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High TWR Scifi Impossible Scifi Drives... Overrated?


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It is ironic that giving a free pass as it were for space travel WON'T make it easy for us even then. Here we go.

 

So this is a scifi scenario:

We have a scifi drive that has a very high thrust to weight ratio and can accelerate virtually indefinitely (so long there is electricity). It uses said drive to reach orbit and also land SSTO style.

The thrust cannot be throttled down, so to reduce overall thrust from killing human occupants with excesdive g-force, the mass of vessels is increased accordingly to reduce engine thrust.

The overall thrust of the drive can be calculated by the fact that if you put it on a tomahawk missile the missile would accelerate at 900g.

More than enough to obliterate it in the air.

Tomahawks weigh this much:

Mass

2,900 lb (1,300 kg), 3,500 lb (1,600 kg) with booster

 

 

So the question is... how massive do we need to make our spaceship to lower the ridiculilous amount of thrust to enough to reach orbit and also travel around interplanetary?

The true irony is that the lowest you would want the acceleration to get is 4g probably. Which is enough to reach orbit and more than enough for interplanetary.

Yet constant acceleration at 4g? No thanks.

At best maybe you just coast to the moon and cut out a big enough rock to tow so that you can reduce thrust to 1g.

Otherwise we are talking 4g engines that cannot be throttled down.

That's all I can think of.

What about you?

 

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Short answer: You just described magical Solid Rocket Booster. With emphasis on "magical".

Long answer: You really like to complicate things, do you? Even three oldest methods of propulsion humans were able to develop can be throttled up or down. Drifting with the stream by the use of pushing pole. Rowing by applying more or less strength to oars. Sailing by reefing the sails.

Surprisingly, humans can be reasonable and practical species, applying principles of economy and efficiency to most aspects of our endeavors. Thus my conclusion below:

Nobody would bother.

 

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12 minutes ago, Scotius said:

Short answer: You just described magical Solid Rocket Booster. With emphasis on "magical".

Long answer: You really like to complicate things, do you? Even three oldest methods of propulsion humans were able to develop can be throttled up or down. Drifting with the stream by the use of pushing pole. Rowing by applying more or less strength to oars. Sailing by reefing the sails.

Surprisingly, humans can be reasonable and practical species, applying principles of economy and efficiency to most aspects of our endeavors. Thus my conclusion below:

Nobody would bother.

 

 

Yes. Many things can be throttled... as well as many that cannot.... like the sun for example.

It's not like you can turn it off and on. You either kill it or let it live.

 

We have hardly reached tge point where we have created anything that cannot be shut off or throttled, something that can only be destroyed to stop it.

Yet to do so would be to copy some of the most powerful things we know of in the universe. 

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In such case, first question should be: "Do i want to use it?"

Followed by the second question: "Do i have to use it?"

We do not like to encounter binary choices. Mostly because they put hard limits on our ability to control things.

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4 hours ago, Scotius said:

In such case, first question should be: "Do i want to use it?"

Followed by the second question: "Do i have to use it?"

We do not like to encounter binary choices. Mostly because they put hard limits on our ability to control things.

 

For what it's worth Scotius, the drive is for and from...

Spoiler

Scifi aliens.

Did not see the need to mention it at first, as they are humanoid, but behavior itself is definitely skewed and bent far more toward accepting, providing, and supporting responsibilities.

Does not mean everyone of them is what you would call a 'good guy'. It just means I don't automatically paint a whole fictional race as villians. If anything I like to give them positive qualities and show how those can be used both for good and evil.

Spoiler

Extra spoller I made by accident

 

Edited by Spacescifi
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This is not a difficult question and may be better suited to the thread for questions that don't merit their own thread.

As a general comment, provided you're accelerating at more than 1g, that will be sufficient to reach Earth orbit. No need to go hell-for-leather at 4g. Accelerating at 1.001g (to take an absurd example) wouldn't be the most efficient way to get to orbit but with your indefinite-acceleration drive, efficiency isn't a concern. And in some respects, low acceleration is a positive advantage in that it reduces aerodynamic stress and aerodynamic heating on your spacecraft during its ascent to orbit.

