Hello, and thruster question!

[SpaceMouse]

Good news everyone! I have joined your forum!

I absolutely love the variety of thrusters available for KSP but, i have to wonder, why has no-one combined the high-ISP advantages of electric thrusters with the high thrust advantages of rockets? By my reasoning, you could use both to boost your efficiency greatly but, then, the nitty-gritty-fiziks have never been my thing.

And hello again!

[Claw]

Hi SpaceMouse,

Ion engines and chemical thrust rocket engines work very differently. Ion engines are highly efficient because they accelerate nearly all of the propellant, which extracts as much kinetic energy as possible from them. Chemical engines, on the other hand, spend a lot of energy accelerating the exhaust, plus they lose a lot of efficiency due to all the heat generated.

Because Ion engines work by accelerating small particles, it's very difficult to make one large enough that pushes enough small particles to give a higher thrust output. Plus the energy (electricity) requirements for an Ion engine are fairly high. Especially considering that a chemical rocket pretty much only needs electricity to get started, then can generate electricity as it runs (through thermocuples). In some cases, a chemical engine might not even need electricity to get started.

Anyway, both engines have tradeoffs. That's what forces you to decide what you need!

Welcome to the forums!

~Claw

Edited October 23, 2014 by Claw
Kowtowing to NK

[NathanKell]

Welcome!

Claw nailed it; I just want to add a bit more context. Ion engines in KSP are about 1000-100,000x more powerful than real ones (and have 20-1000x the thrust to weight ratio), and use less electricity than real ones too. In real life, an ion engine might need the equivalent of two or more of those Gigantor solar panels (to get its 2200W electricity), and generate only about 0.00012 kN of thrust out of its 6kg mass. At an Isp of 1800s, not 4200.

[SpaceMouse]

Hmmm. I was aware some of the number in KSP were.... altered, to help there fun value, i was hoping the mods were a little more accurate at least. I've been following that 'holy grail' microwave thruster and was aware the energy requirements for that are pretty insane.

[Xannari Ferrows]

Amazing to hear! Welcome to the forums! I wish you the best of times here!

Fairly generic introductions aside, to answer your question of rocket efficiency, there are a few things that come into play.

Each of these topics have been touched by Claw already, but I can add in a bit more detail.

ISP is very dependent on the propulsive element you are using. For example, small Ion engines kind of work in the same context as a particle accelerator, but does have it's differences. These similarities, though, include the method of acceleration and the sheer amount of power required to do it.

The reason this is not a powerful engine, however, is because the amount of thrust these particles produce is laughable, due to the mass and energy transferred from them being so small, regardless of being in a decent sized field. It's like a car taking 5.8 hours to make one right turn. However, they are efficient due to the fact that the propellant used transfers all propulsive energy to the probe in question, which allows this so called "probe" to use every ounce of energy this propellant has to offer.

I can go into more detail about the chemistry if you want, or I can leave it at this.

Chemical rockets are as powerful and inefficient as they are do the fact that a lot of propulsive kinetic is lost in the exhaust. To explain this properly, lets go back to the basics.

Specific impulse is describing the efficiency of an engine by referring to the amount of thrust you get from each unit of fuel/energy expended. Or chemically, how much energy is being transferred from one body to the next per unit of energy expended. So chemical rockets do not exploit all the propulsive energy from the fuel as they could, thus must use a very high amount in order to get that sweet thrust we find so appealing. This is why I love the nuclear rocket. Provides a fair amount of thrust while keeping that lovely Delta-V so high.

The upside to chemical rockets is their alternator.

Chemical rockets are much more complicated than I'm putting down, but that's the basic gist.

And greetings again!

Edited October 23, 2014 by Xannari Ferrows
Must fix grammar

[SpaceMouse]

Most of these thoughts originated from playing with Near future and thinking, Since i've brought up all these solar panels to drive electric thrusters, and nuclear also for the heavy lifting, why not use that electricity to augment a standard rocket?

Before this thread dies, i must ask one final question that im sure has been asked a dozen other times. There's a big push for nuclear thrusters now, and i must ask, isn't this a rather bad idea? I know without a atmosphere the radiation dissipates considerably faster, but isn't even a professional orbit transfer going to release a more than lethal dose for the several burns you'd have to make, or are we just going to ship up with these rockets a special lead-lined can to put our astronauts in while firing? isn't this still a bad idea? Or is the information i've looked up a few decades out of date?

Edited October 26, 2014 by SpaceMouse

[Xannari Ferrows]

I'm not exactly sure what you're trying to ask. From what I gather, you're worried about the doses of radiation given from a burn from a nuclear rocket while in orbit, correct?

If that's the case, yes, a considerable amount of ionized inherent-reactive gases will be released, but the Earth will be protected by the repulsive forces of the magnetic field.

A lead-lining would be a very bad idea, because it would be super heavy. I like to think of the old televisions that used electrons to line energy on the screen. Those held huge lead plates, and weighed about 750lbs (340.2Kg)

If you're talking about a nuclear engine used for a lifting stage, than yes that is a very bad idea.

[SpaceMouse]

I was talking about radiation concerns in a interplanetary stage, well out of the atmosphere. I would of thought radiation would still be a concern. Also, aware lead is a bad example, it was just the first shielding material that came to mind.

[Xannari Ferrows]

Ah. I see what you're saying now. To answer properly now, think of the radiation like a flume effect. No matter the amount of water, it will always slow down to speed in the same spot when flowing down a hill. No matter the amount of radiation, it will dissociate into individual atoms at the same distance from the core point of reference.

As for lead, I'd take soil over all else.

I now have ideas for more space probes and rovers. Time to get that Ion engine!

[SpaceMouse]

Hmmm. I supose im just concerned of my kerbals gaining superpowers. XD

On the subject of soil as a radiation shield, there has been concern about radiation mars, (err, I mean Duna) mission, wouldn't the easiest solution be to just dig underground?

[Xannari Ferrows]

Well, if Duna is modeled after Mars, then the surface is composed of mostly Iron oxide, or rust. Rust isn't an ideal shield for radiation. If soil was on Mars/Duna, however, it would still have to be compacted to a marginal density, though it is very efficient.

But hey, if Jebediah were to have a power, it would probably be to have the power to take others (Oh please get the reference)

[SpaceMouse]

Sorry, not getting the refrence. Relatively new here. Havent even left creative mode yet in the game.

[sal_vager]

I don't get it either :/

[SpaceMouse]

Someone needs to write fanfic of a Kerbal with superpowers.

[Starwaster]

"Water Breathingâ€â€This is a wimp power. Face it. When the great super hero parties happen and Ulterior Motive Man asks you what you do and you say 'I breathe water,' you end up wearing a full fishbowl." â€â€Marvel Super Heroes Advanced Set (TSR, 1986).