2,500lb Thrust Booster Motor

A large 6" diameter rocket motor for amateur experimental rocketry

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I recently moved out to California which gave me access to the Friends of Amateur Rocketry (FAR) site, which has amazing resources.

My previous site limitations in Maryland left me wanting, and now I can stretch me engineering to some rather large motors in my foray into amateur experimental rocketry.

This booster will have a high thrust to mass ratio and be used a payload lofting single stage or multi-stage launch systems.

Project Overview

This booster will be a 6" ID rocket motor with a high thrust to mass ratio capable of heavy payload lofting or multi-stage use. It will utilize approximately 78 pounds of PNCP rocket propellant to produce around 2,500 pounds of thrust for 3.5 seconds. This will come in at a full O-class rocket motor, just under the FAA class 3 permit requirements.

Once flight worthiness of the design is proven, it is possible to add an additional 2 grains (8 feet total propellant) and increase the nozzle throat to increase thrust. For heavy lifting, 4 of these boosters can be strapped together similar to a Nike Hercules, providing nearly 10,000 pounds of thrust at a full Q designation to be used as the first stage of a 3 stage sub-orbital 3U cubesat mission at significantly lower costs when compared to existing services.

Project Stages

  1. Grain casting jig and large quantity propellant casting methodology fortification
  2. Construction of 6" ID steel thick walled single grain testing rig for use in propellant characterization testing, grain geometry deformation testing, and grain slump testing. Measurement of chamber pressure only.
  3. Full scale motor casing and nozzle construction to include motor case, forward bulkhead, low KN nozzle, and high KN nozzle. Nozzles will not have mass reduced for flight until after verification of design specification to avoid unnecessary loss in manufacturing time.
  4. Full scale motor grain casting (78 pounds of fuel)
  5. Full scale low KN sonic nozzle static test (no divergent cone or expansion), measuring chamber pressure
  6. Full scale high KN nozzle static test, measuring chamber pressure and thrust
  7. Full scale flight test
  8. Possibly adding 2 more grains into tests


  • Will the PNCP propellant scale to this large scale diameter without compromising integrity?
    • Initial grain casting and characterization testing will let me know.
  • Will the large surface area to ignite pose an issue to ignite?
    • Will be answered after the first full scale static testing
  • Steel, Titanium or Aluminum casing material?
    • Further research to see if titanium's added weight is worth the hassle
      • My first price quote came in at $325/ft for the grade 2 titanium, so looks like it'll be steel or aluminum, with steel for the testing rig.
  • Final nozzle diameter and expansion
    • To be decided after initial full scale propellant characterization testing

Other Thought

This project will be long in duration, as family and work comes before rockets not to mention the complicated nature of rocket science. Stay tuned!

  • Ordering Parts for Grain Casting Jig and Physical Testing

    J. M. Hopkins10/06/2016 at 03:31 0 comments


    The first round of testing is grain casting and physical properties verification. Using my PNCP propellant I am concerned about the physical strength at the dimensions required for my motor and that the propellant is too malleable and may sag under its own mass.

    Testing Specifics

    The grains cast for physical properties verification will consist of the nominal full size grain at 5.75" diameter, 12" long, with a 3.5" core. If there appears to be a slumping/sagging property at this dimension it might be necessary to add fiber at small percentages (0.5%) to combat this issue, and possibly alter the propellant chemistry to combat the issue.

    Parts Needed

    For physical casting we need items for propellant preparation and for grain casting. First for the preparation we will need:

    • Presto Electric Multi-Cooker 06003 - $35.23 (Amazon)
    • Habor Digital Cooking Thermometer - $6.99 (Amazon)
      • -58°F - 572°F
      • 6" 304 stainless steel probe
    • Silicone Utensil Set (Spatula and Whisk required) - $10.98 (Amazon)
      • For mixing of propellant ingredients

    For the casting jig we will need the following:

    • 14" length of 3.5" OD, 0.125" wall 6061 Aluminum Pipe - ~$20 (Local Metal Supply)
      • Coring tool
      • Outer surface needs to be sanded down to a fine finish
    • 14" length of 6.065" ID, 0.28" wall 6061 Aluminum Pipe - ~$40 (Local Metal Supply)
      • Main casting container
    • 8"x8" square wood board x2 (3/4" ply or whatever you have handy) - Free, local scrap
      • Bottom of our jig double stacked
      • Router used to create center 3.5" hole and 6" diameter ring
    • Kraft Paper - 24"x900' - $14 (Amazon)
      • Large roll of Kraft paper, used to make casting tubes
    • Wood Glue - $5 (Lowes/Home Depot)
      • Used for gluing the casting tubes
    • Aluminum Foil - $5 (grocery store)
      • Used for lining bottom of grain

    Total costs: $140.00 ($35 in cooker, $60 in pipe, $50ish in miscellaneous, no propellant costs added)

    Description of Casting Jig

    A 6" ID pipe, with a 3.5" OD pipe centered inside, attached to a wooden base.

  • Testing Rig Specifics

    J. M. Hopkins07/09/2016 at 22:54 2 comments


    The testing rig will use a grain similar is size to the final motor, but slightly different to facilitate accurate testing. Each grain will be 10.375" long, 5.75" in diameter (leaving 0.125" on the outer circumference to be used as insulation), and 3.5" diameter core. This will give a neutral burn BATES grain profile with a web thickness of 1.12".

    This post will cover the propellant mixture, case liner, instrumentation, and nozzle specifics to be utilized during testing.

    Propellant Mixture

    Each testing grain will have the mass of about 11.3 lbs (5,125.6 grams) and require 3331.6g( (7.3lb) of potassium nitrate, 922.6g (2lb) light corn syrup, 435.6g (0.96lb) sorbitol, 435.6g (0.96lb) sucrose, 38.4g carboxymethyl cellulose, and 12.8g potassium bitartrate. This is the final composition, which will need to be increased 5-10% to account for manufacturing waste after initial propellant casting methodologies are tried and confirmed.

    The addition of a fiber to increase tensile strength might be needed in the range of 0.5% after initial grain physical property testing has commenced to insure that the grain does not slump.

    Case Insulation / Liner

    Phenolic liner or a custom aramid based fiberglass liner will be utilized. After the first few tests it might become necessary to change the liner to a hardier version.


    The testing rig will utilize a single pressure transducer with a maximum chamber pressure of 1,500 psi.

    It will run off of 5VDC excitation and have 0.5V to 4.5V linear output

    Data will be captured at the rate of 1kHz Via DATAQ system.

    Nozzle Specifics

    To properly characterize the propellant and to test the physical properties of our grain there will be a total of 8 test burns, the sonic nozzle ranging from 0.8125" diameter to 0.625" in diameter.

    Nozzle DiameterMin KNMax KNMax Pc (psi)

    It might be that my initial characterization of the propellant is significantly different than expected, and that additional burns or a limit of the high KN burns may be in order. Depending on the propellant burn rate plateau the projected chamber pressure is between 800 and 1000 psi.


    A post explaining the mechanical design and the interchangeable nozzle concept will be coming along shortly.

  • Ultimate Tensile Strength vs Temperature

    J. M. Hopkins07/08/2016 at 04:07 0 comments


    In my previous log we went over the mechanical aspects of using retaining rings for securing the bulkhead and nozzle by calculating the shearing and deformation values related to the retaining ring in question. I knew that temperature plays a large factor in this strength rating and this post will delve deeper into this aspect of materials engineering.

    Material Choices

    I have several choices of metals ranging from aluminum and stainless steel to titanium. They each have their points and counter points from ultimate tensile strength, heat, and materials cost.

    Aluminum is a the most wanted material, because it is both light and inexpensive, however it is also the least strong of the options.

    Stainless Steel makes a good choice for the testing rig, because weight is not an issue. However, due to its weight does not make the best of cases for flight.

    Titanium is heavier than aluminum, but also really quite strong. It would be the choice if cost was not an issue.

    Inconel is... amazing but outside my scope and budget.

    We can see from this graphic that as temperature increases tensile strength reduces drastically in most materials, and that titanium, stainless steel, and inconel lead the pack.

    We can easily tell that proper thermal insulation via ablative materials is needed in most of these materials to make them functional as a casing material.

