How Heavy is Reasonable In This Setting And What is TOO Heavy?

[Spacescifi]

There is arguably a weight limit to how heavy one will want to make a spaceship, even if it has KSP antigravity and warp drive.

 

Why? The heavier the ship, the larger and more powerful your reaction control maneuver rockets need to be. Also more propellant is required unless you do not mind it taking hours to roll, yaw, or pitch (I imagine most KSP gamers would). Make a ship too heavy and mere RCS begins to look like proper chemical to orbit rocket engines LOL!

At that point I will call the ship too heavy.

Main Question: How heavy in tons can a spaceship become and still use RCS that is underpowered compared to the main rocket engines (either chemical or NTR chemical)?

What would be the max crew of a vessel that was not too heavy like this? My guess? Not a lot, since crew require a lot of reaction mass to maintain that dwarfs them in mass anyway. My guess? Fifteen sounds reasonable, which coincidentally is the same crew complement of a small klingon bird of prey from star trek. A 120 crew complement ike the original Enterprise seems unlikely for a SINGLE vessel. Since it is far easier to make smaller vessels that are easier to propel. Another great irony is that it seems that even with KSP antigravitt and warp, spaceship weight is arguably ALWAYS lower than ocean vessels and with less crew as well.

At what tonnage is a spaceship too heavy? Precisely where it's reaction control rockets have as much boost as chemical to orbit booster rockets!

Any more thoughts to add?

Thank you for your answers!

Edited October 11, 2020 by Spacescifi

[kerbiloid]

If the crew is enough large, they may just run from port to starboard and back, to change the CoM position, instead of RCS.

[Scotius]

If you have enough grip on the laws of physics to create antigravity and warp engines, you can easily build superior control system for your spaceships. And it can be equally exotic as main drive.

[Spacescifi]

6 hours ago, Scotius said:

If you have enough grip on the laws of physics to create antigravity and warp engines, you can easily build superior control system for your spaceships. And it can be equally exotic as main drive.

Sure. But what will it react with? All mass requires reaction mass to work or energy.

Space only affords radiation. So I suppose one could use an ablative hull and use extendable lasers powered by solar and nuclear to ablate it as an RCS system.  

 

Getting more hull would be for landing. Make a liquid paste of dirt of any world and stick it to the hull for RCS reaction mass.

Wow... that could save on all the trouble with rocket propellant!

 

Edited October 12, 2020 by Spacescifi

[kerbiloid]

If have cows onboard, they can provide not only ablative cover but also methane.
ISRU.

Edited October 12, 2020 by kerbiloid

[Scotius]

38 minutes ago, Spacescifi said:

"Sure. But what will it react with? All mass requires reaction mass to work or energy.

Space only affords radiation. So I suppose one could use an ablative hull and use extendable lasers powered by solar and nuclear to ablate it as an RCS system."

Yet you specifically mention antigravity and FTL propulsion as working in-Universe. What's stopping future spaceshipbuilders from using Iron Man-esque repulsors as RCS? Or other flavor of reactionless propulsion related to antigravity? Or highly localized area of spatial curvature generated by warp drive?

 

[sevenperforce]

16 hours ago, Spacescifi said:

At what tonnage is a spaceship too heavy? Precisely where it's reaction control rockets have as much boost as chemical to orbit booster rockets!

The gas-gas RCS thrusters on Starship will have three times as much thrust as the Kestrel rocket engine on the Falcon 1. By your standard, Starship is "too heavy".

The primary RCS thrusters (not the OMS engines) on the Space Shuttle Orbiter produced 7.8 kN of thrust...more than the upper stage of the Juno I booster that launched Explorer 1. By your standard, the Space Shuttle is "too heavy".

The R-4D RCS thrusters used on the Apollo lunar module produced 490 N of thrust...more than four times the thrust of the engine on the Electron's upper stage. By your standard, the Apollo lunar module was "too heavy".

And as @Scotius pointed out, a civilization with antigravity is going to have no trouble coming up with attitude control SLIGHTLY more refined than pushing compressed gas out of a tube.

