Fuel Grain Casting Method (HTPB)

Owned by Cameron Sexsmith
Last updated: Dec 11, 2023
4 min read

The Odyssey engine makes use of an HTPB fuel grain with a simple cylindrical port geometry. To accomplish this, many small test batches were completed featuring various ratios of resin and hardener, additives, and vacuuming conditions. Further details of the lessons learned through these tests can be found at the bottom of this page.

Mixing Materials

Chemicals

Resin - Rocket Motor Components - Hydroxyl-terminated Polybutadiene Resin

Hardener - Rocket Motor Components - Modified MDI Isocyanate Curative

Defoamer - Silicone based defoamer (need to update with more info)

Tools

  • Cement mixing drill attachment

  • Drill (preferably Ryobi)

  • 5 gallon bucket

  • Nitrile gloves (5 mm minimum)

  • Hot glue gun

Method

The total volume of the fuel grain is calculated prior to starting the pour. The mixture ratios we have found work best for us are 87% HTPB and 13% MDI isocyanate by mass. Using a rough density of 920 kg/m^3, the masses of each chemical can be found.

Liner Prep

We use a stainless steel mandrel that is completely coated in Vaseline for removal later. Prepping the mandrel is best shown in pictures shown below.

Materials for fuel grain mixing

The phenolic liner is hot glued into place as shown below.

After the hot glue is placed, the liner is slid over the mandrel and pressed firmly in place. After placing the mandrel cap in position, the final setup becomes the following.

Any gaps in the base between the liner and the mandrel are filled with additional hot glue to minimize the chances of leaks. A couple rounds with duck tape are usually added for extra security.

Mixing

  1. The HTPB is added to the 5 gallon bucket until the desired mass is attained.

  2. The MDI isocyanate is shaken in the bottle for about 10 seconds.

    1. We found this helps especially with older bottles of the chemical.

  3. MDI isocyanate is added into the bucket until the correct mass is attained.

  4. The total mass of the pour is recorded and 1% of this recorded mass will be the portion of defoamer added.

  5. The bucket is transitioned to the floor and mixed for 6 minutes using the drill cement mixer with a low speed.

    1. Trying to minimize the amount of air entrained into the HTPB.

  6. The bucket is poured into the top of the fuel grain mounted in the mandrel until the desired volume is attained.

The fuel grain is left to sit at room temperature without the use of a vacuum chamber. The grain is usually ready to remove the mandrel after 4-5 days of hardening.

Lessons Learned

As mentioned earlier, we conducted many test batches with various mixture ratios and curing methods. This list has all the major lessons learned from that testing period.

  1. Vacuum chamber for this size grain is very difficult to pull off. Not able to pull all the air bubbles out of the mixture after poured. Seems to be too viscous for us to pull it off with our current setup. The defoamer we have is meant to combat this issue but still doesn’t solve the problem.

  2. Use a high temperature hot glue gun for securing a mandrel to the phenolic. This gives a lot more time before it sets.

  3. Use Vaseline for the mold release. We tried multiple commercial mold releases and none worked as well as the Vaseline does. Doesn’t seem to interact much with the pour at all and slides right out when cured.

  4. Use very high grit sand paper on any 3D printed parts that will be used to modify the grain geometry. We added more space for the igniter at the head of the grain and successfully accomplished this with a PLA print that was sanded to 1000 grit and covered with Vaseline. Popped out very easily.

  5. When trying to remove the mandrel, twist and pull instead of only pulling. The twisting breaks any bonds that remaining to the mandrel and usually comes right out after the initial break. We have flats machined on the top of the mandrel for this reason. We should’ve had flats on the mandrel base as well.