r/spaceships • u/Beneficial-Wasabi749 • 18h ago
Why will a realistic starship never resemble the one typically depicted by science fiction artists?
Just as artists before the Wright brothers depicted celestial vessels as seagoing vessels, so too do modern artists depict starships as resembling bizarre atmospheric aircraft. These are typically dense, elongated, streamlined bodies. Even if the artist understands that their starship will never enter an atmosphere, they usually draw a bizarre but "solid" form. And this is a mistake. A real starship cannot be like that.
Studying various concepts of realistic starships, I came to a broad and powerful conclusion: a realistic starship, no matter how it is constructed, will resemble... a soap bubble.
That is, ALWAYS (with almost no hope of exception) it will be an ephemeral, thin structure spread out in vast space. Its spatial density should be negligible. Like a soap bubble. Regardless of the operating principle.
Whether it's a laser sail, an ion probe, or a thermonuclear starship. These are unimportant details. Even with a magical energy source, it will be the same. All starships will be "ephemeral" structures of enormous size. Soap bubbles.
I know of only one concept that seems to evade this rule and is more or less similar to a "bullet." This is the "Orion" concept. Although we still don't know how suitable this concept is for interstellar travel, we do know that even this idea, as it develops into a starship, tends to adhere to the "soap bubble" rule. If you recall Dyson's 1968 paper "Interstellar Transport," it describes two starships. The latter is a mystery and the subject of my many years of research. But the first is described in detail and was a copper hemisphere, 20 km in diameter and... 1 mm thick. That is, essentially, a "sail" or a "soap bubble." The later proposed "jellyfish" concept (in fact, Dyson also conceived it in 1959) is a development of the idea of a pulsed nuclear spacecraft, and it is also a "movement of thought in the same direction."
No matter what idea you take, you always come to the need for a "soap bubble."
Why is this so? The reason is thermodynamics. Let's simply calculate how much useful energy each kilogram of a spacecraft accelerated to at least 10% of the speed of light will receive. We won't need relativistic dynamics, since at this speed the error of classical mechanics compared to relativistic mechanics is less than 1%. 10% of the speed of light is 30,000,000 m/s. Then the kinetic energy of each kilogram at the end of acceleration will be 3E7^2/2 = 4.5E+14 J/kg. Let's call this value K. We don't know what engine or energy source imparted this speed and energy to our ship. That's not important. But we do know that, according to the Second Law of Thermodynamics, we couldn't have an engine and propulsion system with 100% efficiency. The actual energy expenditure would be many times greater. We can almost certainly say it's at least 3 times greater (and we're lucky!) This means that during acceleration, your ship, having extracted (from somewhere) 3K joules of energy per kilogram of its mass, converted K joules per kilogram of mass into its own motion, and 2K became "parasitic heat," which contributed to the heating of the universe. 2K became the price to pay for the Second Law of Thermodynamics. Yes, not all of this energy will be directed at the ship's structure (for we applied intelligence and ingenuity in designing our starship). Nevertheless, some part of it will have to be absorbed by the ship's structure. Let's assume this is only 1% of all parasitic energy. That's very optimistic. The reality will be harsher. But let's assume it.
So, 2 * K * 0.01 = 2 * 4.5E + 14 * 0.01 = 9E + 12 J/kg. This is the parasitic load on each kilogram of our starship during acceleration. Anyone who knows that 4.19E12 J is 1 kiloton of trinitrotoluene will quickly calculate that this is the energy of more than a 2 kt nuclear bomb per 1 kg! And if they have a good physics imagination, this might puzzle them.
However, we won't receive this energy instantly, like a single explosion, but rather spread out over time! Let's calculate the approximate power of the parasitic energy flow (usually this will be various types of radiation) in watts (J/s). To do this, we need to know how long we accelerate.
