Constructing the model of the MMX spacecraft

Figure 1: The model of the MMX spacecraft

(Translated from an article by Takane Imada, MMX Spacecraft System Designer.)

Recently, I have become intrigued by civil engineering. On one of my cycle routes through the mountains, there is a point where the road curves and a dam wall becomes visible. There are many different types of dams due to differences such as location, the construction period, the type of material (rock) and available transportation to the site. But in each case, the engineer has done their best to store the largest amount of water with the least amount of effort using the technology obtainable at the time. It is always a result I can appreciate!

Not bounded by gravity and free from air drag, our spacecraft does not suffer from the same technical restrictions as dam building. For our technicians, weight poses the most severe restrictions on the design. Even essential controls and the equipment to be used on the mission must be built without any surplus bulk. By considering these needs and restrictions, we can create a plan for what the Martian Moon eXploration (MMX) spacecraft will look like. Figure 1 is a technical model of the spacecraft I sketched in three dimensions, overlaying a hand drawn background created with an artist tablet.

To create the spacecraft image, I used a three-dimensional software package called “Shade”. Although highly precise models that can be used in construction analysis are created using the full-scale CAD engineering imaging software, three dimensional sketches to examine the spacecraft layout can be done more easily with Shade.

Figure 2: The arrangement of the heaviest part of the spacecraft.

In this 3D sketch, we first draw the parts whose dimensions are already known. The heaviest part of the MMX probe (and therefore the first parts to be installed) are the propellant tanks. Propellant is a mixture of fuel and oxygen which combine to produce gas for the spacecraft propulsion. The mixing ratio of these two ingredients is one of the factors that determines the tank size, along with the required change in velocity of the spacecraft, ΔV, and the density of the propellant. Since the tanks are heavy, their position on the spacecraft must be carefully balanced by the rest of the structure. In this image, I did this by joining the propellant tanks to a conical shape like a Christmas tree that can withstand strong lateral stresses during launch (see Figure 2). As the MMX spacecraft will carry more propellant than any previous Japanese scientific satellite to date, I think this kind of structure will be suitable for lifting such a heavy load.

The size of the solar cells on the spacecraft is determined by the required electric power. The solar energy available at Mars is less than half that at the Earth, as the red planet is more than 1.5 times further from the Sun. We therefore need solar panels that are more than twice the size needed for a satellite operating around the Earth. In the three dimensional image data previously created with Shade, there was already a solar battery design. I used this same image and enlarged it for the MMX spacecraft model. The lightweight thin-film solar panels developed at JAXA have a slight curvature to improve their rigidity. On the image, this wave is emphasised to show this property but the actual panel structure may not be like this.

Figure 3: The spacecraft landing module.

A completely original part of the MMX spacecraft model is the landing gear and connected structures on the spacecraft’s landing module. For the “Hayabusa” and “Hayabusa2” asteroid missions, the spacecraft briefly touches down on a small celestial body with a weak gravitational field. There is therefore no need to have any leg-like structures. But the MMX spacecraft will hit the ground as a reasonable speed and risks bouncing unless there are legs to prevent a rebound. Although the shock from the landing will be absorbed by cushioning material, the spacecraft will still experience a force of up to 0.3 G (1 G = Earth’s gravity). This is enough for a spacecraft more than 1.5 metric tons to need a strong and rigid structure system. By referring to Moon-landing spacecraft, I attached the rigid legs with cushion cylinders in Figure 3 to the structure of the MMX spacecraft to absorb the almost all the landing energy and keep the spacecraft on the moon surface. However, the legs currently drawn will not allow the spacecraft body to clear the surface, so these will be extended in the next version.

The dark surface of the spacecraft body is the black MLI (Multi-Layer Insulation) that reduces heat lost via radiation and prevents the spacecraft becoming electrically charged. This is the same material that was used for the lunar orbiting satellite, “Kaguya”, and the Mars probe, “Nozomi”. For the MMX spacecraft, we have not yet decided whether black or gold MLI will be used. However, the surface the Martian moons are covered with fine regolith (soil) which is likely to be kicked up before and after landing. This risks sticking to the spacecraft surface due to its small electric charge. I think it is therefore highly probable that the black MLI will be used as it should be superior against such electrical adhesion. In the model of the spacecraft, parts of the propulsion tank are covered in the gold MLI. In fact, this is only to accent the appearance of the different components. The entire spacecraft must be covered by the same black MLI type if electric charge prevention is required. From the technical point of view, this two-tone view is incorrect! In Figure 4, you can compare the spacecraft all in black and all in gold, but I felt it looked clearer when both colours were used.

Figure 4: MLI colour comparison

Although I considered many technical aspects to create the model of the MMX spacecraft, I think the image is not yet right. But if we allow for this being a first sketch, you can see the spacecraft system already has many cool and interesting features. I am not sure if this design will become the familiar image of the spacecraft, or if it will be significantly modified as the technology is optimised further. However, as many engineers discuss conflicts with the design, the MMX spacecraft will become more refined and get cooler and cooler. Do keep an eye out for these changes in the future!