On 12 May 2020 at 11:09, Ethan McKinney wrote:

> Remember the rocket equation: extremely high exhaust velocity = very
> low thrust = very high ISP.
>
> If you want decent thrust, your ISP goes down the toilet.

Huh? You are seriously confused about this.

Exhaust velocity has no effect on thrust.

Exhaust velocity and rate of fuel flow pretty much *define* thrust.

The (dimensionaklly inaccurate, but still workable) equation is this:

Isp = thrust/fuel flow rate.

Thrust is in mass units
fuel flow is mass units per second

Ve = Isp * "gravity"

"gravity" is a conversion factor not actual gravity. It's needed to
correct the dimensional goof of measuring thrust as mass units
instead of force units.

So "gravity" is 9.8 m/s^2 or 32.16 ft/sec^2

Anyway, from the above it should be obvious that for a given exhaust
velocity the thrust goes up in direct proportion to the fuel flow
rate.

What you are likely confused by is that all existing really high Isp
engines are ion drives, which have a high exhaust velocity but an
inherent low rate of fuel use.

Theoretically, a fusion rocket would have an Isp of around 600,000
(figure from an old NASA poster about rockets). So for 600 *tons* of
thrust you'd have a fuel flow rate of one kg/sec.

But since the exhaust velocity would be 5880 km/sec. That's almost 2%
of the speed of light.

And that means the engine would be generating 180 *gigawatts* while
it was running.

Just a bit unhealthy to be close too :-)

--
Leonard Erickson (aka shadow)
shadow at shadowgard dot com