Richard Nakka's Experimental Rocketry Web Site

Impulser and Impulser-X Rocket Motors

Impulser & Impulser-X

  • Introduction
  • Basic Dimensions and Configuration
  • Propellant Grain
  • Nozzle
  • Bulkhead
  • Motor Casing and Thermal Liner
  • Motor Ignition
  • Engineering drawings
  • Performance
  • Photos and Video Clips
  • Introduction

    The Impulser rocket motor was conceived in 2013 as the primary motor for my then-planned Zeta rocket. It was envisioned as an I-class motor powered by a choice of either KNSB or KNDX sugar propellant. The Impulser was subsequently static tested in October of 2013 and first flown exactly one year later. The Impulser-X, a stretched version of the Impulser, was developed soon after. Since that time, the Impulser motors have proven to be ideally suited to my experimental rocket flight activities, performing with a high degree of reliability and having a "just-right" impulse range.

    Impulser is configured with a 4-segment BATES grain and has a nominal capacity of 300 grams of KNSB or KNDX. Impulser-X is configured with a 5-segment BATES grain with a nominal capacity of 375 grams of KNDX. Nominal impulse of Impulser is 400 Newton-seconds, and 500 Newton-seconds for Impulser-X.

    Impulser is capable of launching a 2kg (4.4 lb), 2.5 inch (64mm) diameter rocket to an altitude of 2800 ft. (850m.). Impulser-X can loft the same rocket to about 3500 ft. (1100m).

    To date (August 2017), the Impulser motor has been fired 30 times (27 flights; 3 static). The Impulser-X motor has been fired eight times (7 flights; 1 static).

    Impulser Rocket Motor

    Figure 1 -- Impulser rocket motor

    Impulser-X Rocket Motor

    Figure 2 -- Impulser-X rocket motor

    Basic Dimensions and Configuration

    The Impulser motor has an outside diameter of 1.50 inches (38.1mm) and an overall length of 11.8 inches (300mm). Impulser-X has an overall length of 14.0 inches (355mm). Both utilize steel nozzles and aluminum alloy casings and bulkhead. A pair of standard o-rings seal the nozzle and a single o-ring seals the bulkhead. The nozzle and bulkhead are both retained with snap-rings. To protect the aluminum casing from combustion heat, a thermal liner is employed. The motors are designed for unlimited re-use. Minor nozzle throat erosion does occur with each firing, resulting in a small deviation in performance over time.

    Impulser cross-section

    Figure 3 - Cross-sectional view of the Impulser motor assembly

    Propellant Grain

    The Impulser motor can utilize either sorbitol-based KNSB or dextrose-based KNDX propellant. Four BATES segments make up the propellant grain, with combustion initially occuring along the core and at both ends of each segment. A bonded inhibitor prevents combustion from occuring on the outside surface of the grain segments.

    The Impulser-X was designed solely for use with KNDX propellant. Five BATES segments (identical to those of Impulser) make up the propellant grain.

    Propellant segments are separated within the motor by spacer rings. The spacer rings serve an essential purpose, ensuring that all the segment ends start burning immediately upon motor start-up. To maximize performance and reliability, the grain segments are cast using a dedicated mould system which allows for propellant curing under spring pressure. This is done to ensure a good bond between the casting tube and propellant. A secondary reason for curing under pressure is to eliminate all trapped air, maximizing density and providing consistent burn characteristics. After casting and trimming, the propellant surfaces are coated with Combustion Primer to ensure rapid ignition of all grain surfaces and to provide for swift motor start-up.

    Grain segments

    Figure 4 - BATES grain segments (prior to priming)

    To produce the grains, molten propellant slurry is poured/scooped into casting tubes, which serve the additional role as inhibitor to control which surfaces are exposed to burning. The casting tubes are fabricated from poster board (or tagboard). Cut out as strips, the poster board is rolled around a mandrel to form a two-ply tube. The plies are bonded together with glue-stick adhesive.

    The Spacer Rings are made in an identical manner to the casting tubes, using the same mandrel.


    Both Impulser and Impulser-X nozzles are conical profiled, deLaval supersonic type machined from low-carbon steel, with 30o convergent and 10o divergent half-angles. The nozzle is "free-floating" within the motor casing, retained within the casing by a N1300-137 snap-ring.