 

In a sci-fi world, a spaceship is precisely as large as it needs to be. Remember that the square-cube law always favors larger vehicles over smaller ones for pure performance (although you might want smaller vehicles for other reasons).

[Nuke]

just having your anti gravity engines operating asymmetrically can induce rotations. 

i think your real limits are structural. because a big structure is a complicated structure. you even run into this problem in ksp where you get huge oscillations that make things like docking and precision maneuvering impossible. huge cross sections make you susceptible to atmoshperic drag (in low orbits) not to mention space debris and dust at interstellar velocities. 

Edited October 12, 2020 by Nuke

[Spacescifi]

1 hour ago, Scotius said:

Yet you specifically mention antigravity and FTL propulsion as working in-Universe. What's stopping future spaceshipbuilders from using Iron Man-esque repulsors as RCS? Or other flavor of reactionless propulsion related to antigravity? Or highly localized area of spatial curvature generated by warp drive?

 

 

Antigravity requires mass to push against. Not interested in reactionless, as there are easier options that work just as well. Ironman repulsors make little sense as depicted, mainly for propellant reasons as it never seems to run out.

29 minutes ago, sevenperforce said:

The gas-gas RCS thrusters on Starship will have three times as much thrust as the Kestrel rocket engine on the Falcon 1. By your standard, Starship is "too heavy".

The primary RCS thrusters (not the OMS engines) on the Space Shuttle Orbiter produced 7.8 kN of thrust...more than the upper stage of the Juno I booster that launched Explorer 1. By your standard, the Space Shuttle is "too heavy".

The R-4D RCS thrusters used on the Apollo lunar module produced 490 N of thrust...more than four times the thrust of the engine on the Electron's upper stage. By your standard, the Apollo lunar module was "too heavy".

And as @Scotius pointed out, a civilization with antigravity is going to have no trouble coming up with attitude control SLIGHTLY more refined than pushing compressed gas out of a tube.

 

In a sci-fi world, a spaceship is precisely as large as it needs to be. Remember that the square-cube law always favors larger vehicles over smaller ones for pure performance (although you might want smaller vehicles for other reasons).

 

You got me there... yet I realize now that a very simple solution to the propellant problem exists that I doubt even KSP allows for... yet.

Even luminal warp is good enough to fly to an asteroid in our system in a reasonably fast time frame. Most asteroids orbit slowly (a few kilometers per sec) so it won't take much rocket propellant to match their speed for landing.

If at least part of the ship's surface is either adhesive or has a way to attach an asteroid each on at least four points for axis control, that is enough.

Powerful movable lasers do the rest. Maybe on mechanical tentacles?

The main benefit is that it is ULTRA cheap, and a virtually unlimited resource of high thrust propellant. Moons only provide even more.

So why take tons of finite redources with you when you can live off the land so to speak? It's cheap (at least as cheap as luminal warp) and the resources are more or less endless. So long you have working lasers.

Asteroid and moons are now viable propellant. High thrust at that! No need to do chemical extraction of water from rock... that is tedious.

Nor any need to sit by a sun frying yourself as in Elite Dangerous just to collect hydrogen. When asteroids are unlimited and have higher thrust.

You just have to warp to them.

Edited October 12, 2020 by Spacescifi

[sevenperforce]

34 minutes ago, Spacescifi said:

Antigravity requires mass to push against.

Why? You can define antigrav anyway you want.

34 minutes ago, Spacescifi said:

If at least part of the ship's surface is either adhesive or has a way to attach an asteroid each on at least four points for axis control, that is enough.

Powerful movable lasers do the rest. Maybe on mechanical tentacles?

I am pretty darn certain that grappling asteroids and using them to provide RCS via laser ablation thrust will NEVER be the most efficient option for RCS.

[Rakaydos]

19 hours ago, Spacescifi said:

 

At what tonnage is a spaceship too heavy? 

Building a gas-giant colony ship is not as difficult as it looks.

