Earth To Orbit

This is the sticky wicket of all space.

Getting up there in the first place.

The first thing running against you is the gravity.  If your drive makes 1g you sit there making noise.

The Saturn V stack made barely 1.1g at ignition, but got lighter as it used fuel so acceleration increased as it went on its way.

But we don't want to use staging because we're gaming a sci-fi world and the paradigm there is 100% reusable.

Now, you don't need more than 1g acceleration to get off the ground if you have wings.  A 747 sure as heck doesn't have that much acceleration (0.27g at max load at sea level) and they certainly can fly.  An F-15C can pull 1.12g with a typical loadout.  Air breathing engines have an advantage in that they don't carry their reaction mass onboard, but you get decreasing thrust as you gain altitude.  And because you need air, you can't use them to get to orbit.

Atmo is a drag.  Literally.  When you watch NASA launch something you hear about "max Q" that's when the air is pushing as hard as it ever will during the launch.  In some cases the vehicle will throttle back to keep from damaging itself.

Using wings means you can get off the ground with less thrust, but it also means spending more time in the atmosphere and using more fuel to get out.

Using wings means you need to have a runway and that will have a limiting effect on the size of your vehicle.

Pretty much every rocket initially goes straight up before pitching to get clear of the worst of it.

Let's take our F-15 and give it engines that don't need air.  1g of our acceleration is eaten up canceling the pull of Earth leaving us with just 0.12g to affect our velocity.  How long does it take us to get to escape velocity?  We need to get to 11.2 km/s.  Math again: Velocity is acceleration multiplied by time. So with an effective acceleration of 0.12g the equation is t=112,000m/(0.12*9.81m/s).  On the order of 2.6 hours.

It's actually not as bad as all that.  You can pitch over and start utilizing more of your thrust to accelerate in exchange for a shallower climb.

Traveller maneuver drives include three miracles.  First is the reactionless thrust itself.  Second is that it seems to move a volume and is unaffected by the mass of the vehicle.  Third is that it appears to be unaffected by gravity.  GURPS: Vehicles called that third item contragrav.

That means our Traveller Type-A Free Merchant with a Maneuver-1 drive can apply its entire 1g to acceleration.  That means it can outrun our F-15 to escape velocity taking just under 20 minutes to get there.

Harder science fiction would use a more F-15 like vehicle.  Oh, acceleration also increases as fuel is consumed.  An empty F-15C would pull 1.78g as the fuel ran out.

Albedo is harder than Traveller, but it's still running some miracle.  The fusion thrusters shown in canon are mentioned pulling 20m/ss and doing so for days at a time.  That's a wee bit more than we think you can get fusion to do with realistic fuel consumption; but it might be plausible.  I have an excellent drawing I need to scale and see what the volumes of their sample scout ship come to and extrapolate the drive from that.  At present the best numbers I have for a realistic fusion drive are from GURPS: Vehicles; and I don't trust those because a 135hp small block Chevy is a much smaller and lighter motor than a 350hp SBC where in the real world they are pretty much identical.  Their earth to orbit vehicle is called an aerodyne, and those things are shown thrusting at 8 or 9g.  Plus have thrust vectoring for hover.

The fuel consumption vs acceleration curve needs to be plotted out so it's easily usable.  There's math that will do that.  I always fudged and calculated the acceleration at 100%, 90%, 80% etc. fuel.  You can throw that on a graph and get the ball-park easily.

Oh, acceleration is a nice simple equation.  It's thrust over mass and gives g.

Are we bored yet?