    The nozzle is pressure sealed with a pair of -123 o-rings (Buna-N nitrile), well-lubricated with silicone grease.

    The Impulser throat diameter is dependant upon whether KNSB or KNDX propellant is utilized. Due to its faster burn rate, KNDX requires a slightly larger throat to achieve the same max chamber pressure. Otherwise the nozzles are very similar in design.

    The inlet to the throat is radiused in order to accelerate the combustion products more gradually. This reduces two-phase flow performance loss (see SRM Theory section for details). It has been found that a small amount of nozzle throat erosion occurs during each firing. For the Impulser, the erosion per firing amounts to about 1 thousandth of an inch, or 33 microns (Click for chart). For the Impulser-X, the erosion is greater, about 3 thousandth of an inch, or 80 microns (Click for chart).

    Impulser Nozzle

    Figure 5 -- Impulser nozzle


    The bulkhead forms the forward closure of the motor. The bulkhead is machined from 6061-T651 aluminum alloy. For sealing, a -123 o-ring (Buna-N nitrile) is used in conjunction with silicone grease. The bulkhead is retained with a N1300-137 snap-ring.

    Impulser Bulkhead

    Figure 6 -- Impulser bulkhead, with o-ring and snap-ring


    The motor casing is made from 1.50 inch (38.1mm) 6061-T6 aluminum alloy drawn tubing with a wall thickness of 0.065 inch (1.65mm). Grooves are internally machined near each end to seat the snap-rings which retain the nozzle and bulkhead. To protect the casing from the hot combustion gases, a thermal liner is fitted inside the casing. The thermal liner is fabricated from a sheet of posterboard, rolled around a mandrel. The liner is two-ply, with the overlapping portion bonded together with glue-stick adhesive. To provide added resistance against charring, a single coat of polyurethane varnish is applied to the interior surface of the liner, and allowed to fully dry, prior to rolling. Further thermal protection is achieved by covering the outer surface of the liner with a single layer of aluminum foil, bonded (prior to rolling) with glue-stick.


    To achieve rapid ignition and optimum performance of the motor, an electrically initiated pyrogen type igniter is employed. Inserted through the nozzle up to the head end of the motor, the pyrogen igniter fires a jet of flame along the motor core toward the nozzle, rapidly igniting the exposed surfaces of the propellant grain. The igniter consists of a 6cm length of plastic tube ("drinking straw") filled with MDP or Black Powder pyrolant, sealed at both ends with hot glue. A short length of nichrome bridgewire initiates the charge. Two slits in the tube wall near the aft end of the igniter control the burn rate and allow for escape of the hot gases in the form of flame jets. (Click for full details).

    Engineering Drawings

    1. Impulser Motor Assembly
    2. Impulser-X Motor Assembly
    3. Impulser Nozzle (KNSB)
    4. Impulser Nozzle (KNDX)
    5. Impulser-X Nozzle
    6. Bulkhead
    7. Impulser Motor Casing
    8. Impulser-X Motor Casing
    9. Impulser Thermal Liner
    10. Impulser-X Thermal Liner
    11. Thermal Liner Mandrel
    12. Propellant Grain
    13. Grain Spacer Ring
    14. Propellant Grain Mandrel

    Motor Performance

    The Impulser and Impulser-X motors were designed with the aid of SRM_2104.xls "Solid Rocket Motor Performance" software. The design parameters are presented in Figure 7.

    Impulser parameters

    Figure 7 - Design Parameters for Impulser and Impulser-X motors

    Photos and Video Clips

    1. Machined nozzle o-ring grooves
    2. Turning external profile of nozzle
    3. Machining snap-ring grooves
    4. Tool bit for cutting snap-ring grooves
    5. Stash of Impulser grain segments
    6. Primed KNDX grain segments for Impulser-X
    7. Thermal Liner
    8. Video-clip Impulser (prototype) static test (3.3 Mbyte)
    9. Video-clip Impulser-X static test (2 Mbyte)

    Last updated

    Originally posted July 2017

    Last updated August, 2017

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