1. Build a fusion candle. It's called a "candle" because you're going to burn it at both ends. The center section houses a set of intakes that slurp up gas giant atmosphere and funnel it to the fusion reactors at each end.
2. Shove one end deep down inside the gas giant, and light it up. It keeps the candle aloft, hovering on a pillar of flame.
3. Light up the other end, which now spits thrusting fire to the sky. Steer with small lateral thrusters that move the candle from one place to another on the gas giant. Steer very carefully, and signal your turns well in advance. This is a big vehicle.
4. Balance your thrusting ends with exactness. You don't want to crash your candle into the core of the giant, or send it careening off into a burningly elliptical orbit.
5. When the giant leaves your system, it will take its moons with it. This is gravity working for you. Put your colonists on the moons.
6. For safety's sake, the moons should orbit perpendicular to the direction of travel. Otherwise your candle burns them up.
7. They should also rotate in the same plane, with one pole always illuminated by your candle (think "portable sunlight")
8. The other pole absorbing the impact of whatever interstellar debris you should hit (think "don't build houses on this side"

[RCgothic]

In Star Trek the Impulse Driver Coils envelop the ship in a low-grade subspace field that lowers the effective mass of the ship. Congratulations, now a miniscule quantity of propellant can propel a ship to high fractions of c.

In Mass Effect a ship's drive core generates a negative mass effect field that actually lowers the vessel's mass. Same outcome.

 

You've got a setting with warp drives and anti-gravity. Unless you've got a compelling story reason, hand-wave it.

Edited October 15, 2020 by RCgothic

[KSK]

If you’re using an Alcubierre style warp drive then you have the ability to change the local curvature of spacetime - that whole ‘expand space behind your ship and contract space in front of it’ idea.

Gravity is the curvature of spacetime caused by mass. So if you can change the local curvature of spacetime you’re effectively controlling gravity. Since the Alcubierre drive can both expand and contract space, that implies that it can raise or lower local curvature and therefore raise or lower gravity.

So warp drive and antigravity are linked. More to the point, if you can control gravity, why bother mucking around with reaction thrusters. Just create a gravity field in a desired location and let that change your ship’s course.

For docking at a space station or other larger vessel just use a tractor beam - which would be another helpful application of controlled gravity fields.

 

[kerbiloid]

35 minutes ago, KSK said:

For docking at a space station or other larger vessel just use a tractor beam

Or a pattern of beams on that tractor.

Spoiler

 

[Superfluous J]

4 hours ago, sevenperforce said:

Why? You can define antigrav anyway you want.

I love this concept. I had a throwaway line in a story I wrote once where one character said "Why don't we just use anti-grav?" and the other said "That just lowers weight. We need to lower momentum."

I have no idea what it means but the guy saying it was very smart so I assume he knew what he was talking about.

[kerbiloid]

If make an antigrav flying carpet, we can just bend its corners for attitude control.

[KSK]

7 hours ago, kerbiloid said:

Or a pattern of beams on that tractor.

  Reveal hidden contents

 

Beam me up, Sc... oww, owww, owwww - NOT THOSE BEAMS SCOTTY!

[Nuke]

22 hours ago, Rakaydos said:

Building a gas-giant colony ship is not as difficult as it looks.

1. Build a fusion candle. It's called a "candle" because you're going to burn it at both ends. The center section houses a set of intakes that slurp up gas giant atmosphere and funnel it to the fusion reactors at each end.
2. Shove one end deep down inside the gas giant, and light it up. It keeps the candle aloft, hovering on a pillar of flame.
3. Light up the other end, which now spits thrusting fire to the sky. Steer with small lateral thrusters that move the candle from one place to another on the gas giant. Steer very carefully, and signal your turns well in advance. This is a big vehicle.
4. Balance your thrusting ends with exactness. You don't want to crash your candle into the core of the giant, or send it careening off into a burningly elliptical orbit.
5. When the giant leaves your system, it will take its moons with it. This is gravity working for you. Put your colonists on the moons.
6. For safety's sake, the moons should orbit perpendicular to the direction of travel. Otherwise your candle burns them up.
7. They should also rotate in the same plane, with one pole always illuminated by your candle (think "portable sunlight")
8. The other pole absorbing the impact of whatever interstellar debris you should hit (think "don't build houses on this side"

wouldnt the space debris hitting the moons do a plane change on them? as soon as the orbital plane wasn't perfectly perpendicular to the angle of motion, you would get a prograde/retrograde component. your orbits would either spiral out and you could lose them or spiral them into the roche limit. keeping them in place might require they have their own thrusters to cancel out any thrust they get from incoming space debris, which always hits one side.