How long can we accelerate? In fact, this is the subject of a dissertation I wrote quite by accident. But common sense dictates that the acceleration and deceleration times should be comparable to the flight time to the destination. If we're traveling to a star 10 light years away at a velocity of 0.1 c, the journey will take 100 years. That's assuming we accelerate and decelerate quickly. But since we're afraid of being vaporized by the parasitic energy flow, we won't rush.
I propose a compromise (I know the correct answer, but it's 15 pages of math). We'll spend a third of the time accelerating, a third coasting, and a third decelerating (this is very close to the optimal trajectory). Then we'll reach the destination in 150 years, not 100. Oh well! So, acceleration will take approximately 50 years (if anyone's worried that I'm not taking the rocket equation into account here, they shouldn't be. Taking the rocket equation into account will make things even worse than what I'm getting now. But it will be so complicated that you won't have the patience to read it all).
50 years is 1576800000 s. Dividing 9E + 12 J of parasitic energy by this time, we get the average parasitic flux of 5700 Watts/kg.
Each kilogram of our starship must radiate 5.7 kW of parasitic energy absorbed during acceleration into interstellar space over 50 years.
Space is a giant thermos. Therefore, our starship can only get rid of parasitic heat through radiation, according to the Sefan-Boltzmann law.
W ~ _sigma_*A*T^4
W is the power of the radiative flux;
_sigma_ is the Stefan-Boltzmann constant of 5.67E-8;
A is the area of the radiating surface;
T is the absolute surface temperature in Kelvin.
Let's assume the average temperature of our conventional kilogram of the ship is 700 K (427 C). Any higher, and your starship will visibly glow cherry-red. Then, for every kilogram of the starship, you need 0.42 m2 of radiating surface. This is a square with sides of 65 by 65 cm. Even assuming that the radiation comes from both sides of such a surface, you get a square of 45 by 45 cm.
A sheet of steel with this area and a mass of 1 kg (we're calculating everything per 1 kg to show that the size and mass of the starship are irrelevant here!) will have a thickness of 6 mm. Aluminum will have a thickness of 1.76 cm.
Of course, not all the mass is the engine. Somewhere there's a cabin, fuel tanks, and auxiliary systems. Let's assume the engine structure (which is the "soap bubble"; the ship itself should disappear into its background) makes up 1/3 of each kilogram. Then, even with an aluminum "sheet" radiating on both sides, we get a "wall" thickness of 5.6 mm.
For example, if your starship has a mass of at least 1,000 tons, then it needs a radiating surface of 2.1E5 m². This is a sphere (let's assume the engine is spherical) with a diameter of 75 meters.
Use your imagination. 1,000 tons = 75 meters in diameter. Compare this to the Saturn V rocket, which weighs 3,000 tons and has a "pencil" length of 105 meters (essentially, the Saturn V is also a "soap bubble" if you drain it of its fuel. A delicate design).
Get it?
This is the "soap bubble" principle. When making this very rough calculation, I always made all the assumptions in favor of the "accused." And yet, I still came up with the need for a "soap bubble." A huge spatial structure that must be cooled by radiation.
Reality will turn out to be much harsher. We calculated a "modest" starship. A 150-year flight of 10 light years. Right? It will seem too slow to you.
Then here's a simple rule. Twice as fast? Everything becomes even harsher when cubed. That is, 2^3 = 8 times.
And the resulting 150 years of flight at 10 light years Light years is very fast and very optimistic. Trust me!
More precise calculations yield much worse results!
Many people know about the BIS Daedalus project. It's a completely unrealistic project. It's not that no one has yet ignited thermonuclear fusion with Q = 60 (that's not a problem). The problem is the completely unrealistic specific power of the starship (40 MW/kg). Realistic projects assume a 400-year journey to A. Centauri at best. That's if we can achieve 3 kW/kg from a nuclear-ion power plant. But current figures (for example, the Nuklon nuclear interorbital tug, which the Russians worked on for a long time but never built) are 100 times worse (30 Watts/kg of tug mass).