As usual, you haven't given us enough information for a definite answer (is your drive capable of pushing a Tomahawk or a Tomahawk + booster at 900g for example?) but for the sake of argument, I'm assuming its capable of pushing a Tomahawk and its booster, so 1.6 metric tons, at 900g. I'm also assuming that relativistic effects are negligible - which seems reasonable for interplanetary velocities - so that we can use Newtonian physics.

Force is mass multiplied by acceleration (Newton's 2nd Law).

Therefore a drive capable of pushing 1.6 tons at 900g is capable of pushing 3.2 tons at 450g or 6.4 tons at 225g or 12.8 tons at 110.5g.... etc. etc.

So if you want that drive to accelerate a spacecraft at 4g, the spacecraft needs to weigh 1.6 x 900/4 =  360 tons. The same drive will accelerate a spacecraft weighing 720 tons at 2g, which seems like a more reasonable acceleration. For comparison purposes, 720 tons is approximately 1/3 of the mass of the first stage (S1C stage) of the Saturn V rocket.

Personally, if I was building a ship around your drive, I would go with a 720 ton vessel and dock it in orbit with another 720 tons of shielding, giving me a 1440 ton vessel capable of a constant 1g acceleration. It's by no means an original idea (I forget where I read it) but the shielding would probably be made of ice since that's cheap, readily available and easily workable into whatever shape you need. Assuming my arithmetic is correct, 720 tons of ice is approximately 774 cubic metres, which should provide a nice thick shield against radiation as well as dust and micrometeorites.

 

 

Edited by KSK
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I would make it so it gave one g trust, more is pretty much overkill.
You might want 2 g for takeoff and landing unless you make an spaceplane. 
This is obviously an starship an real one. 

You main problem will probably be power, on the other hand we have an tricks. 
This thing is also an perpetuum mobile, how? well mount it on the end of an fairly long arm with an counterweight. As the energy used to power this thing is constant, say twice the energy to accelerate something at one g the kinetic energy in this rotor increase rotation speed^2. 
Once spun up to speed simply use the dynamo at the bearing to break it and counter the engine, you could easy get out say 5x the energy you added, use this to power the engines. 

This was also pointed out as an argument against EM-drive as all reactionless drives shares this problem.
Note that an anti gravity device does not as far as I know say you need 1.5 times the minimum energy to generate an force who counter gravity to eliminate gravity effects on your device. You would get stuff like 0 g rooms and VTOL would be very easy. 

This is very common, lots of stuff in Star Trek would have way more uses. Why don't Star Trek teleporters save you as in savestate in an MMO, if you die on mission you are recreated. 
Yes this could easy be exploited making copies of expensive arts or even peoples. You might want multiple Musk level characters around, or simply you as captain falls in love with an lady who also like you but she is going to an planet, you port her down, then a bit later things heat up as the Borg or somebody attacks so you recreate the lady, telling her the transfer failed. and we have to run away. 
Many years later after an nice marriage she finally get to the planet and run into herself :)

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10 hours ago, KSK said:

This is not a difficult question and may be better suited to the thread for questions that don't merit their own thread.

As a general comment, provided you're accelerating at more than 1g, that will be sufficient to reach Earth orbit. No need to go hell-for-leather at 4g. Accelerating at 1.001g (to take an absurd example) wouldn't be the most efficient way to get to orbit but with your indefinite-acceleration drive, efficiency isn't a concern. And in some respects, low acceleration is a positive advantage in that it reduces aerodynamic stress and aerodynamic heating on your spacecraft during its ascent to orbit.

As usual, you haven't given us enough information for a definite answer (is your drive capable of pushing a Tomahawk or a Tomahawk + booster at 900g for example?) but for the sake of argument, I'm assuming its capable of pushing a Tomahawk and its booster, so 1.6 metric tons, at 900g. I'm also assuming that relativistic effects are negligible - which seems reasonable for interplanetary velocities - so that we can use Newtonian physics.

