Horus Lupercal
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This one is for you Blazer
An oft pondered question in SFS, and the 'Holy Grail' of challenges.
Can you get to Low Earth Orbit using nothing but Ion engines.
The short answer, if you don't want to read my now infamous long winded explanations, is no. Not a fucking chance.
If you wish to know why not, then read on and I'll explain the genius (or coincidence) behind the Ion engines biggest design flaw.
First, for some numbers.
An out of the box ion engine weighs 0.6 tons, generates 24kN of thrust, burns 0.0024tons of fuel a second and thus has a huge specific impulse of 1020.40 seconds and a turd base TWR of 4.08.
Getting to LEO generally requires a TWR of between 1.00000001 and 1.6 and a Dv of above 2650m/s. For the purposes of this, I'm going to assume a requirement TWR of 1.2 and a Dv of 2800m/s (the low TWR needs more Dv, hence 2800 instead of 2650).
Also for reasons I'll go into later, an estimated burn time of 150 seconds to LEO (which is roughly how long it takes, burn time, at that TWR with chemical engines.)
Now, to the genius malevolence of the ion engine. Yes, an out of the box ion has a TWR of 4.08. However, an Ion doesn't work out of the box. You need fuel and you need power. Fuel we'll get onto later, lets deal with power first.
As you're all aware, power is either generated or stored using batteries, RTGs or solar systems. And as previously mentioned, from pad to orbit takes 150 seconds. A single Ion at 100% throttle burns 2 units of power per second. Which means that either the rocket requires 300+ units of power storage per Ion, or needs to generate 2 units of power per second per ion.
How much does that weigh then?
Broadsword can lift 40t and Hawk 115t.
It gets worse as well. 150 seconds is based upon chemical rockets. These dispose of large amounts of consumables, thus making the rocket markedly lighter / faster in flight. An empty battery is just as heavy as a full one and because of the huge efficiency, it doesn't burn much weight in fuel. So an ion powered rocket doesn't get much lighter in flight, so that TWR doesn't increase much and thus the flight time will be much higher requiring more Dv as gravity is going to be having a longer say in the matter. This means not only more fuel, but also more power, making batteries the less efficient to weight option.
For this same reason, RTGs are ruled out.
Solar it is then.
Whether or not you use arrays or panels is a matter of personal preference, it weighs the same. 2 tons of solar generation and 0.6 tons of ion = a TWR of 0.94. This is a negative number, even before you add a control unit or fuel. It doesn't matter how many engines you add to balance off the weight of a module (2.5 tons), the TWR will never be higher than 0.94 and depending on how much fuel you need for the Dv you require, the real maximum TWR is even lower than that.
Using the numbers above, let's test this. Feel free to follow along at home.
2,800m/s Dv required.
2.5 ton 'payload' of a probe module for control
X = number of engines,
Y = total engine thrust
Z = Fuel tank mass
A = TWR
Ions are awesome for super long distance burns in vacuum. But as a challenge breaking lift engine from Earths surface, they're useless. There is no way through art or design to get an electrically powered rocket into LEO.
Addendum
'But what about the other power methods?' I hear you ask.
Just for you and because I'm a thorough Warmaster, I'll tell you the maximum TWR of RTG and battery powered rockets, and also theorise the maximum mass of batteries you can have to break the TWR duck.
An oft pondered question in SFS, and the 'Holy Grail' of challenges.
Can you get to Low Earth Orbit using nothing but Ion engines.
The short answer, if you don't want to read my now infamous long winded explanations, is no. Not a fucking chance.
If you wish to know why not, then read on and I'll explain the genius (or coincidence) behind the Ion engines biggest design flaw.
First, for some numbers.
An out of the box ion engine weighs 0.6 tons, generates 24kN of thrust, burns 0.0024tons of fuel a second and thus has a huge specific impulse of 1020.40 seconds and a turd base TWR of 4.08.
