Voaskhod-D as it would appear from my window (Click to Enlarge)
Yes: Восход-Д is large and massive. Well, in a sense less massive than large, but anyway that’s what it is: a large, massive structure. From end to end, the spacecraft is 632 meters long, and at its widest it measures 125 meters. To give you an idea, in picture number 1 I tried to figure what it would look like if it was somehow dumped near my home, in via Gran Sasso in Milan, with the HAB resting in the middle of Piazza Piola. A beast. For trekkies out there, it is more than twice as long as a Constitution class cruiser (yes, good old NCC-1701), albeit not as wide. For starries, it is three fourth the length of a Star Destroyer.
I’ve designed it as a modular structure, the idea being that its components would have been launched to Low Earth Orbit to be assembled in space, much as it has been done for the International Space Station. The HAB itself is built from 48 sections, spanning an arc of 7.5 degrees each, each made up by 12 modules (total: 576 modules). Forward of the HAB bearing are three habitable microgravity modules (i.e.: they don’t spin) which I modeled after the Mir core and Kvant elements.
Its size is dictated essentially by two factors: first, the size of the crew, which requires a lot of space, and second, the fact that its main drive is a nuclear reactor, therefore it should be kept as faraway as possible from the crew itself. Beyond that, the reactor is not completely coated by shielding, to save mass: the shadow-shield leaves a radiation-free cone of 20° in aperture, therefore the HAB has to be removed a long way from the engine to be completely within this cone.
Most of the length of the spacecraft is filled by lightweight truss structures which only have the function of linking the payload to the main drive, distribute acceleration forces when these apply, and sustain the engine heat radiators, the propellant tanks and the four high-gain communication antennas.According to my crude computation, the launch mass of the spacecraft is in excess of 20,000 metric tons, most of which, though is the propellant.
To those who might be interested in these matters, you should know that the whole engineering behind astrounautics is concerned with the optimization of a single, very simple equation: the Tziolkovsky Rocket Equation. Since in space you basically always move along an orbit, the performance of a spacecraft is not determined by how long it can run (which it can forever), but by how much it can change speed, or accelerate. Acceleration is what you need to escape the gravity of Earth, to surpass Earth itself and launch yourself into the outer Solar System, and to match velocity with your destination planet. The total amount of meters per second by wich the spacecraft can change its speed is called its Delta-V (or total difference in velocity). Now, the Tziolkovsky Equation basically states that the delta-v of a spacecraft depends on two things alone: (a) the speed at which its engine ejects propellant mass into space (or exhaust velocity), and (b) the ratio between the mass of the spacecraft with and without propellant. In the case of the Восход-Д, we wanted a total delta-v of 20,000 m/s. From the (estimated) performance of my nuclear reactor, the required mass-ratio is 3.4, therefore at launch about 70% of the total mass of the spacecraft is propellant.
To avoid extremely large propellant tanks, Soviet engineers used methane instead of the commonplace hydrogen. Methane has a lot of advantages, so many in fact, that it looks quite strange that it is not commonly used as a propellant in real life (more to do with my incompetency, I suppose). Anyway it is much denser than hydrogen (more or less half the density of water), requires less energy for storage in liquid form (liquefies at higher temperatures than hydrogen), and most importantly, it can be found, already liquefied, in lakes on the surface of Titan, where Восход-Д is directed. The initial project was even to refuel at destination, but there’s a big problem with this: you have to lift so much mass from Titan to orbit, as the spacecraft itself is never going to land anywhere, and to do so you need a launcher (which you have to bring alongside), and more delta-v (at least enough for the landing, assuming the launcher itself is reusable and can refuel in-situ, using methane as well). That means more mass on one side, and an enormous surface-to-orbit lifting capacity.
Now the 20,000 tons of the Восход-Д are not that much, on Earth: any average-to-small cargo ship can load as much without much effort. But launching that into orbit is an entirely different story. The good old Space Shuttle (or rather Buran) had the cargo capacity of an average TIR lorry: 25 small tonnes. If the whole material and propellant for our spacecraft was to be launched into space by Buran alone, we’d need 800 launches. In its whole operating life, which spanned thirty years from 1981 to 2011, Space Shuttle only made 133.
For what concerns the Earth orbit assembly, we can safely assume our Soviet Unit went fearlessly on building something like a heavy-lift nuclear-powered vector such as the Liberty Ship (thank you again, Atomic Rockets). In case you think I’m too optimistic, I’m just as optimistic as NASA scientist in the nineties. Anyways, this way we might have put Восход-Д in orbit in twenty or so launches – but the problem would turn up exactly the same way on Titan. True, we wouldn’t have to lift the whole ship into orbit, but as said above, the ship’s just 30% of the story: you need that 70% propellant, plus the propellant needed to propel the propellant in orbit, and all in a reasonably short period of time.
If you’ve got a solution, please comment. Otherwise, whatever lands from Восход-Д onto Titan, stays there.