    This graphic shows some of the other material choices vs temperature. we can see that even titanium drops in strength after a certain amount of heat soak.

    Rocket Motor Implications

    Since the rocket motor itself is only burning for 3 - 4 seconds, thermal conductivity plays a roll in the amount of heat soak as well. Thick walled steel provides a lot of mass to heat up, but on the motor case itself we need to pay attention to our liner.

    Retainer Ring vs Bolt Retention

    We can start to see that this severely reduces our ultimate tensile strength, and that a retaining ring might not be the best option. The orthodox use of multiple bolts might be more appropriate to insure a successful design.

  • Internal Retaining Ring Mechanical Validation

    J. M. Hopkins07/07/2016 at 05:08 0 comments


    I'm in progress of designing the 6" ID BATES grain testing rig to fully characterize my propellant, test grain brittleness under pressure, and insure my liner is sufficient. To that end I want to utilize snap rings for mechanical connections of my bulkhead and sonic nozzle. It needs to be determined if this method is strong enough to run the rig at my target of 1,000 psi.

    There are two things to consider with snap ring load capacity, and this involves calculation of ring shear and groove deformation.

    I'll be using a 6" bore diameter Internal Retaining Ring, from McMaster-Carr (99142A775). Calculations are from Smalley Steel Ring Company [1]

    We will now need to calculate our values to insure that this will sufficient in our design.

    Acting Force

    Our case will be operating at a target 1,000 psi, our safety factor will be in our snap rings, so will not be included in this value.

    Total force will equal the operating pressure multiplied by our surface area, or:

    So our bulkhead will be pushing with 28,274.3 pounds of force against our casing. We need to insure our retaining ring can handle this via ring shear and groove deformation.

    Ring Shear


    P_R = Allowable thrust load based on ring shear (pounds)

    D = Shaft or housing diameter (inches)

    T= Ring thickness (inches)

    S_S = Shear strength of ring material (psi)

    K = Safety factor (3 recomended)


    This means that we can apply 35,342.9 pounds with a 3x safety factor to this snap ring before shearing would occur. Since our 28,274.3 pounds is under this value we are safe from shearing our ring.

    Groove Deformation

    Aluminum 2017 has a material yield strength of 40,000 psi. We will be using steel in our testing rig case, but our eventual rocket case could be using aluminum, so I wanted to insure it would be strong enough in these calculations.


    P_G = Allowable thrust load based on groove deformation (pounds)

    D = Shaft or housing diameter (inches)

    d = Groove depth (inches)

    S_gamma = Yield strength of material (psi)

    K = safety factor (2 recommended)


    Again we find that our groove deformation value is lower than the force we will be applying, so we should be good to go.


    At this point it would appear that using large 6" snap rings is mechanically feasible. However it is worth noting that during motor burn the temperature of the case will decrease the yield strength of the material significantly, and thermal insulation is inherently important.

    [1] -

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Enjoy this project?



Mike Maluk wrote 07/06/2016 at 04:40 point

Looks like one hell of a project! I'm excited to follow along!

  Are you sure? yes | no

J. M. Hopkins wrote 07/06/2016 at 23:07 point

It will be similar in scale to some of the larger tests of the SS2S (Sugar Shot to Space) program, and in fact will probably have some collaboration with that team during some the initial grain testing.

A lot of the issue of scale will be in how large these grains can be without failure. I'm hoping that my variant of flexfuel will allow this to happen.

Possible finocyl grains are on the table, but all has to do with the casting paradigm that works. I'm going for inexpensive (in comparison to other large motos), repeatable, and easy to manufacture.

We'll see :)

  Are you sure? yes | no

Mike Maluk wrote 07/07/2016 at 03:41 point

That sounds great, I saw Rick posted about a launch this past week on the SS2S program. I've been so swamped I haven't been able to get on too much. I'm on the cusp of separating so things are busy. I'll have a bit more time soon, though!

  Are you sure? yes | no

J. M. Hopkins wrote 07/07/2016 at 03:45 point

I was able to watch Rick and the SS2S team successfully launch their "mini" 2-stage at FAR, spectacular flight to over 30k on sugar.

They'll be on to attempt another 6" diameter here after another verification test of this design.

  Are you sure? yes | no

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