No, you do not need 50 km / s delta ve. You Need Additive Technologies (Part 1)

Space flight is still expensive. Even if we take the seemingly overly optimistic one, the ability to launch a fully reusable carrier of 100-150 tons for $ 7 million - we get about $ 50 per kilogram of payload. A flight to the Moon or Mars using the same StarShip will increase the minimum cost of cargo delivery by about 6 times (5 refuelers will be added) to ~ $ 300 per kilogram.

Usually, from such calculations, it is concluded that industrial space exploration is impossible without the development of fundamentally new sources of energy or even non-jet motion or finding something very valuable in space. It’s just that it overlooks the fact that most of the celestial bodies in the Solar System have a runaway speed much lower than on Earth, where we, in theory, were going to import the mined, and the Earth has an atmosphere that slows down spaceships and ballistic capsules without spending reactive mass.

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Not enough vespen gas

The idea that it would be good to get fuel for the return flight on the spot a long time ago. I would venture to suggest that in science fiction it was not new back in the 1960s. But perhaps R. Zubrin in the Mars Direct project was the first to decide to promote it as the basis of a promising manned mission. Then came Elon Musk, who decided to take yes and try to do so (work in progress).

It is curious that in the production of fuel from local resources by electrolysis or by the Sabatier reaction, solid-phase NRE become economically disadvantageous. Yes, the methane NRE has a specific impulse that is about twice as high as that of the methane-oxygen LRE (see the book “Electric Interplanetary Ships” or the game Children of a Dead Earth). That's just for every kilogram of methane, the Sabatier reactor gives 4 kilograms of oxygen. Excess fuel is usually used in LRE, but, for example, in the case of the Raptor and Zvezdolet, 860 tons of oxygen are accounted for 240 tons of methane.

On the graph, the blue columns correspond to the final masses for four rockets with a characteristic speed (aka delta ve) of 5 km / s and fuel reserves equivalent in energy costs to the synthesis of 1100 tons of methane-oxygen. The yellow columns are the payload minus the mass of the rocket, provided that each technology has 10 tons of construction per ton of fuel. Orange - payload taking into account the density of the fuel (methane-oxygen - 20 tons per tonne of rocket, methane - 15 tons, hydrogen-oxygen - 10 tons, nucleus - 5 tons). A delta of 5 km / s was taken because it is the second space velocity of Mars. In the case of the Moon and its 2.5 km / s, the advantage of chemical rockets will be even more pronounced.

As can be seen from the graph, methane-oxygen outperforms the rest of the technologies without options due to the larger initial mass. A methane nuclear engine could argue with a hydrogen-oxygen liquid fuel engine, only if methane can be synthesized will there be anything to refuel a methane engine. For methane and hydrogen NRE to be able to compensate for the use of only part of the fuel plant's products, they need a specific impulse of about 10 and 30 km / s, respectively. Conclusion: for space transport using extraterrestrial sources of the working fluid, solid-phase NREs are unpromising. Only gas-phase engines can be of some interest, even in the best times of nuclear optimism, papers that did not advance further. Methane-oxygen is a more preferable pair than hydrogen-oxygen, however, if there are no carbon deposits on the celestial body, you will have to use what is.

Not enough minerals

So. We want to build a plant on the Moon that will send something useful to the Earth at an acceptable cost. In the beginning, you need to calculate this very cost.

The road map of the cislunian space. Taken from here .

According to the scheme, for a flight from a low near-Earth to the first point of Lagrange, we need 3.7 km / s delta ve. And another 2.5 km / s for landing. A fully charged Starship will land on the Moon with no payload with 130 tons of fuel. Having loaded ~ 50 tons of regolith into the ship, we will still have a reserve of deltas for flying off to Earth. Considering that the cost of the expedition, along with tanker launches, was $ 50 million (Mask himself promised "like Falcon-1 due to reusability", ie 5-7 million per flight), we get 1000 bucks per kilogram of regolith. What is curious, at such a price and delivery volumes, it is already quite possible to trade simply regolith for souvenirs and educational material for universities.

But on Earth, no one extracts minerals, having flown into a pure field by helicopter, and leaving everything that is badly lying in it. Instead, transport and mining infrastructure is being built at the beginning. If we consider the same "Starship" as the transport infrastructure, we will have a bottleneck in the form of +1000 $ / kg for transportation. In principle, you can live with this if you find something that can be pushed for more than $ 2000 / kg (taking into account non-transportation costs and a non-zero margin). And such substances exist - see the price list [1]. ULA in its CisLunar Economy wanted to bring materials for the construction of satellites and solar power plants into low Earth orbit. But still try to expand the bottleneck.

We will expand the bottleneck by optimizing transport. From the Moon’s point of view, the Starship shuttle shuttle scheme is not optimal - a reusable ship constantly dives into the gravel, from where it has to be pulled out and, at the same time, it takes fuel for flights in the same pit. Moreover, on the moon, most likely there is water, the solar constant is twice as high as on Mars, in the absence of clouds. In impact craters, metals can be found, including iron. The latter is convenient in that it can be scanned from a satellite in a magnetic field and selected from regolith using it.

You can launch cargo from the Moon to Earth in the following ways:

  1. Rockets fueled from local resources.
  2. Electromagnetic gun.
  3. Somehow else.

Let us dwell on the first option, considering that NASA was not mistaken at the expense of water. According to the latest data, water at the North Pole alone is not less than 600 million tons [2], so that the exhaustion of this resource in the near future does not threaten.

A missile can either be built on the spot or imported from Earth. In the first embodiment, one-time use is possible, in the second only reusable. In both cases, it is necessary to master the production of disposable ballistic capsules from local resources.

Consider the option with an "import" rocket. 2 tons of dry weight, 14 seasoned. Worse than the Centaurus with 20 tons of hydrogen-oxygen per 2 tons of dry mass, but the Centaurus has no legs to land on the moon. Without PN, the tug will have a delta of 8.5 km / s, which is enough to land for landing on the moon at the start with NOU. On which the boat will throw all the same "Starship" of the associated PN. Back to Earth, the ship will be able to push out a ballistic capsule weighing 10 tons and return empty.

The cost of one tugboat voyage will be equal to the cost of building a tugboat and putting it to the DOE divided by the number of uses. For the first, the very same $ 50-60 million seems to be a completely adequate estimate from above - this amount is on the same order as the cost of launching a whole Falcon-9 or manufacturing a Dragon capsule. According to [3], the RL-10 engine in the early 1960s could run up to 2.5 hours with 50 restarts, after improvements it could last more than 11 hours, unfortunately, there was no information about the number of starts. But it is known that the J-2 withstood 103 launches and 6.5 hours of operation, and then the engineers got tired :) So the resource of 50 flights on the engine does not look fantastic. Total we have about a million dollars for a tugboat flight. In one flight, the tug kicks a 10-ton capsule to the Earth, assuming that the capsule has a “fill factor” of only 50%, we get a million for 5 tons or $ 200 per kilogram. Five times less than Starship. The most interesting thing is that if instead of the Starship, a tugboat is launched with the usual Falcon-9 with a used stage and a stage return, the price will increase only to $ 400 thousand per ton.

But will not the whole creation of a gas station spoil? Yes, and along with the production of ballistic capsules and the production of rare earths. About this in the sequel, which follows.




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