(spaceships) rocket (super)science

[jacobmuller]

In a space opera setting - a "drive" system that "alters inertia" such that your 3G drive effectively gives you 3,000G but you still only feel the 3G, what does this do for delta-v?
Instead of 1.5mps per fuel tank, is it 1,500mps?

[ericbsmith]

Technically... nothing.

You calculate Delta-V as:

eV * ln( FM / EM)

eV = Exhaust Velocity
FM = Full Mass, with fuel
EM = Empty Mass, once fuel is exhausted
ln is the Natural Log of the FM/EM

Notice what's missing from the calculation? Neither Acceleration nor Burn Time of the engine matters in the equation. It doesn't matter if you're accelerating for 3 years at 0.001G or 1 hour at 3000G, the final velocity change will be the same if the ships engines have the same Exhaust Velocities, Full Masses, and Empty Masses.

In the real world, engines with higher accelerations almost always have lower Exhaust Velocities, meaning it's a trade-off between acceleration and Delta-V. The reason why high Delta-V rockets aren't used most of the time is because the primary rocket needs a minimum acceleration of 1G just to leave the Earth (and really needs an acceleration of at least 3G to leave the Earth before it runs out of fuel, because coasting into orbit at 1.01G for 3 hours means you run out of fuel before you make it). For rockets on satellites or probes they do sometimes use the low acceleration high Delta-V rockets.


All of that said, if you can alter effective inertia you can alter the exhaust velocity of the fuel. So if you want to keep it simple you can increase Delta-V by the same multiplier that you apply to Acceleration. Realistically this would mean Relativistic exhaust velocities (that is, exhaust at significant percentages of the speed of light - as in 99%+ of C), but this is Space Opera, so woosh!

[Anthony]

Originally Posted by jacobmuller

In a space opera setting - a "drive" system that "alters inertia" such that your 3G drive effectively gives you 3,000G but you still only feel the 3G, what does this do for delta-v?

It does whatever you define it as doing. I would be inclined to say it multiplies delta-V by the same amount, but you can't really get realistic answers out of a system that starts with an impossible assumption.

[malloyd]

Originally Posted by Anthony

It does whatever you define it as doing. I would be inclined to say it multiplies delta-V by the same amount, but you can't really get realistic answers out of a system that starts with an impossible assumption.

The derivation of the rocket equation starts with conservation of momentum, so assuming you alter the "inertia" and hence momentum of the ship and not the exhaust, then multiplying delta-V by the same factor you reduce your inertia makes some sense. And no this doesn't do anything to the exhaust velocity of the rocket, it violates conservation of energy instead.

But sure, if you can mess with conservation laws, depending on where you do it in the process of burning and expelling your fuel you might be able to change the exhaust velocity too, doing pretty much whatever you wanted to delta-V. On didn't want - the point your ship gets accelerated is after all where the pressure of the expanding gas acts on the inside of your nozzle, so it's maybe technically inside the field of your inertia reduction, so maybe your delta-V actually gets *worse*.

[johndallman]

Originally Posted by jacobmuller

In a space opera setting - a "drive" system that "alters inertia" such that your 3G drive effectively gives you 3,000G but you still only feel the 3G, what does this do for delta-v?

As an alternative, you might want to adopt the "Doc" Smith inertialess drive, the "Bergenholm". That gets rid of your inertia entirely, so any thrust you apply immediately accelerates you to a velocity where the resistance of the medium you're passing through (which is rather low in space) equals your thrust.

Smith did not go into details of just how this allowed travelling faster than light, but it certainly did in the Lensman series of space-operas.

[jacobmuller]

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

[ericbsmith]

Originally Posted by jacobmuller

Inertia is telling me I should just accept that space travel is going to take a while - excepting the "jump drive" option.

"Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space." - Douglas Adams, Hitchhiker's Guide to the Galaxy

Any realistic propulsion system is going to take a long time to travel to other planets, let alone to the stars. The quickest travel times to Mars right now are on the order of 9 months each way when the planets are in correct alignment to use Hohmann transfer orbits. An advanced realistic propulsion system might get that down to a couple months. A superscience rocket/reactionless drive with 1G thrust can go from Earth to Mars in 2-4 days using constant acceleration, which is quite fast all things considered.

The 3000G acceleration you were thinking of using just means that a ship can get to light speed in a few minutes, making travel times between planets on the order of minutes for the inner solar system of a few hours for the outer solar system. If you're going to go that far just use a lightspeed drive of some wort, whether it's warp, jump, or hyperspace.

[whswhs]

Originally Posted by johndallman

Smith did not go into details of just how this allowed travelling faster than light, but it certainly did in the Lensman series of space-operas.

As I understood it, the argument was that what stopped you from reaching the speed of light was that as you accelerated, your effective mass increased without limit, so that no amount of thrust could give you sufficient acceleration. Smith seems to have thought that if you didn't have mass (which was the effective result of inertialessness), you could just go on accelerating right past c. He doesn't seem to have thought about the fact that particles with rest mass zero such as photons travel at exactly c and cannot be further accelerated . . .

Technically, what was limiting your superluminal speed was that as you pushed forward, whenever you hit a particle, no matter how tiny, it stopped you dead. But your continuing thrust would then push you forward, accelerating the particle sideways. So this went on until your thrust exactly equalled the force you needed to shove aside all the particles in the local medium. Effectively you were experiencing the interstellar or intergalactic vacuum as a very tenuous fluid and overcoming its resistance. (It's not as simple as that, either, but that's a different story.)

[Fred Brackin]

Originally Posted by whswhs

Ac. He doesn't seem to have thought about the fact that particles with rest mass zero such as photons travel at exactly c and cannot be further accelerated . . .

T.)

Inertialess flight does not appear until the discovery of sub-etheric particles (and or radiations). These do travel faster than light and detection ssytems based on them are necessary for safe navigation (i.e. not blind) when traveling at FTL speeds.

I won't claim that the system is blasterproof but it covers more thna might be thought at first glance.

[malloyd]

Originally Posted by ericbsmith

The 3000G acceleration you were thinking of using just means that a ship can get to light speed in a few minutes,

Hours. The speed of light is about 8500 G-hours, so 2.8 hours, and for interplanetary distances you still need a few - at constant acceleration with a midcourse flip you'll need 1.2 hours to cover an AU. Even with insane levels of acceleration like this, space travel is still an actual trip, not a daily commute.