Getting to LEO generally requires a TWR of between 1.00000001 and 1.6 and a Dv of above 2650m/s. For the purposes of this, I'm going to assume a requirement TWR of 1.2 and a Dv of 2800m/s (the low TWR needs more Dv, hence 2800 instead of 2650).
Also for reasons I'll go into later, an estimated burn time of 150 seconds to LEO (which is roughly how long it takes, burn time, at that TWR with chemical engines.)
Now, to the genius malevolence of the ion engine. Yes, an out of the box ion has a TWR of 4.08. However, an Ion doesn't work out of the box. You need fuel and you need power. Fuel we'll get onto later, lets deal with power first.
As you're all aware, power is either generated or stored using batteries, RTGs or solar systems. And as previously mentioned, from pad to orbit takes 150 seconds. A single Ion at 100% throttle burns 2 units of power per second. Which means that either the rocket requires 300+ units of power storage per Ion, or needs to generate 2 units of power per second per ion.
How much does that weigh then?
- 300 units of power storage = 1.5 tons
- 2 units of generation:
- RTG = 4 tons
- Solar = 2 tons
Broadsword can lift 40t and Hawk 115t.
It gets worse as well. 150 seconds is based upon chemical rockets. These dispose of large amounts of consumables, thus making the rocket markedly lighter / faster in flight. An empty battery is just as heavy as a full one and because of the huge efficiency, it doesn't burn much weight in fuel. So an ion powered rocket doesn't get much lighter in flight, so that TWR doesn't increase much and thus the flight time will be much higher requiring more Dv as gravity is going to be having a longer say in the matter. This means not only more fuel, but also more power, making batteries the less efficient to weight option.
For this same reason, RTGs are ruled out.
Solar it is then.
Whether or not you use arrays or panels is a matter of personal preference, it weighs the same. 2 tons of solar generation and 0.6 tons of ion = a TWR of 0.94. This is a negative number, even before you add a control unit or fuel. It doesn't matter how many engines you add to balance off the weight of a module (2.5 tons), the TWR will never be higher than 0.94 and depending on how much fuel you need for the Dv you require, the real maximum TWR is even lower than that.
Using the numbers above, let's test this. Feel free to follow along at home.
2,800m/s Dv required.
2.5 ton 'payload' of a probe module for control
X = number of engines,
Y = total engine thrust
Z = Fuel tank mass
A = TWR
- 5 engines - 120 kN of thrust - 5.7 tons of fuel tanks - 0.58 TWR
- 10 - 240 - 10.61 - 0.63
- 100 - 2400 - 97.75 - 0.68
- 1000 - 24,000 - 969.18 - 0.69
- 1100 - 26,4000 - 1066 - 0.69 <~~~~~~ This is the maximum TWR of an electric rocket with a Dv of 2,800m/s
- 10,000 - 240,000 - 9683 - 0.69
- 100,000 - 2,400,000 - 96825 - 0.69
Ions are awesome for super long distance burns in vacuum. But as a challenge breaking lift engine from Earths surface, they're useless. There is no way through art or design to get an electrically powered rocket into LEO.
Addendum
'But what about the other power methods?' I hear you ask.
Just for you and because I'm a thorough Warmaster, I'll tell you the maximum TWR of RTG and battery powered rockets, and also theorise the maximum mass of batteries you can have to break the TWR duck.
- Max TWR of Battery powered rocket (300 units of power per ion) = 0.85, achieved at 250 ions
- Max TWR of RTG powered rocket (4 tons of RTG per ion) = 0.39, achieved at 75 ions
- Max amount of battery power per engine capable of being lifted with fuel and payload = 200 units
- This assumes one probe module, 10t of fuel, 15 ions each with 200 units of power on top and a TWR of 1.01. The max TWR if you increase this set up exponentially is about 1.1 with about 100 ions, it will go to 1.12 but with several thousand ions. This however means that you have 100 seconds before the power runs out. Which is 150 seconds less time than you need to burn the fuel on board and won't get you to LEO even with no gravity and drag off.
- Number of Ions required for 1.2 TWR to gain LEO without power constraints = 3
