Ion engines and other low thrust vs Ship mass engines

[Tomato3017]

Hello,

I am having a problem when it comes to ships that have a low thrust vs the ship mass(ie the Perseus which is very very massive because of fuel and other ships like the ion engine class).

The problem is that I can't seem to orbit eject properly with these types of engines because they require multiple burns at the PeA to achieve escape velocity. Now I have done many escapes with higher thrust ships like the DGIV or XR-2. But all of those only require one Escape burn to achieve the TMI TVI or whatever planet you are going to.

I can get the Perseus to leave but the problem with that is that I have to go to 2M so that IMFD doesn't abort the burn due to the PeA passing too quick. When I use transx at LEO of around 200km it has me start the burn too early and I end up going deep into the atmosphere. Which obviously puts me way off course and makes me burn almost all my fuel.

I guess my problem is that I don't know how to setup a multiple burn escape yet . Can anyone help?

 

[Andy44]

Perseus has nuclear thermal engines for higher thrust maneuvers within deep gravity wells. Eject using the NTRs, then switch to ion jets for cruise power.

If you insist on using ion jets alone, then you need to spiral out over several days or weeks until you reach escape velocity, and this will be tougher to plan properly. I have some ideas of how I would do it, but I haven't tried them yet so I don't know if they are too stupid to post. But spiraling out is easy enough; you just set the prograde autopilot, throttle up the ion jets, and speed up time accel. What you do when you are close to escaping is the question.

 

[Tomato3017]

Perseus has nuclear thermal engines for higher thrust maneuvers within deep gravity wells. Eject using the NTRs, then switch to ion jets for cruise power.

If you insist on using ion jets alone, then you need to spiral out over several days or weeks until you reach escape velocity, and this will be tougher to plan properly. I have some ideas of how I would do it, but I haven't tried them yet so I don't know if they are too stupid to post. But spiraling out is easy enough; you just set the prograde autopilot, throttle up the ion jets, and speed up time accel. What you do when you are close to escaping is the question.

It happens with the NTR's as well. If i'm in a LEO of around 200km the IMFD autopilot will abort when the dV required starts to rise after passing the PeA which the engines can't compensate enough for. Now that I think about it I have had the same issue with the Smerch tug as well. Lots of mass on the ship because of fuel with an engine that is much weaker(therefore requiring longer burns)

 

[Andy44]

You can break it up into more than one burn. I don't use IMFD so I can't help you there, but doing more than one burn is nothing new.

 

[sputnik]

Had any luck with IEAT MFD? Seems to work pretty well.

[ame="http://www.orbithangar.com/searchid.php?ID=3815"]IEAT MFD[/ame]

 

[francisdrake]

I would try this method:

1. Determine the theoretical ejection point with IMFD or TransX.

2. When approaching the ejection point perform a manual prograde burn (not using the MFD burn function) for a few minutes.

3. Repeat this prograde periapsis burn, until the orbit becomes a real elongated ellipse. The periapsis should stay close to the ejection point.

4. Finally perform the automated ejection burn. The elongated elliptical orbit has already a lot of speed near the periapsis, so the low thrust of the engines may now be sufficient to provide the final delta V required.

This method is easier to handle than spiraling up, because it does not require continous prograde orientation at high time compression levels.

 

[flaugher]

I've been messing with this stuff for many weeks now, simulating a constant 1-G thrust from future ion engines because I think this is the way we will really get into space someday. Thrusting at 1-G toward a destination then turning around at the halfway point and decelerating at 1-G solves all sorts of health problems for astronauts plus weight and supply problems because not only do you live in a 1-G environment but you get to a places FAST, like Mars in less than four days! Furthermore, I think that as ion engine tech and battery / supercapacitor tech continue, we will eventually reach a meeting of technologies where we will HAVE the 1-G capability. AND, I think it might be within our lifetimes (hopefully).

For now what I do is use the AGMFD (available on the site) and set my ships for unlimited fuel. I know, unlimited fuel sounds unrealistic, but you have to project yourself to the day when we WILL have ion engines and battery/capacitory technology that will break us free of the bonds of liquid fuel thrusters.

AFMFD has a couple of quirks still in it, but some really nice features, too. If you use the SET button to enter EITHER "1" or "9.8" it will understand that you want to accelerate at 1-G of gravity (9.8 meters/sec/sec). If you enter a value like 7.5 it will understand you want 7.5 m/s/s.

Doug Keenan made AGMFD and his (I think) tutorial is at http://www.hemptrek.com/orbiter/agmfd/tutorial/agmfd.html

Lately I've been using the Ares launcher and experimenting with the Perseus because they both have their main engine axis straight up rather than forward, as one would anticipate in the design of low-acceleration 1G ion engines.

