S1 SRB-KD25k "Kickback" Solid Fuel Booster

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S1 SRB-KD25k "Kickback" Solid Fuel Booster
Part image
Solid rocket booster by

Radial size Small, Radial mounted
Cost (total) 2 700.00 Funds
(dry) 1 140.00 Funds
Mass (total) 24.00 t
(dry) 4.50 t
Drag 0.2-0.3
Max. Temp. 2200 K
Volume  ?
Impact Tolerance 7 m/s
Research Tech tree heavy rocketry.png Heavy Rocketry
Unlock cost 8 000 Funds
Since version 0.23.5
Part configuration part.cfg
Solid rocket booster
Maximum thrust (1 atm) 593.86 kN
(vacuum) 670.00 kN
Isp (1 atm) 195 s
(vacuum) 220 s
Fuel consumption 41.41 Units of fuel/s

Thrust vectoring No
Fuel 2600.0 Units of fuel
Burn time (1 atm) 55.7 s
(vacuum) 62.8 s
Testing Environments
On the surface No
In the ocean No
On the launchpad Yes
In the atmosphere Yes
Sub orbital Yes
In an orbit Yes
On an escape No
Docked No
Test by staging Yes
Manually testable Yes

The S1 SRB-KD25k "Kickback" Solid Fuel Booster is the largest 1.25 m solid rocket booster (SRB). It has slightly more than twice the thrust of the smaller BACC "Thumper" Solid Fuel Booster and three times as much fuel.

It comes as a single unit. The engine and fuel tank are one single part which means that it isn't possible increase the burn time for it by adding more fuel tanks.

It is the largest 1.25m solid rocket booster, larger than the BACC "Thumper" Solid Fuel Booster.

Product description

This super heavy booster is designed to be recovered after jettisoning. Once recovered, it is refurbished and refueled for another launch.



Like other SRBs, the Kickback can be fired once, and cannot be shut down or throttled once active. While assembling a vessel, it is possible to adjust the amount of solid fuel and thrust using tweakables in the VAB or SPH. If planned properly, this can provide some control during flight.

The minimum thrust required for the Kickback to lift its own weight off Kerbin's surface at sea level is about 40%, or about 237.5 kN.

The length of the Kickback makes it difficult to use it as a first stage mounted below the rocket stack. It can be radially attached using radial decouplers (such as the TT-38K Radial Decoupler) to solve this problem.

At full thrust, the Kickback will burn for 62.8 s before running out of solid fuel and burning out. If radially attached to a larger SRB, the thrust can be adjusted so that the Kickback burns out at the same time at the other SRB, removing the need for expensive radial decouplers. For a S2-17 "Thoroughbred" Solid Fuel Booster, the Kickback will need to be set to 78.5% thrust; for a S2-33 "Clydesdale" Solid Fuel Booster, the Kickback will need to be set to 73% thrust.


  • The Kickback is the only solid rocket booster with a flag painted on it.
  • The Kickback was added in version 0.23, which also added several parts related to the Space Shuttle and Space Launch System. It can be inferred that the Kickback is meant to resemble the Space Shuttle SRBs, which are reused in the Space Launch System.
  • The product description is likely a reference to the Space Shuttle's practice of recovering and reusing the Space Shuttle SRBs.
  • However, actually recovering the Kickback ingame is quite impractical. It does not have a high impact tolerance, and cannot be restarted for a soft touchdown (like all SRBs). It is also quite heavy when empty, requiring three Mk2-R Radial-Mount Parachutes (or two Mk2-R parachutes and one Mk16-XL Parachute) for a safe velocity - costing more than the empty booster itself!


  • Added ESA skin variant
  • Initial release