Thursday, October 29, 2015

Interstellar Baggage – Part 4 – Landers

When a seeding vessel from the originating solar system arrives in the destination solar system, there are several options it could follow to try and improve the probability that it would be able to accomplish its mission. In the best of all possible situations, the new planet is just waiting for the settlers to arrive. It is more than just an ordinary sweet spot world, it is one in which the biology is right as well. The best analogy on Earth would be a large island, undiscovered by humans, but full of edible flora and fauna, and free from any highly lethal bacteria or viruses. Humans arriving on that island, provided they had some minimal hunting equipment, could simply move in and make themselves at home. They of course would need a boat or canoe to get them to the island. There also would have to be a beach or a harbor where they could bring the boat and leave it, while they explored the island.

The seeding vessel was not the first starship to arrive, but at least the second, one or more probes having preceded it. They would have found some sites that would be good to set up an initial camp in. This just leaves the problem of getting there from some orbit around the star, when the deceleration engine was turned off, leaving them in the right solar system. Running the engine a bit more might bring them somewhere into the gravity well of the destination planet.

One of the key elements of the baggage the seeding vessel would have to bring involves the descent to the planet’s surface. There would seem to be no point to leaving some supplies up in planetary orbit, to be dropped down in a second lander. Having everything present at the initial target landing site seems to be the most practical method, assuming the probe or probes did their task correctly and ascertained that there is a good landing site and there would be no need to try and move to another one.

There are two options at this point. One is to bring the entire star vessel down to the landing site so that metals or other construction materials can be salvaged from it, and the other is to reduce the landing weight to the minimum, so that less payload weight would have to be devoted to the task of entering the atmosphere of the destination planet. A middle solution might be best, leaving all the portions of the starship which were irradiated during the voyage, such as the power plant and some propulsion system parts, up in orbit and bring down the rest. It would be beneficial for the settlers to have power there, so the question of whether to bring the starship power source down depends on the design details. Otherwise solar cells or other power sources would need to be part of the payload.

On any possible sweet spot planet, with the right gravity, which is dictated by the need for the right kind of atmosphere, orbital speed is very high compared to the rotation of the planet, so a great amount of speed would have to be burned off. One way is to use some remaining fuel in the starship, changing the vector of thrust so that the vessel does not fall into the atmosphere before enough speed is reduced. This requires a large amount of thrust, and if the starship was designed to have low thrust for a long time, there would not be enough thrust for this option. A second engine could be added to the payload for this, but the engine would have to be large and all this payload weight would be expensive.

The other obvious alternative is to use the atmosphere itself for speed reduction, which produces a large amount of heat. Some heat shield is needed able to withstand the short duration high heat levels caused by aerobraking. Then, at the end of the trajectory through the atmosphere, where speed is sufficiently low, balloons and /or parachutes are needed to soften the impact. Coming down on water is not an option, as this would require the mean density of the starship, even if it were possible to shed the heatshield, to be less than that of water.

The larger the parachute, the slower the final impact on land. A faster impact means more shock and sturdiness in the landing vessel, which means more weight. A slower impact means larger parachutes, which means more weight. Just one more of the obvious tradeoffs that would have to go into the design of the landing system.

Because there will necessarily be some residual impact, the choice of the landing area will have to be made with this in mind. Landing on sand will be easier than landing on rock, and landing in an area where there are protuberances, for example rocks larger than a basketball, would likely add further damage to the craft. Navigating to this area through the three stages of landing: reverse thrust, aerobraking, and parachute, will be challenging. Aerobraking is a somewhat unstable process, meaning that slight corrections of the orientation of the landing vessel will lead to large perturbations on the landing spot. Parachute motion is determined by wind, and the slower the descent, the more susceptible the motion will be and the larger the discrepancy between desired landing site target and actual site. Parachutes are certainly navigable, but they are always at the mercy of the winds, especially near the surface.

Navigation would have to be done based on some type of inertial navigation system, as the remaining portions of the starship in orbit cannot provide sufficient references. Optical tracking and communication could help in the parachute descent, but the orbiter might be out of position by that time.

A perfectly smooth, rockless sandy area would be the ideal landing site for accomplishing a landing with minimal damage and the highest probability of success, but such a site might be far from any area that the landing party wanted to be in. Large pastures or meadows might be suitable sites, as long as there were not mountains nearby or even outcroppings with hard rock exposed.

A lot of payload mass, in terms of required starship propulsion, has to be devoted to the descent to the surface, and that mass would be roughly proportional to the real payload mass that is useful for the landing party. In science fiction there is invariably the magic of high thrust propulsors plus infinite fuel, but when the alien civilization has to make a realistic design, there are some very obvious tradeoffs that they are forced to make, meaning sacrifices. The resulting lander may not look too different from those in our primitive era of space transport, except for a great difference in scale.

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