Imagine ... all the way to Mars or Saturn, sitting at a table and eating with silverware, drinking from glasses, and flushing the toilet when you're done. Yes, that's the way I want to go to space. And it's the ONLY way we're ever going to do it for the masses.

Watch your times. The first hour or so go by so slowly because you don't get very far from Earth. Remember that your speed will be constantly building because of the fixed acceleration. At the halfway point it will surprise you how fast you're going. Low-acceleration = high speeds. Sounds ... futuristic ... doesn't it?

Once you've learned how to target (roughly) using AGMFD, you can also control your direction and speed by altering trajectory and changing the decel rate. 9.79 or 9.61 m/s/s is not much less than 1-G, so passengers probably wouldn't notice very much difference, especially if you were flying a big cruise ship like Perseus carrying a hundred or so holliday patrons.

I've experimented with other G's also, like half-G for cargo only computer-controlled flights, and multi-G trips for unmanned probes attempting to reach light speed, etc.

AGMFD doesn't like time-warping beyond X100. I sometimes just turn off the guidance and let the ship go X1000 or better, then made course corrections after midpoint. I have lately been using the Encounter MFD pretty effectively at corrections, except you have to lead the planet you're going to because almost everything we have nowadays is designed around the Hohlmann Transfer Orbit, which of course is for space loosers. :yes:

My understanding is that NASA can already do 0.1-G or better with existing engines, and considering how much progress we're making every year, it MAY not be long before this type of flight becomes feasible. What we need are other people to push the technologies along, like the satellite community pushing ion engines for tugs, and perhaps electric car technology pushing supercapacitors.

The next twenty years might be very intersting.

 

[RisingFury]

We won't see 1G ships in our lifetime and ion engines probably aren't gonna get us to 1G or anywhere near it. I don't know where you've heard of 0.1G. Where ever it was, it wasn't true. The most powerful experimental ion engines have been run in the range of 100N of thrust. That roughly as much force as you need to hold 10 kg or 20 pounds. 100N of thrust powering a 500 kg prove would give you 0.02G.

These devices probably won't be used to power and space craft any time soon as their power requirement is in the range of 10 MW. A typical nuclear power station is 1000 MW. Such power requirements cannot be met on a prove weighing in at 500 kg.

Besides... why would you wanna fly at 1G? It's extremely inefficient. You need a lot of fuel that could have been spent as cargo and you need a lot of power to accelerate that fuel.

 

[Linguofreak]

I've been messing with this stuff for many weeks now, simulating a constant 1-G thrust from future ion engines because I think this is the way we will really get into space someday. Thrusting at 1-G toward a destination then turning around at the halfway point and decelerating at 1-G solves all sorts of health problems for astronauts plus weight and supply problems because not only do you live in a 1-G environment but you get to a places FAST, like Mars in less than four days! Furthermore, I think that as ion engine tech and battery / supercapacitor tech continue, we will eventually reach a meeting of technologies where we will HAVE the 1-G capability. AND, I think it might be within our lifetimes (hopefully).

Do you understand how much engine power you'd need to make a constant 1-G flight to Mars? Your engine would have to put out 18000 horsepower for every pound your ship weighed. You're talking something like the total engine power of a Boeing 777 for every three pounds or so of ship. Assuming that the ship weighs as much as a 777, that comes out to a total power output of 10 terawatts, or 2/3 of the electrical power consumed by the entire world, or the equivalent of a Hirsoshima bomb every five and a half seconds. And you have to pack that onto something the size of an airliner.

For now what I do is use the AGMFD (available on the site) and set my ships for unlimited fuel. I know, unlimited fuel sounds unrealistic, but you have to project yourself to the day when we WILL have ion engines and battery/capacitory technology that will break us free of the bonds of liquid fuel thrusters.

I doubt we'll ever get to that point, and if we do, it won't be with ion engines. You'd need fusion or antimatter.

---------- Post added at 06:35 PM ---------- Previous post was at 06:17 PM ----------

We won't see 1G ships in our lifetime and ion engines probably aren't gonna get us to 1G or anywhere near it.

Well, certainly we won't see ships that can do 1G for four days anytime soon. 1 G (or even three) for a few minutes is trivial. But, yes, it can't be done with Ion engines.

I don't know where you've heard of 0.1G. Where ever it was, it wasn't true. The most powerful experimental ion engines have been run in the range of 100N of thrust. That roughly as much force as you need to hold 10 kg or 20 pounds. 100N of thrust powering a 500 kg prove would give you 0.02G.