Force is mass multiplied by acceleration (Newton's 2nd Law).

Therefore a drive capable of pushing 1.6 tons at 900g is capable of pushing 3.2 tons at 450g or 6.4 tons at 225g or 12.8 tons at 110.5g.... etc. etc.

So if you want that drive to accelerate a spacecraft at 4g, the spacecraft needs to weigh 1.6 x 900/4 =  360 tons. The same drive will accelerate a spacecraft weighing 720 tons at 2g, which seems like a more reasonable acceleration. For comparison purposes, 720 tons is approximately 1/3 of the mass of the first stage (S1C stage) of the Saturn V rocket.

Personally, if I was building a ship around your drive, I would go with a 720 ton vessel and dock it in orbit with another 720 tons of shielding, giving me a 1440 ton vessel capable of a constant 1g acceleration. It's by no means an original idea (I forget where I read it) but the shielding would probably be made of ice since that's cheap, readily available and easily workable into whatever shape you need. Assuming my arithmetic is correct, 720 tons of ice is approximately 774 cubic metres, which should provide a nice thick shield against radiation as well as dust and micrometeorites.

 

 

You were right. With the booster for the tomahawk.

You were quite thorough but there is one thing you did not know that is worth considering.

The scifi drive still uses engine nozzles.

Look like this:

s-l300.jpg

 

Note the reflective nozzles. Because the scifi drive is propelled via momentum transfer of FTL energy rays, so the nozzle size should effect the thrust.

 

Bigger nozzle, more thrust flow?

Smaller nozzle, less thrust flow?

Right?

Just one more way to manipulate the thrust for smaller vessels without killing everyone perhaps?

19 minutes ago, kerbiloid said:

If take two such engines, producing 899 g and 901 g respectively, and mount them at the opposite ends of the rocket, we'll get the required 2 g.

 

Maybe... perhaps one IRL factor I did not take into account is waste heat and kinetic energy.

Any rocket exhaust with enough kinetic energy AND mass to provide such high thrust might melt the nozzle. Unless it was more mass flow flowing through than energy, in which case the engine is going to leave shockwaves and craters in it's wake on every launch and landing.

If it is more energy than mass flow providing the thrust, then we would need a lot of heat rejection equipment to prevent the engine from overheating. Won't change much about the shockwaves and craters. Except now they are smoking from all the heat.

Edited by Spacescifi
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Why are you constantly coming up with these convoluted ideas with improbable and arbitrary restrictions?

Anyway, make the ship a bajillion tons and install multiple engines. Then turn them on individually as needed to get whatever acceleration you want.

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14 minutes ago, Shpaget said:

Why are you constantly coming up with these convoluted ideas with improbable and arbitrary restrictions?

Anyway, make the ship a bajillion tons and install multiple engines. Then turn them on individually as needed to get whatever acceleration you want.

 

TWR has diminishing returns when you get into really high tonnage.

Let's say we want to fly ship that is 5000 tons.

It would defintely take multiple engines to pull that off, and big nozzlee at that, unless someone really rather put on hundreds of smaller nozzles ala kerbal.

 

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6 hours ago, Shpaget said:

Why are you constantly coming up with these convoluted ideas with improbable and arbitrary restrictions?

Anyway, make the ship a bajillion tons and install multiple engines. Then turn them on individually as needed to get whatever acceleration you want.

This is what Saturn V did.  F1 engines can't throttle, and they wanted to fire them longer than the 3g the rocket (more likely astronauts) could handle.  So they turned the center on off after acceleration exceeded 3g.

5 hours ago, Spacescifi said:

 

TWR has diminishing returns when you get into really high tonnage.

Let's say we want to fly ship that is 5000 tons.

It would defintely take multiple engines to pull that off, and big nozzlee at that, unless someone really rather put on hundreds of smaller nozzles ala kerbal.

TWR has diminishing returns after a couple minutes of flight.  Most of the reason SSTO isn't feasible is how wasteful it is to get those high thrust engines into orbit, and it takes even more mass to bring them down safely.

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