A model of a scramjet (or ramjet) engine. The model also includes a de Laval (convergent-divergent) nozzle which would be used on this type of engine.

There are two print profiles:

• The big boy that measures 776mm (almost 80cm) from the cone tip to the nozzle end
• Its little brother at 50% scale

The assembly process is simple and it shouldn't take you more than an hour.

I didn't try to scale below 50% but it should work if you scale it down even more (or scale it up).

The model consists of:

• The main body with a cutout to display its internals
• Cone assembly with fuel injectors
• Diffusor and liner

Some theory

What is a scramjet or ramjet engine

This is still a mostly experimental technology with a lot of potential. Both versions achieve extremely high speeds which can't be achieved with normal jet engines. Above Mach 3-4, this engine type becomes more efficient than other types of jet engines.

They have little to zero moving parts which also means, that for them to work, they need to be already moving. 

These types of engines require airflow to work. They are installed in vehicles that are dropped from an airplane or are propelled using an alternative method of propulsion first.

The air coming in is slowed down in the first part of the engine.

Next fuel is injected and ignited in the second part.

Last, the ignited fuel-air mixture is propelled through the nozzle which speeds it up to supersonic speeds.

Difference between scramjet and ramjet engine

The scramjet engine is a variant of the ramjet airbreathing engine. The difference between the two is that in the scramjet engine, combustion occurs at supersonic airflow while in the ramjet engine, combustion occurs at subsonic airflow.

Assembly

Required tools

No special tools are required except for glue. 

I recommend using a 5-minute 2-part epoxy to give you time to align parts. Alternatively, you can use slow-acting superglue but the gaps may be too large to hold properly.

In addition, some masking tape and clamps can be helpful.

Assembly instructions

Before you glue anything make sure it fits together. 

Make sure the glue is completely cured before you join more parts together.

Step 1

Assemble the three body parts. 

I recommend you apply glue to the marked corner to minimize the risk of it spilling out of the part.

Step 2

Assemble the cone.

Glue the two parts together.

Step 3

Install the injectors on the cone. They will likely hold without glue but I recommend you glue them in place. Install them without glue first, remove them, and install them again with glue. Make sure you keep them aligned so it looks nice.

Step 4

Insert the liner in the body assembly. Do not glue it yet!

Push it down enough to make room for the next step. About 70mm below the glue line of the top two body parts should be enough.

Step 5

You will now glue the cone assembly to the body assembly. This is the most difficult step.

Place the body assembly on its side. See pictures of where to apply the glue. Use the glue line of the top two parts as a reference for where to place the cone assembly. This line should be just covered by the cone assembly ring.

Slide the cone assembly in and align. The cone tip should be 73mm from the surface.

I used clamps to hold the assembly in place and the masking tape was used to keep the cone tip at the right height (73mm from the surface).

Step 6

Push the liner to about 40mm below the bottom of the injectors. Apply glue in the back where it touches the body. Then pull up the liner to about 30mm below the bottom of the injectors. 

Step 7

Apply glue to the perimeter of the diffuser part. About ⅓ of the perimeter is enough. 

Apply glue to the top edge of the liner. 

Place the diffuser on the liner

Step 8

You are done. Time to display your model.