Edited October 13, 2020 by Nuke

[Rakaydos]

1 hour ago, Nuke said:

wouldnt the space debris hitting the moons do a plane change on them? as soon as the orbital plane wasn't perfectly perpendicular to the angle of motion, you would get a prograde/retrograde component. your orbits would either spiral out and you could lose them or spiral them into the roche limit. keeping them in place might require they have their own thrusters to cancel out any thrust they get from incoming space debris, which always hits one side.

Statistically, debris should be striking evenly on all points of the orbit. It's no worse than the planet the moon's orbiting flying out from underneath them. If a piece of debris is large enough that the impact could significantly alter the inclination at a single point, you should be able to see it coming far enough out to Miss it if you try hard enough.

[Nuke]

1 hour ago, Rakaydos said:

Statistically, debris should be striking evenly on all points of the orbit. It's no worse than the planet the moon's orbiting flying out from underneath them. If a piece of debris is large enough that the impact could significantly alter the inclination at a single point, you should be able to see it coming far enough out to Miss it if you try hard enough.

well if you assume the plane is and will remain completely prependicular to the motion vector. and if you assume that impacts will be infrequent. but take this up to near relativistic speeds and every little hydrogen atom represents a lot of energy and the forces on the moon would be fairly consistent. assuming a prependicular starting condition, the first impulse will slant the orbit such that there will be a nonzero prograde component to the force vectors. it will be tiny and would take a long time to build up any meaningful orbital energy.

there will also be natural plane tilt caused by the acceleration of the host planet. the moons will accelerate to keep up and this will tilt in the forward direction. impacts here translate into a retrograde component. managing throttle and velocity might find a breakeven point, but if you wanted to accelerate faster (and you would) you would have to re adjust the orbits of the moons to compensate for any prograde/retrograde force components.  but hey if you are moving a gas giant, so you can probably figure it out and have the tech to make it work. 

Edited October 13, 2020 by Nuke

[Spacescifi]

On 10/12/2020 at 2:34 PM, KSK said:

If you’re using an Alcubierre style warp drive then you have the ability to change the local curvature of spacetime - that whole ‘expand space behind your ship and contract space in front of it’ idea.

Gravity is the curvature of spacetime caused by mass. So if you can change the local curvature of spacetime you’re effectively controlling gravity. Since the Alcubierre drive can both expand and contract space, that implies that it can raise or lower local curvature and therefore raise or lower gravity.

So warp drive and antigravity are linked. More to the point, if you can control gravity, why bother mucking around with reaction thrusters. Just create a gravity field in a desired location and let that change your ship’s course.

 

 

 

.From Google:

People also ask

Do we really know what gravity is?

However, if we are to be honest, we do not know what gravity "is" in any fundamental way - we only know how it behaves. Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is not just the attraction between objects and the Earth.

NASA (.gov) › gsfc › starchild › docs

What is gravity? - StarChild - NASA

 

Alcubierre requires what may as well be called fictional matter, inasmuch negative matter as far as I know remains an idea that works out mathematically but has never been detected.IRL.

 

Asteroid ablation RCS is not most efficient, but on a long 5 year mission like the enterprise, it is a cheap trick that would work.

 

 

Edited October 14, 2020 by Spacescifi

[KSK]

3 hours ago, Spacescifi said:

From Google:

People also ask

Do we really know what gravity is?

However, if we are to be honest, we do not know what gravity "is" in any fundamental way - we only know how it behaves. Gravity is a force of attraction that exists between any two masses, any two bodies, any two particles. Gravity is not just the attraction between objects and the Earth.

NASA (.gov) › gsfc › starchild › docs

What is gravity? - StarChild - NASA

Alcubierre requires what may as well be called fictional matter, inasmuch negative matter as far as I know remains an idea that works out mathematically but has never been detected.IRL.

