13
$\begingroup$

I am trying to design spacecraft with realistic mass estimates, so that I can "accurately" design fuel requirements and calculate potential speed.

Obviously the mass of a ship is going to vary widely based on in its dimensions and purpose (a 500m long freighter will have a very different mass than a 500m long battleship). That being said, I am trying to come up with some reasonable estimates and real world comparisons so that I could quickly estimate the mass of any ship.

I have looked at the mass of nautical ships, aircraft, real spacecraft, and fictional space craft; the numbers seem like they vary so much that I am not really sure where to go from here.

Here are some rough approximations I have gathered so far as a starting point.

  • The space shuttle (empty) ~= 75 metric tons.

  • A 757 at take off ~= 100 metric tons.

  • The ISS ~= 450 metric tons.

  • The Seawise Giant (largest ship ever built) has a full load displacement ~= 654,000 metric tons.

  • The Starship Enterprise ~= 4,500,000 metric tons (this seems absurd to me compared to the other measurements).

As for technology level, I am thinking something like 300-400 years in the future. I want to create something with elements of space opera, but based more on hard sci fi/realistic constraints. Because FTL is out, I am imagining a solar system where travel between plants and satellites is relatively easy and quick (like a few months to outer solar system instead of years), where the OORT cloud is the untamed frontier, and if humans have left the solar system, its only in generation ships which effectively are cut off from the rest of humanity.

Edit After digging around on the site that Jim2B listed, I found this excellent section on interstellar trade: http://www.projectrho.com/public_html/rocket/basicdesign.php

There are a couple of paragraphs on estimating the size a ship based on the tonnage of its cargo, which could of course be used to calculate tonnage from size. This is really perfect for my purposes, because even though I asked about mass specifically, the real problem I am trying to solve is how large do ships need to be to accomplish "x" task. I was probably going about it slightly wrong as I was trying to think in terms of "how large is a plane, or an ocean liner, or a battleship, etc." The approach from Atomic Rockets is much more systematic.

$\endgroup$
  • 2
    $\begingroup$ Do you want to have your spacecraft be able to land on planets, or just remain in space indefinitely? Presumably you want real, chemical fuel. How often do you want to refuel? These will dictate a lot of the construction of the ship. You should also be looking more at the ratio of fuel/mass rather than just mass. $\endgroup$ – Lacklub Apr 11 '16 at 17:21
  • 2
    $\begingroup$ If you're not using chemical fuel, then basically every real world comparison goes out the window. And if fuel isn't a big concern, then your mass can (almost certainly) become much larger than the Starship Enterprise. Cost is always limiting, but structural requirements are ridiculously low (in terms of size dependence) in space. $\endgroup$ – Lacklub Apr 11 '16 at 17:31
  • 2
    $\begingroup$ You can make these calculation only knowing exactly all the materials used. That's why your search results seem to be all over the place. Obviously you wont be in front of a CAD developing all the parts of your spaceship. Get a comparable (use the Shuttle) and just multiply or divide according on dimensions of your ship compared to that. Then add or subtract a fudge factor depending of your made up materials. And the Enterprise could push those numbers, its a made up spacecraft, so it could have fantasy materials of any density one wanted. $\endgroup$ – Erik vanDoren Apr 11 '16 at 17:37
  • 2
    $\begingroup$ You'll have to provide a lot more in the way of specifics. The mass of spacecraft today is generally dictated by the Tsiolkovsky equation, but that's contingent on rocket drives. If you don't want to expand on the drive technology, the only reasonable advice I can give you is to calculate the density of your examples and use that to determine mass based on size. $\endgroup$ – Mike L. Apr 11 '16 at 17:58
  • 2
    $\begingroup$ If you are mostly interested in calculating fuel requirements and attainable velocities, then you might want to start from the other end. You can start with either time and acceleration, or time to and final velocity, and get the missing variable. Then you can pick the engine thrust, anything from a Saturn F-1 burning tons per second (with massive thrust) to an ion thruster needing next to no fuel (but minimal thrust). Then you can calculate how massive your spacecraft can be. You don't need to decide the spacecraft mass first unless you want to. $\endgroup$ – a CVn Apr 11 '16 at 19:14
8
$\begingroup$

Please read the Atomic Rockets: Basic Design. It has everything you need to know.

