5 ms·
The most important parameter of an engine like that is velocity of particles emitted as reaction mass (translate to specific impulse). Thermal rocket is going t
by lmilcin 6y ago
The most important parameter of an engine like that is velocity of particles emitted as reaction mass (translate to specific impulse). Thermal rocket is going to loose to well designed electric one. Electric rocket can propel ions to huge velocities basically functioning as a small particle accelerator.
The issues currently are ability to get enough thrust (density of the particle stream) and preventing materials from decaying due to energetic particles.
Thermal rocket will be limited to very small, thermal velocities.
- p_l 6y agoThe electric will finally reach much higher velocity, yes. The thermal one will reach usable velocity within your lifespan.
- bryanlarsen 6y agoJust run a lot of ion thrusters in parallel to get your thrust. They're tiny, that's feasible for any ship trying to go further than Mars/Venus.
- m4rtink 6y agoThis works to a degree - each thruster weights something and has cooling requirement. Also wear and tear if you run them preatty much continuously.
- bryanlarsen 6y ago> This works to a degree - each thruster weights something A lot less than the fuel for chemical or propellant for nuclear thermal, though > and has cooling requirement. That's for the electricity generation, not the thrusters > Also wear and tear if you run them preatty much continuously. The whole point of using thrusters is to run them continuously. No moving parts.
- m4rtink 6y agoIIRC some ion thrusters experience grid erosion over very long runs - stuff can wear out even without moving parts.
- dwaltrip 6y agoAdding more of them doesn’t meaningfully change the very low thrust to weight ratio, which is what makes it hard to get to closer places quickly.
- rbanffy 6y agoYou still need to get rid of excess heat, which scales linearly with power. The more engines, the bigger the radiators you need to carry, which will increase your total mass.
- laverya 6y ago> Thermal rocket is going to loose to well designed electric one. Depends on your metric. For a given tech level, you'll get higher thrust/weight out of a Nuclear Thermal Rocket than you would from a Nuclear Electric Rocket, even if the specific impulse is lower. And 'very small velocities' here is still double the ISP of hydrolox, so not exactly shabby...
- lmilcin 6y agoI assume neither design is going to lift off Earth's surface. Most likely any nuclear engine is going to be lifted cold and disassembled and we'll packaged in case of mishap. Since you are going to have months to run the engine, thrust to weight is less important than ISP. Small accelerations do wonders if you can run them continuously.
- valuearb 6y agoThrust to weight still will matter. NTR and NERs still have to drag all that dead mass of engine, radiators and shielding wherever they go.
- m4rtink 6y agoNot to forget NTR dumping most of the heat during the burn kinda by definition while NER has to radiate lots of heat continuously to power the engine. Also without high trust you can't really make use of the Oberth effect: https://en.m.wikipedia.org/wiki/Oberth_effect https://en.m.wikipedia.org/wiki/Oberth_effect
- lmilcin 6y agoOberth effect is for long term missions where you have time to swing by other planets or moons just to get some free velocity. This is fantastic, but costs huge amount of mission time. The nuclear engine is about getting so much delta v that you can cut the crap and power directly to your destination.
- bernulli 6y agoIt's really not the most important parameter, though. Unless you get the necessary thrust levels, the efficiency (Isp) of electric propulsion will get you very efficiently nowhere. And don't get me wrong - electric is amazing and well working, but if you want to move serious payload to Mars, that's not going to work for a while.
- handol 6y agoPowering those ion thrusters is a bit of a problem at the mass ranges you need for peopled interplanetary flight. Either you have an absolutely enormous solar array, or an absolutely enormous radiator array for your nuclear reactor.
- valuearb 6y agoI think STR is the best possible competitor to chemical rockets for trips inside Jupiter’s orbit. NTR is what we will need out past Jupiter unless a few brave souls use chemical rockets for the fame of being first to make those extremely long expeditions to Jupiter and beyond. Much like early explorers in the age of sail.
- m4rtink 6y agoOr even the Age of the Black Friday Sale! ;-)
- bryanlarsen 6y agoThose aren't necessarily all that massive, though. Enormous in size, yes, but the mass would be a lot less than the mass of the fuel for less efficient rockets like chemical or nuclear thermal. Either way, any vessel travelling beyond Mars or Venus in a time short enough to be safe for humans is going to require in orbit assembly, so size in terms of volume becomes less of a constraint. But as long as we're boosting all of our mass from Earth, that's what costs the money.
- baybal2 6y agoThe hotter your reactor is, the higher is your efficiency, and the smaller radiators you need.
- marcosdumay 6y ago> The most important parameter of an engine like that is velocity of particles emitted as reaction mass (translate to specific impulse). Hum... That's true for simple designs. Once you start transforming one kind of energy into another, you have to deal with a more complex form that is how much total momentum you can eject from a fixed amount of fuel (and weight it by the mass of the engines that stays on the rocket).