/sci/ - Science & Math

>>15404071
Like that 50,000-s Isp lithium-fueled gridded ion thruster that has been posted here before. IMO power sails remove one of the main obstacles of preventing greater adoption of electric propulsion, which is that high Isp doesn't give you much of an advantage when you can't accelerate fast enough to make it to your destination in a reasonable amount of time because the mass of the power system is so god damn high.

>>14883800
>kinetic torpedoes propelled by an ion engine
The core advantage of ion engines is that they typically have very high specific impulse which allows you to move a lot of mass with little propellant or perform high delta-v maneuvers, but it's at the expense of acceleration. Spacecraft using them would make for very good kinetic impactors for asteroid defense but poor kinetic weapons for intermediate range warfare or ballistic missile defense, which is often suggested for Starlink. Allow me to make a few quick and dirty assumptions and kindly point out any mistakes.

Torpedo full mass: 600 kg
Torpedo dry mass: 500 kg
Power 200 kW (very light thin-film panel unfurled from around torpedo, alpha 1 kg/kW)
Thrust/power: 200 mN/kW
Thrust: 40 Newtons
Isp: 1000
Delta-v: 1,800 m/s

So the Isp was gutted to aid acceleration, but how quick is it? It would accelerate at only 0.066 m/s, the full burn would take 7.5 hours. The Oberth effect is reduced since it can't do impulsive burns and if it was in LEO the panel would be blocked from sunlight part of the time. Clearly it would be far better to spam chemical torpedoes

>>14754565
Nope, nuclear electric propulsion just converts the thermal energy of the reactor into electricity which then feeds the ion thrusters. The reactors that have been proposed over the years, most notably in Project Prometheus, actually had a specific power multiple times worse than state-of-practice solar near Earth so they would actually accelerate slower if the rest of the spacecraft was kept the same. You can however trade Isp for thrust but it would still be painfully slow over a short period of time.

>>14603860
That person was only joking, it's absolutely impossible. The upper limit for methane/liquid oxygen rocket engines is about 460 seconds of Isp. Ignoring that, the ship wouldn't necessarily be extremely large because the fuel efficiency would be so great but it's acceleration would be incredibly slow that it would be useless for interplanetary trips.

This is why both the thrust meme and Isp meme needs to die, and it's unironically not that easy in rocketry. You want a high exhaust velocity if you need a lot of delta-v to reach your desired orbit but it has to be low enough to reach it within a reasonable period of time. You want high thrust low in a gravity well to take advantage of the Oberth effect, outside that you would ideally be burning half way to your destination, then turning around and burning through the other half as that is most efficient.
>>14603935
What matters is the specific power of the reactor or solar array, most electric thrusters are actually tuned to be higher thrust, lower Isp and they already can reach 70%+ efficiency. Like with VASIMR you can make them better at throttling but that does little to improve electric propulsion in general.

>>14594810
>Gatekeeping power sources with the same tired fallacies against intermittent power and the cost of electricity in places where literally everything is expensive while ignoring actual metrics e.g. LCOE
Deranged.
>Raptor and traditional chemical rockets will do 99% of what we need
That's because 99% of near future requirements are related to the Earth, Moon and Mars, however most of the solar system is largely inaccessible with chemical due to the high dV requirement outside flyby missions and essentially inaccessible with nuclear due to the inherently high cost. Luckily for everyone you have no sway in the industry otherwise your nuclear power only autism would hurt the advancement of spaceflight and space colonization. Cheap power is essential.
>Electric propulsion is a waste of time outside of niche station keeping uses.
Tell that to SpaceX/Starlink or all of the probes which now use electric propulsion. You want high thrust burns when you're stuck deep within a gravity well such as LEO, otherwise the Oberth effect gains are small and you're decreasing the total dV because specific impulse is directly proportional to thrust, provided you're throwing reaction mass from the spacecraft.

>>12030128
Very basically, thrust (and thus acceleration) and exhaust velocity (specific impulse, "efficiency" of the rocket) are coupled to one another by the amount of power you put in. It's a very simple relation: Thrust x Exhaust Velocity x 0.5 = Power Output.

If you have a 10 gigawatt fusion powerplant (in reality you'd need much higher, because powerplants are inefficient in converting their power output to thrust power), you can either use that power on better Isp or higher thrust. You can't do both. For comparison: the space shuttle produced about 12 gigawatts of power at launch, most of that going to thrust, with a mere 250-450 Isp. Your fusion drive can massively jack up the Isp, but thrust is going to seriously suffer (you can very simply calculate that with the formuula above).

Now you should see the problem if you want both high thrust AND high Isp. Your rocket will be a monstrous energy hog. You might need TERAWATTS of power, the power output of our entire civilization at once, to get a fusion rocket with the space shuttles proportions and high efficiency/exhaust velocity at once. The only design we have that would be capable of this is literally throwing out a kiloton nuke every second (equals 4 terawatt of power output) out the back and having them accelerate you.

REEEEEing at reality edition

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>>11849687

cool chart