Post
by **Iscander** » Sat Aug 11, 2018 3:13 pm

Unfortunately thrust(force) and power are not so easy to interchange.

I don’t disagree with 2049’s assessment that treating Star Wars repulser lift as rocket is incorrect, but lacking more information on the efficiency of their anti-gravity engines or baring someone actual inventing anti-gravity this is probably the best method we have of approximating it with tech we understand. And we can at least draw and equivalence.

Even give that though it does come with its own problems, like introducing more unknowns into the equation. Notably Specific Impulse, or effective Specific Impulse, since the actual lifting mechanism is in question.

Knowns and assumptions:

Mass = 40 Million tons, treating as metric tonnes (Disney Star Wars comics)

Local gravity = 1 gee = 9.81 m/s^2

For **** Thrust = F = m * a = ṁ_dot * v_exhaust ****

m_dot is the mass flow rate and v_exhaust is the leaving velocity of the thruster exhaust

Isp is related to exhaust velocity by **** I_sp = v_exhaust / g_0 **** so once we select an Isp value, we can solve for m_dot from the previous equation. Then solve for Energy with **** KE = 1/2 m_dot * v_exhaust^2****

For a range of Isp (Specific Impulse) values

Isp1 = 1 sec (effectively moving full own mass each second with engines) 1.92E+12 watts or 460 Tonnes/sec

Isp2 = 21,400 sec (Dual stage ion thruster) 4.11E+16 watts or 9.8 MT/sec *

Isp3 = 30,570,000 sec (Photon rocket) 5.88E+19 watts or 14 GT/sec

I will rule out number 3 out of hand baring incredibly poor efficiency, since the energy to launch from surface to LEO is only 60MJ/kg (573.6 MT) for a 40 Million-ton ISD.

For a rough confirmation of these values, I’ll look at potential energy

PE=mg∆h

Where ∆h is from the time dependent **** d = V_0 * t + 1/2 a * t^2****. For a single second, the ISP would fall 4.095m without its engines firing in a 1gee field.

Per second the ISD will need to overcome 1.92E+12 watts or 460 Tonnes/sec of ∆PE/sec.

Assuming a 100% efficient engine and no transfer losses, you need at least 460 tonnes/sec and an effective Isp of 1.

Let’s look at inefficiency real quick.

A quick web search gives me about 6% losses for transmission and distribution of electrical power in the USA. So, 94% efficient. And a NEMA premium efficiency electric motor runs from 85.5-96.2% so let’s say 90% for arguments sake.

That knocks up total power to around 544 tonnes/sec for staying aloft.

Starting to feel like Disney may have gone overboard on the Nerf bat since they took over, because GJ turbolasers are starting to look more reasonable than kT. Between this, Rebels, and the underwhelming power of the Dreadnought assault cannon in TLJ.

* Had to correct the typo 4.11E+11 watts to the correct value 4.11E+16 watts

Last edited by

Iscander on Wed Aug 15, 2018 2:04 am, edited 1 time in total.