Ion-engine, low trust? -Conservation of energy, newtons law

[shynung]

18 minutes ago, fredinno said:

BTW, how much energy would you need to get 100kN (around RL10 level) ION drive, with a 4000s ISP?

That's an exhaust velocity of around 39 km/sec. 100 kN would mean a mass flow of a touch above 2.5 kg/sec. Accelerating that propellant would need just over 1.9 MW, plus energy needed to ionize 2.5 kg/sec of propellant.

[eddiew]

44 minutes ago, fredinno said:

How about compounds? CO looks like a great idea, with a Xenon-level ionization energy, a gas at STP, and a great ISP, though the oxygen's reactivity and Carbon soot probably kills the idea.

I suspect by the time you've reached the ionisation energy of an element, you've passed the point at which you smash up any molecule it was attached to... (might be wrong, prepared to be educated )

[Bill Phil]

4 hours ago, shynung said:

That's an exhaust velocity of around 39 km/sec. 100 kN would mean a mass flow of a touch above 2.5 kg/sec. Accelerating that propellant would need just over 1.9 MW, plus energy needed to ionize 2.5 kg/sec of propellant.

The real issue is energy density. 

Maybe fusion engines, or broadcasted power, could help. But fusion engines would likely be more effective.

[PB666]

42 minutes ago, Bill Phil said:

The real issue is energy density. 

Maybe fusion engines, or broadcasted power, could help. But fusion engines would likely be more effective.

Once again we reenvent the wheel, Big giant ray gun is the way to power those panels........

[sevenperforce]

6 hours ago, fredinno said:

Unfortunately, Beryllium costs an arm and a leg, so...

How about compounds? CO looks like a great idea, with a Xenon-level ionization energy, a gas at STP, and a great ISP, though the oxygen's reactivity and Carbon soot probably kills the idea.

Once you ionize the CO it will want to recombine. Xenon, like all noble gases, has the handy attribute of not ever forming compounds with anything. 

If you're willing to play with something as deadly as beryllium hydride, use it to build an SSTO.

[PB666]

2 minutes ago, sevenperforce said:

Once you ionize the CO it will want to recombine. Xenon, like all noble gases, has the handy attribute of not ever forming compounds with anything. 

If you're willing to play with something as deadly as beryllium hydride, use it to build an SSTO.

Coupling one really bad idea to another. 

[YNM]

I think this is practically asking for coefficient of restitution.

Kinetic energy may not be conserved, but momentum is always conserved. If you look at propellers, the two colliding object (propellers and air) were separate, and both have initial momentum. After the collision, the two is also separate, with the propeller losing a slight momentum (which is resupplied by the engine) and the air molecules gaining a load of momentum - it happens continously, resulting in difference in momentum, hence thrust...

Ion engines (and rockets in general) carry their initial propellants with them. The process that results in thrust is practically the reverse of inelastic collision. And you do know collisions wastes a lot of energy in other forms. Be that heat, ionization energy, sound wave...

[John JACK]

On 12.05.2016 at 6:09 PM, QuesoExplosivo said:

You're confusing force with energy. Conservation of momentum is the relevant principle here.

That.

Energy is measured in joules. Energy (kinetic) is mass * velocity squared / 2. Momentum is just mass * velocity, and is measured in kg-m/s, no names unit here.

There is law of conservation of momentum and there is law of conservation of energy. That laws are different. With two bodies, impulse of one equals impulse of other. But sum of energy of one and energy of other is constant.

If you throw a stone with 10 joule in one direction, you got an energy of 10 joule in the other direction — only if your mass is exactly same as the stone's. If you are heavier, your velocity will be just as slower, as the stone is lighter. And the energy of throw will be divided — most go to stone, some go to you. Ratio of energies equals ratio of velocities, and equals ratio of masses. In ion engines exhaust velocity is very high, so exhaust get much, much more energy than spacecraft. But high velocity allows to use low amounts of propellant mass for same impulse.

[fredinno]

On 2016-05-13 at 11:26 AM, shynung said:

That's an exhaust velocity of around 39 km/sec. 100 kN would mean a mass flow of a touch above 2.5 kg/sec. Accelerating that propellant would need just over 1.9 MW, plus energy needed to ionize 2.5 kg/sec of propellant.

How large of a 50% efficiency solar panel would you need?

Edited May 16, 2016 by fredinno

[shynung]

15 hours ago, fredinno said:

How large of a 50% efficiency solar panel would you need?

