the specific impulse (Isp) stays constant over the full range of application, being rated at 4000 s, which is the same efficiency as the J-404 "Panther" Afterburning Turbofan in wet mode, but less than half of the Wheesley. However, as the thrust/height curve of this engine allows for usage in already quite thin air, it uses only little more fuel per distance traveled when flying high and not too fast. As the possibility of travelling much faster compared to the Wheesley comes with the need of flying really high, this engine is best used for long range missions, as it is somewhat thirsty during the ascent.

Combining the height and the speed behaviour, this engine gives remarkable thrust even at great heights as long as it moves fast enough. Practically, you need to have in mind both curves for two different reasons:

  • Overpowering: from 6000m up to 12500 m thrust is nearly not reduced by height. As it is speed boosted (100%+ thrust) up to mach 5, level flight at full throttle quickly leads to disintegrating.
  • Underpowering: Flying very high, the engine may have enough thrust to maintain high speed (> mach 2.8) due to the speed boost, but getting slower than mach 2.8 quickly leads to a non-stoppable deceleration as the engine gets weaker.

In conclusion this means: The engine needs thick air to get really strong, but then the craft desperately needs thin air to maintain integrity. Carefully flying at the right ascent-path, you can easily (and quickly) reach 1100 m/s at 20000 m (still rising fast), which is quite a useful base for continuing towards stable orbit with the same stage. A normal ascent path for a turboramjet spaceplane ends up looking something like this: climb at a 30 degree angle at full throttle until 10,000 meters, then nose down to 20 degrees or even 10 degrees ascent. The best compromise between angle of attack, rate of climb, and drag is reached somewhere in this range. Keep this attitude constant, even nosing down a little bit more, until the turboramjet flares out at 26km. If it flares out at a lower altitude, then more intakes are required. However, unless one is extremely lucky or extremely thrusty, the apoapsis at this point will not be out of the atmosphere (this, along with the need to make a circularization burn at apoapsis in vacuum, is why a rocket is still required for a turboramjet spaceplane). Immediately pitch up (being careful to avoid creating huge drag from an extreme angle of attack) and light up the rocket(s).

The exact altitude where you should drop the nose (trading the rate of ascent for acceleration) varies for each individual craft design, as does the optimal angle of attack in both the lower and upper phases of the ascent.  Some experimentation is normally required to find the optimal ascent profile for each individual design of spaceplane.

Note that currently, the only planets which this engine will work on are Kerbin and Jool's moon Laythe.

Product description

A highly advanced turbine engine using space-grade materials for very high performance. This new model features the latest in thrust vectoring and compression technology and is designed to operate best at higher altitudes and speeds. The advanced turbo ramjet design bleeds air around the compressor at high speed allowing a far maximum speed than regular jet engines, so engineers from C7 Aerospace assured us all those leaks are intentional.

Created by: C. Jenkins


The internal part's name is turboFanEngine which is another kind of jet engine. In real life, a turbojet engine's fan pulls all the air from the intake through a combustion chamber, as opposed to a turbofan in which some of the air sucked in bypasses the chamber and is shot out of the back without being burned at all. The turbojet is more efficient at high speeds and in thinner atmospheres, although turbofans are much quieter and are very efficient when moving slowly. This is why turbofans are almost exclusively fitted on commercial airliners, and turbojets are more common on supersonic planes.


Appearance before 1.0.5

  • Remodeled and retextured
  • Isp halved, thrust reduced
  • complete overhaul of most characteristics
  • Initial Release