The Main Difference Between IRL and Scifi Spaceships

[Bamax]

Shuttles and SSTO's Are The Same:

There is little one cannot simulate IRL that is scifi so long it is optimized for the job.

For example, I figured out that an SSTO is basically a shuttle, no matter how large or heavy it is purely a means for getting to orbit without a disposable first stage rocket. Since gravity is a fundamental force that cannot be played with or adjusted IRL, that means that the lighter weight an SSTO is the better.

Ideally you want minimal life support, since a shuttle is merely a ferry to a larger vessel in orbit with better life support.

Adding on extra unneeded features costs weight and makes an SSTO less capable of it's job. Namely, reaching orbit.

Manned Spaceships Should Be Pure Orbiters Or Low Gravity Landers:

Spaceships most be optimized purely for orbit to orbit transfer, or landing on low gravity moons as well.

Do note that a pure orbiter that cannot land has more room for cargo than any orbiter that also uses part of it's hold for propellant to land on low gravity moons or asteroids or comets.


Long Story Short: There is a cost for every advantage with spaceships.

It is common in scifi for vessels to be jacks of all trades, but it really does not pay IRL when how far you go depends on the engine and how much of your vessel is propellant plus how heavy it is.

[Demarquis]

History would appear to support you. Specialization correlates with effectiveness and efficiency in all domains of technology pretty well.

[avatar576]

It's been many years since my college days in aerospace engineering, but when designing any launch vehicle (single or multiple stage) the two primary constraints are going to be payload capacity and delta-V. That is, how much "stuff" do you want to put into orbit, and how high up do you want to put it?
The problem with our current level of technology is that as payload and delta-V increase, the amount of propellant required to achieve the desired outcome increases exponentially. The best we can do is increase efficiency (in relatively modest proportions) by using multiple stages with each stage configured to optimize performance at the altitude at which it is used. You still have relatively few options when doing this. Your main "control levers" will be propellant type, engine selection (dictated by propellant selection), and nozzle geometry.
The ultimate goal is to minimize your propellant mass fraction (how much of your total vehicle mass is propellant) for putting a given payload into a desired orbit. For most modern rockets, that number is usually between 0.8 and 0.9. Meaning 80-90% of the vehicle as it sits on the launch pad is just fuel.

Another key difficulty with current technology is the optimization of fuels. There are some incredibly powerful and efficient hypergolic liquid fuels which, if used, would make rocketry far more efficient (high thrust and high specific impulse). The problem is, such fuels are very, very toxic. Think hydrazine, MMH, UDMH, dinitrogen tetroxide. LOX/LH2 is another potent combination. Very, very clean (its only combustion byproduct is water). But also very, very expensive, especially the liquid hydrogen part. And liquid hydrogen has the unfortunate tendency to "leak" out of any container because hydrogen, being the smallest atom on the periodic table, can slip past pretty much any barrier. That's why a lot of earlier rockets and even the Saturn V (1st stage) burned RP-1, which is basically kerosene. Cheap, although not very efficient (high thrust, low specific impulse). Finally, there are means of propulsion (think ion thrusters) that have extremely high specific impulses but very low thrust. So they will get you where you need to go with very, very little fuel required, but it will take a long, long time to get there. Useless in a launch vehicle.

Sci-fi vehicles just tend to gloss over all this by using sufficiently advanced means of propulsion (read: theoretical, or even just a raw product of imagination with no known science behind it whatsoever) and assuming it is powerful enough to get done whatever job needs to get done. Takeoff to orbit? Antimatter engines. Interplanetary/Interstellar travel? Bigger antimatter engines. Landing? Antimatter engines with vectored thrust capability. They do this to overcome other physical barriers as well, like "subspace radio" to get around the problem of FTL comms. But frankly, how it works or whether it's even possible is not really important in my view. The technology is just a plot device to make the story possible in our imaginations and keep the story moving.

[Bamax]

True... I simply find that reality is both fascinating and exciting.

And at the same time I find making overpowered rockets all too easy which invites RKV's for everyone despite how awfully dangerous they are.

The interesting thing about rockets is that the more energetic a reaction the more thrust it grants for less fuel applied.

As an example, a shaped blast nuke blown up under a pusher plate Orion vessel is vastly more energetic than the chemical reactions rockets normally rely on.

Yet we all know nukes are radioactive so that spoils that idea.

Nonetheless I quite like pure fusion pulsed rockets as an idea.

The irony is that the more powerful the reaction in your engine, the larger both it and it's components need to be in order to survive the reaction and pressure waves exiting the nozzle.

Pure fusion reactions can be throttled far below that the energy of a nuke or throttled to match it.

Which ultimately means I honestly think a heavy pulsed fusion SSTO might just pulse fire the main engine on a runway and almost take off like a plane.... using a sloping ramp that goes vertical to fly upward before tilting forward a bit and pulse firing it's engines repeatedly to reach orbit.

[sdfgeoff]

You may find the site 'atomic rockets' of interest. It goes into an awesome of detail about everything sci-fi from fuel types to the layout of the command deck, and is very good at explaining the hard-science in an understandable way. I've spent many hours reading this resource.

For example, here is the page on different types of spaceship, and it covers landers vs orbit-to-orbit and looks at the design considerations of each.

It would be interesting for a SF book to focus on the crew of a shuttle - always ferrying people up to the stars but still being trapped to the one planet.
I wonder how air-traffic-control would be handled for a planet? I guess you need some unified agency for overall orbit planning - maybe they give each spaceport a dedicated altitude? Hmm.

[G. Janssen]

The problem with our current level of technology is that as payload and delta-V increase, the amount of propellant required to achieve the desired outcome increases exponentially.

Loved reading that. You probably know Orbiter? The space sim? I recommend it to anyone who wants to experience space travel. Get a joystick though. http://orbit.medphys.ucl.ac.uk/

[avatar576]

The problem with our current level of technology is that as payload and delta-V increase, the amount of propellant required to achieve the desired outcome increases exponentially.

Loved reading that. You probably know Orbiter? The space sim? I recommend it to anyone who wants to experience space travel. Get a joystick though. http://orbit.medphys.ucl.ac.uk/

Never seen that before...looks cool though. I know what I'll be doing this weekend.

[Demarquis]

Another great realistic spaceship simulator is "Children of a Dead Earth" (https://www.childrenofadeadearth.com/). They claim it's "The most scientifically accurate video game ever made." Well, for interplanetary combat anyway. The blog is really informative too.

[G. Janssen]

Another great realistic spaceship simulator is "Children of a Dead Earth" (https://www.childrenofadeadearth.com/). They claim it's "The most scientifically accurate video game ever made." Well, for interplanetary combat anyway. The blog is really informative too.

Thanks!