I don't understand how much that panel it would produce under what conditions given this bit of information, unfortunately. I'd need power density (in kW/m²).

Or, just point me to the specs of a currently-available solar panel. Given power and mass density, I can calculate total panel area and mass.

Edited May 17, 2016 by shynung

[PB666]

16 hours ago, fredinno said:

How large of a 50% efficiency solar panel would you need?

1300 kw at 1 AU per meter squared at 50% is 650 w per square meter. If your ion drive is 80% efficient, and you have a 5% loss between the panel and the drive, then its 3/4ths that lets say .0005 MW per meter squared. On a panel 10 time as long as wide and 2 of them you would need 2 panels 14 meters wide and 140 meters long. The estimated weight would be 3900 kilograms at best. You could have 4 at 10 meters x 100 meters. 16 at 5 meters by 50 meters.

Its alot. The ISSs panels which are not as big weight 14 tonnes each.

[fredinno]

On 2016-05-17 at 8:42 AM, PB666 said:

1300 kw at 1 AU per meter squared at 50% is 650 w per square meter. If your ion drive is 80% efficient, and you have a 5% loss between the panel and the drive, then its 3/4ths that lets say .0005 MW per meter squared. On a panel 10 time as long as wide and 2 of them you would need 2 panels 14 meters wide and 140 meters long. The estimated weight would be 3900 kilograms at best. You could have 4 at 10 meters x 100 meters. 16 at 5 meters by 50 meters.

Its alot. The ISSs panels which are not as big weight 14 tonnes each.

Just for an RL-10 sized engine?

[PB666]

8 minutes ago, fredinno said:

Just for an RL-10 sized engine?

Didn't i have a nice little thread back there with pictures of my ION drive craft with 30 10 x 100 solar panels struggling for a TWR of 0.002. Dont you remembering me saying these would be good for transporting non perishable materials between staging sites in the inner solar system but very poor for human transport?

 

 

[shynung]

2 hours ago, fredinno said:

Just for an RL-10 sized engine?

An RL-10 with 4000 seconds ISP. The actual RL-10's ISP is about 1/10 of that.

[Lukaszenko]

Might be a bit off topic, but is there a theoretical maximum ISP you can get from a rocket engine? Can you, for example, just keep pumping more and more energy into the ions of an ion engine, raising their exhaust velocity and ISP without bound?

[Sereneti]

6 minutes ago, Lukaszenko said:

Might be a bit off topic, but is there a theoretical maximum ISP you can get from a rocket engine? Can you, for example, just keep pumping more and more energy into the ions of an ion engine, raising their exhaust velocity and ISP without bound?

Theoretical ... yes.
you can pump partikles to the speed of light, if you want to put unlimet energy into that...
Praktikal: no

 

[Lukaszenko]

You're saying that "theoretical ... yes" there is a maximum, or "theoretical ... yes" you can raise ISP without bound?

I understand that the exhaust will never reach the speed of light...it will just start to gain more and more mass at some point as you pump more and more energy into it. But that would still continue to increase the ISP, no?

[PB666]

1 hour ago, Lukaszenko said:

You're saying that "theoretical ... yes" there is a maximum, or "theoretical ... yes" you can raise ISP without bound?

I understand that the exhaust will never reach the speed of light...it will just start to gain more and more mass at some point as you pump more and more energy into it. But that would still continue to increase the ISP, no?

When you are talking about IS travel its best to drop the ISP and use exhaust velocity, also known as ISP (m/sec) otherwise known as exhaust velocity. ISP (sec) = exhaust velocity / 9.806

Yes it will increase ISP and the craft will become less and less efficient, the problem is that no energy process can produce the power to do this, since a nuclear reaction can only convert about 2% of its mass to energy, if you used that energy to accelerate the reactant you would only have 0.14c exhaust velocity. However since energy capture is imperfect its about 0.1c. You could accelerate reaction mass to near c, but then you would be throwing off waste nuclear reactants with no momentum, and so that the reaction mass would be wasted.

To achieve better acceleration you would need to create antimatter and annihilate and capture and convert energy. Using one proton and one antiproton  an exhaust velocity of one hydrogen to near c can be achieved or so you have to do a Lorentz transformation, and that's pushing the issue. I created a table for this, for a perfect energy capture annihilation you want about 1.5 the annihilation mass as propellant.

This has already been discussed in another thread on nuclear solar power and why it can never be better than a generational interstellar space ship. 

Edited May 20, 2016 by PB666
Sorry many errors IPAD could not fix were fixed.