Asteroid ablation RCS is not most efficient, but on a long 5 year mission like the enterprise, it is a cheap trick that would work.

We don't have a complete explanation for gravity, no. We have a very good theory for it which doesn't play nicely with quantum mechanics so we know it can't be complete. But it's still an elegant, experimentally tested, theory.

From Wikipedia:

"General relativity (GR), also known as the general theory of relativity (GTR), is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations."

General relativity makes a number of predictions that have been experimentally tested, most recently, gravitational waves detected by the LIGO instrument.

You're right of course about the need for 'negative matter' to construct an Alcubierre drive but that's largely irrelevant. For the sake of this thread we're already presupposing that we've got a warp drive and antigravity. Therefore whatever unknown materials, technology or science needed to build them also have to be presupposed. 

The Alcubierre drive does have some grounding in current theory though. (Alcubierre's theoretical model for his drive doesn't actually break any known laws of physics although there's been plenty of debate about whether his model is too simplified). Given that we're presupposing a warp drive anyway, why not choose one that we know something about and then following the consequences of having it?

If you want to presuppose some other kind of warp drive and antigravity such that your ship also requires RCS then that's cool. But its just one option and not the only option. 

 

 

Edited October 14, 2020 by KSK

[kerbiloid]

3 hours ago, KSK said:

We have a very good theory for it which doesn't play nicely with quantum mechanics

That's what happens when two dev teams start implementing the same project.
One team was fond of discretization, finite automatons, etc. They made the QM module.
Another one preferred classics, they implemented the GR part.
And now the users have to invent patches to fill the gaps.

Isn't it the best evidence that we live in Matrix?

Edited October 14, 2020 by kerbiloid

[sevenperforce]

11 hours ago, Spacescifi said:

Asteroid ablation RCS is not most efficient, but on a long 5 year mission like the enterprise, it is a cheap trick that would work.

That's....inventive.

[Spacescifi]

8 hours ago, sevenperforce said:

That's....inventive.

Necessity is the mother of invention some say.

Plot is also the the mother of invention.

I always did want to use those asteroids. But waiting days in zero or low spin g to crack hydrogen from a space rock to get propellant seems absurd when I can slice and dice them with lasers,,and then attach them as RCS and spin up or down to my merry content (would take a while to run out of a big asteroid RCS).

 

Zero g is bad for health.

17 hours ago, KSK said:

We don't have a complete explanation for gravity, no. We have a very good theory for it which doesn't play nicely with quantum mechanics so we know it can't be complete. But it's still an elegant, experimentally tested, theory.

From Wikipedia:

"General relativity (GR), also known as the general theory of relativity (GTR), is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time or four-dimensional spacetime. In particular, the curvature of spacetime is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations."

General relativity makes a number of predictions that have been experimentally tested, most recently, gravitational waves detected by the LIGO instrument.

You're right of course about the need for 'negative matter' to construct an Alcubierre drive but that's largely irrelevant. For the sake of this thread we're already presupposing that we've got a warp drive and antigravity. Therefore whatever unknown materials, technology or science needed to build them also have to be presupposed. 

The Alcubierre drive does have some grounding in current theory though. (Alcubierre's theoretical model for his drive doesn't actually break any known laws of physics although there's been plenty of debate about whether his model is too simplified). Given that we're presupposing a warp drive anyway, why not choose one that we know something about and then following the consequences of having it?

If you want to presuppose some other kind of warp drive and antigravity such that your ship also requires RCS then that's cool. But its just one option and not the only option. 

 

 

 

Negative matter is VERY weird stuff. I read that it is pushed away when you pull it, but attracted when you push it.

So if you saw a negative baseballl on the ground and picked it up, you would find getting it off you difficult until you found a way to pull it, which would propel the ball in the opposite direction. Let go completetly and the ball will fall upward into space and eventually reach escape velocity LOL!

A star trek more in line with modeen physics that had both localized gravity fields and negative matter would not use impulse drives.

They would use diametric drives, which split the ship in half while it accelerates chasing the other half that is pushed away just ahead.

Using a combo of negative matter and localized gravity fields.

Edited October 14, 2020 by Spacescifi