The fun of spacecraft system design

Article by Martian Moons eXploration (MMX) Project Lead Engineer, Takane Imada

Introduction

I like to travel all over Japan by bicycle. Each time I go on a trip, I collect together all my equipment. But since my bicycle does not have a carrier, I have to carry any luggage on my shoulders. This limits the weight I can bring along.

For common troubles such as flat tyres, I always bring the necessary tools for repair and replacement inner tubes. But for other potential problems that may occur while on the road, the weight constraint affects what I can bring along. The area of my trip also affects this choice.

When travelling along the Sea of Okhotsk side of Hokkaido or through the mountains of Shikoku, there are no convenience stores or shops within 100 km, and alternative transportation such as buses are also scarce. Therefore, tools such as a chain cutter in case of a chain break are necessary.

I am the Martian Moons eXploration Project Team Spacecraft System Lead Engineer, Takane Imada. The system design for a spacecraft that I do is similar to the above preparation. With limited mass, the key is to prepare the most efficient combination in anticipation of what may happen in the future. For artificial satellites in the Earth’s sphere, rapid support provided from the ground can be expected. But the spacecraft system design for deep space missions such as Hayabusa, Hayabusa2 and MMX, it is similar to the idea of preparing to ride a bicycle in an area where support such as convenience stores cannot be expected at all.

MMX mission overview

The primary goal of the MMX mission is to return a sample of material from Phobos. No spacecraft has ever completed a return trip to the Martian sphere, so if we are successful, we will become a world first!

The spacecraft for the MMX mission will need to be launched by the most capable type of H3 rocket which is currently under development by JAXA. The MMX spacecraft will need to descend into the gravitational well of the Martian sphere and then return from there; a trip that requires two large decelerations and accelerations. The amount of propellent that must be carried is therefore large, and forms a major feature of the MMX spacecraft.

After arriving at the Mars sphere, observations to find a safe landing site on the surface of Phobos will be prioritised over a year and a half, and an area as flat and wide as possible will be selected as a candidate landing site.

Even after the landing point has been decided, the landing itself will not be performed immediately. Rehearsals will be conducted in advance to observe the surface, and the CNES/DLR Rover will be dropped to make preliminary measurements of the properties of the Phobos surface, especially the surface hardness.

The actual landing is the most dangerous stage of the MMX mission, and coordination with the ground controllers back on Earth must be carried out as closely as possible. The landing will therefore take place during the middle of the period that the MMX spacecraft will stay in the Martian sphere, during the time when the distance between the Earth and Mars is shortest and fastest communication can be expected. A total of two landings are anticipated, and samples will be collected from both locations.

The remaining time before returning to Earth will be spent near Phobos to perform more detailed observations of Phobos and of Mars. When we finally leave the Martian sphere, the orbit of the spacecraft will be adjusted to make a fly-by of Deimos for detailed observation. Then, the spacecraft will return to Earth with a valuable sample from Phobos five years after launch, along with a large quantity of observational data.

Main discussion/decision points for the MMX spacecraft design

When designing the MMX spacecraft, there were a number of important decisions points to resolve that determined the design policy. This could be said to be one of the real thrills of system design. Here are two examples.

Example 1: Chemical or electrical propulsion system?

One of the major differences between the MMX spacecraft and that of Hayabusa and Hayabusa2 is that for both the outbound and inbound journeys the MMX spacecraft will use a chemical propulsion system, rather than an electrical propulsion system. This is because the total acceleration that must be generated by the propulsion system of the MMX spacecraft is about twice that of the Hayabusa/Hayabusa2 missions, and the necessity to land on a celestial body with higher gravity than the asteroid meant that there were trade-offs in the design of the propulsion system.

Since the spacecraft on the outbound journey must also carry the propellent required for the return route, a large mass needs to be accommodated during this phase. This must be combined with a high thrust, which is indispensable for placing the spacecraft into the Martian gravitational sphere. But near Mars, the intensity of sunlight drops to less than half of that in the Earth’s orbit. This makes solar cells an unrealistic energy choice to provide power for electrical propulsion. Chemical propulsion was therefore decided for the outbound journey, leaving us with two proposals for the return route: (A) chemical propulsion also for the return trip to Earth or (B) electric propulsion for the return trip. There was also an additional choice as to whether to stay in the Martian sphere for one or three years, giving a total of four possible plans. The results are shown in Table 1.

[Proposals]
A-1) Chemical propulsion for both the outbound and inbound routes. 1 year stay at Mars.

A-2) Chemical propulsion for both the outbound and inbound routes. 3 year stay at Mars.

B-1) Chemical propulsion on the outbound route, electrical propulsion on the return route. 1 year stay at Mars.

B-2) Chemical propulsion on the outbound route, electrical propulsion on the return route. 3 year stay at Mars.