These devices probably won't be used to power and space craft any time soon as their power requirement is in the range of 10 MW. A typical nuclear power station is 1000 MW. Such power requirements cannot be met on a prove weighing in at 500 kg.

Besides... why would you wanna fly at 1G? It's extremely inefficient. You need a lot of fuel that could have been spent as cargo and you need a lot of power to accelerate that fuel.

Fuel usage at 1G is entirely determined by ISP. For an arbitrarily high ISP you can have arbitrarily low propellant usage. The big problem is energy usage, and the fact that if even a small part of the energy going into the exhaust stream is lost as heat, it'll melt the engine off the ship in no time.

I suppose that it could be done with an Orion drive, which is something we could build with current tech, but that would be a bit bumpier of a ride than what Flaugher was looking for (bathrooms and dining rooms would both be nightmares, what with everything jumping around), and, of course, Greenpeace will be all over you if you try building one.

 

[Andy44]

You're talking about a Heinleinian-style torch drive, something that exists only in science fiction. Maybe someday, but not soon.

As for this question:

Besides... why would you wanna fly at 1G? It's extremely inefficient. You need a lot of fuel that could have been spent as cargo and you need a lot of power to accelerate that fuel.

As I sit comfortably writing this with strong bones and muscles and a convenient toilet nearby, the question pretty much answers itself! Although for a journey lasting only a few days, less than 1 G would be fine.

prejectrho.com has some interesting notes about speculative futuristic drives. Some of them, like liquid-core nuclear rockets and salt-water reactors, may possibly give you a fraction of a G all the way to Mars, but they are also scary dangerous and hideously radioactive.

 

[Tomato3017]

I would try this method:

1. Determine the theoretical ejection point with IMFD or TransX.

2. When approaching the ejection point perform a manual prograde burn (not using the MFD burn function) for a few minutes.

3. Repeat this prograde periapsis burn, until the orbit becomes a real elongated ellipse. The periapsis should stay close to the ejection point.

4. Finally perform the automated ejection burn. The elongated elliptical orbit has already a lot of speed near the periapsis, so the low thrust of the engines may now be sufficient to provide the final delta V required.

This method is easier to handle than spiraling up, because it does not require continous prograde orientation at high time compression levels.

The way you described here worked very very well for me thank you! Also, thanks to everyone who tried to help me! The community here is very intelligent and nice(as long as you spell right:lol.

 

[RisingFury]

As for this question:

As I sit comfortably writing this with strong bones and muscles and a convenient toilet nearby, the question pretty much answers itself! Although for a journey lasting only a few days, less than 1 G would be fine.

Even a trip beyond Mars would take such a short time (relatively speaking) that 1G travel would be inefficient. We've shown that astronauts can go for months in microgravity. Granted, there are effects... but that's why blast gigatons of fuel into orbit if you can build part of your ship as a wheel to give you artificial gravity? Hell, make that two wheels for a larger living area and rotating in opposite directions will cancel out the torques.

Also, you have to consider the speeds you're reach after days of 1G travel. Space isn't exactly empty. There's stuff floating around. Even a grain of sand impacting your ship could cause huge damage.



@Linguofreak:

Mathematically, having a huge specific impulse solves your fuel problem, but let's get real here. Producing such huge specific impulse will take something like the LHC, which brings your mass up quite high, which means your acceleration drops off a cliff. Even if you had the huge power available, there's a limit to how light you can build your engine.

LHC is built in a circle because linear acceleration becomes impractical, but then you need huge magnetic fields to keeps your proton/ion beam bending to the curve. And so far we can't produce more then 30T, which limits how small you can make your engine, which limits how light you can make it.

 

[Andy44]

Even a trip beyond Mars would take such a short time (relatively speaking) that 1G travel would be inefficient. We've shown that astronauts can go for months in microgravity. Granted, there are effects... but that's why blast gigatons of fuel into orbit if you can build part of your ship as a wheel to give you artificial gravity? Hell, make that two wheels for a larger living area and rotating in opposite directions will cancel out the torques.

Astronauts can only go for months in freefall so long as there will be people to help them sit up after they land. No one will be waiting for you on the ground on Mars. If you land and cannot get out of your seat you are not only useless, you are likely doomed. Even after landing on Earth it takes months of hard training to get back into shape.

Also, you have to consider the speeds you're reach after days of 1G travel. Space isn't exactly empty. There's stuff floating around. Even a grain of sand impacting your ship could cause huge damage.

If you're afraid of high speeds you've really got no business in spaceflight.