There's no "one size fits all" answer

The reason no one can answer your question specifically is because spacecraft are not designed generically. Each spacecraft is designed to optimally complete its mission. A spacecraft designed for one purpose (e.g. a Pluto flyby) will only be used for this mission. We will also never see that one again, the ship is gone for good because it's too expensive to include fuel/propellant for the return trip.

Make sure the hot end points towards the ground:
Make sure the hot end points towards the ground
If you don't, then you will not go to space today.

The Tyranny of the Rocket Equation

These difficulties with building spacecraft are sometimes called (both jokingly and not jokingly) the Tyranny of the Rocket Equation. For another take on this Tyranny, you might just read the story "The Cold Equations". These equations are unfeeling and don't care about intentions, feelings, or most other "warm" sentiments.

The following is excerpted (with editing) from Atomic Rockets:

As a rule of thumb, a rocket with the highest $\Delta V$ capacity is going to need three kilograms of propellant for every kilogram of rocket+payload. The lower the total kilograms of rocket+payload, the lower the propellant mass required.

Say the mission needs 5 km/s of $\Delta V$. Each kilogram of payload requires propellant to give it 5 km/s.

But that propellant has mass as well. The propellant needed for that original kilogram of payload will require a second slug of propellant so that it too can be $\Delta V$ to 5 km/s.

And the second slug of propellant has mass as well, so you'll need a third slug of propellant for the second slug of propellant — you see how it gets expensive fast. So you want to minimize the payload mass as much as possible or you will be paying through the nose with propellant.

And for the rocket equation, everything that is not burned fuel/spent propellant; counts as payload. That includes such things as engines, structures, radiation shielding, food, people, life support, unburned fuel, unused propellant, etc.

Even in the most advanced and optimistic designs, you're going to have 3 kg of fuel/propellant for 1 kg of everything else combined. Unless you're using very high specific impulse engines, you won't go on a cruise around the Solar System. People will only travel from point to point if they have a specific mission to perform.

Each specific mission will have its own specific requirements ($\Delta V$ requirements, mission payload, etc.) and, therefore, each spacecraft will be designed for its mission.

A possible answer anyway

Many smart people have wondered the same question for a long time. These smart people have developed many different plausible spaceship designs for different missions. You may want to browse through the list and see which of these fit your needs.

One thing these have in commmon, is >2/3 of their starting mass is propellant tankage.

The take away

Spacecraft design is like no other designed craft used on Earth. The closest thing to spacecraft design would be high performance military aircraft. However, spacecraft are more expensive per pound by at least an order of magnitude.

The one possible exception to this would be spacecraft designed using the nuclear pulse propulsion. Those might be built using similar cost per pound of construction to high end naval construction (aircraft carriers and/or submarines).

NOTE: I used fuel and propellant and fuel somewhat interchangeably. However, except in the case of super high efficiency nuclear or chemical engines, this almost always not true. Fuel is the expendable that provides energy. Propellant is the expendable with which the ship exchanges momentum to accelerate your ship.

$\endgroup$
  • $\begingroup$ That website will be a great resource, and looks like exactly what I was looking for, thank you! $\endgroup$ – johny5w Apr 11 '16 at 20:43
2
$\begingroup$

For the dozen or so unmanned space missions I've been involved with, payload density usually ran about the same as water. That does not include boosters.

So, as a first cut, you need to figure out the volume of your spacecraft, then multiply by 1000 kg/cubic meter.

It's pretty clear that this estimate is not off by an order of magnitude, although the density may be somewhat less. Consider surface ships, which by their nature have densities less than water. Manned space vehicles have much heavier walls (pressure containment) but large voids internally. Also keep in mind that density will probably decrease with increasing size, just as it does with ships. Hull thickness remains fairly constant with size, but hull area only goes up with the square of vehicle dimension.

$\endgroup$
  • $\begingroup$ Not downvoting, but i would say the density would be more than water. Not much need for empty space on a large ship because things would be automated. $\endgroup$ – Mathmagician Jul 19 '17 at 4:47

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.