The final conclusion was that despite the extra weight needed for chemical propulsion, this would be used for both the outbound and return journeys. The decision was largely based on considerations for the landing on Phobos, where the larger solar panels required to power electrical propulsion would result in a greater risk of tipping and ground contact. The return journey by electrical propulsion might also take an extra two years and two months to return to Earth, compared to the chemical propulsion system. This was unpopular with planetary scientists who wished to get the samples as soon as possible!

With a prediction for an increase in observation requests in mind, a three year program in the Martian sphere was chosen. Subsequent refinement of the Phobos observation operations uncovered that it was unlikely all objectives could have been achieved within just one year. I feel the choice for three years at Mars was the right one.

Example 2: Three or four legs?

The gravity of Phobos is large compared to that of the asteroids Itokawa and Ryugu. However, this is far smaller than the gravity you normally feel on our Moon. Water in a bucket on Phobos will not curl upwards due to surface tension, but will collect towards the bottom. As we proceeded to consider the landing of the MMX spacecraft, it became clear that full-fledged legs would be required to land in the gravity of Phobos.

Until now, spacecrafts that landed on our Moon have had either three or four legs. Lunar landers from the USA have included many three-legged crafts, including Surveyor 1, which made the first soft landing on the Moon, while the Apollo program used four-legged lunar landers. Table 2 shows the trade-offs in these two designs.

While a three-leg design has the advantage that all legs are always in contact with the ground, there is a greater potential to tip and fall compared to a four-leg design. I chose the high stability of four legs as the surface gravity of Phobos is low enough to result in a rebound.

Passenger cars come to mind when considering this trade-off. Today’s passenger cars are almost always four-wheeled, but there have been a number of elegant three-wheeled passenger cars. These were probably ultimately avoided due to the potential to fall.

Transition of the MMX spacecraft design

Based on the decisions in the previous section, we proceeded with the design of the MMX spacecraft. As the study progressed, the outer shape changed significantly from the first design. Figure 1 shows the transition.

(i) First generation (2015 – 2016)

At this time, we were focussed on the feasibility of the round-trip to and from the Martian sphere. There were two design candidates because we were still undecided about whether we would use chemical or electrical propulsion systems.

As the efficiency of photovoltaic power generation decreases near Mars and a large thrust is still required to leave the Martian gravitational sphere, a large array solar cells would need to be installed to consider electrical propulsion. This was before considerations for the landing on Phobos had become advanced. When I think about the situation now, and the difficulty of having the large array of solar cells during the landing on a celestial body with significant gravity such as Phobos, I realise that the decision not to go with electrical propulsion was the correct answer.

(ii) Second generation (2016 – 2017)

This is the spacecraft form after deciding to use a chemical propulsion system for both the outbound and inbound routes. The choice of chemical propulsion meant that the needed mass of propellent was going to be large, and this period focussed on reducing the weight of the spacecraft. This led to a module configuration, with a selection of a 3-module form that could minimise the launch mass.

In addition, the solar cell paddle was designed to utilise a newly developed type of thin film solar arrays, allowing the panel to curve in order to reduce weight. The legs were placed parallel to the solar panels to prevent the paddles from touching the ground as much as possible when the spacecraft tilts during landing. The structure of the legs themselves had not been studied at this time, and they would prove not to be completely feasible.

(iii) Third generation (2018)

We were now amidst full-scale considerations of landing on Phobos, and this model was used as the basis for the analysis of the landing dynamics. Examinations of shock absorbers was also progressing and the needed stroke and the design of the connecting parts with the main structure had also begun in earnest. Structurally, the shock absorbers here are of the telescopic type that are similar to those on the front wheel of a motorcycle, and also similar to those on the Apollo Lunar Module.

The solar array paddle design remained the same as for the second generation, but with longer legs and more room for contact with the ground. The ground height was also decided so that the design met the sampling requirements on the Phobos surface.

During this period, we were also exploring the possibility of simplifying the system and this version used a two-module configuration that integrated the exploration and return route modules, which differs from the current design.

(iv) Fourth generation (2019 ~)

As the total MMX mass became heavier as the study progressed, we returned to the complex but lighter three module design. The shock absorber consists of the main landing gear and a sub-landing gear that are connected together using a hinge. While still minimising the size of the main structure, the spread of the legs was made as wide as possible to prevent any falls. Exploration of the structure of the legs had also progressed and we found a design that could take the shock from landing on Phobos.

For the solar paddle design, the lightweight paddles of the third generation and earlier were found to easily bend during the landing operation and they could come into contact with the ground. Therefore, a shape closer to that of a square was adopted that uses a 2-stage deployment mechanism even though  it is a little heavier.

Based on this form, we are currently trying further refinements that could result in weight reduction.

Finally

My role in MMX is described as the “Lead Engineer” in English and translated to something like “Chief Designer” in Japanese. Have you ever seen the Ghibli anime, “The Wind Rises”? The hero in that story has the same job as me! In the project, the highest role is that of the Project Manager but I feel the main character in the big story of the project is really the Lead Engineer!