Realistic Interface Transport in (GDW) 2300AD

Introduction

The formulae in the Naval Architects Manual are a mess. The fuel required for orbit is given as, using the correct order of operations:

(1)

 

 and the time to orbit is given as:

(2)

 

Both these are nonsense. There was an attempt to correct (2) in Challenge 37's Three Blind Mice, but it was also wrong.

The cost of lifting a ton to orbit (of payload) is ca. Lv4,500* in a shuttle or rocket or Lv3,000 with a spaceplane, which is about 1% of the current cost (ca. Lv200,000 per ton on a Falcon 9 Heavy to LEO, whereas 2k3 launches seem to be to a higher orbit). This is about in line with the current optimistic projections of what launches with fully reusable rockets would cost. SpaceX optimistically thinks they can get the price down to Lv2,500 per ton to LEO, with a fully reusable vehicle, which is close to 2k3 numbers.

Fuel is Lv100 per ton, and so the cost of fuel is ca. Lv600 per ton of dry weight, and if the vehicle is 50% payload (and non-thrust fuel) then the cost of fuel is Lv1,200 per ton of payload, or 27-40% of the cost to orbit. This is roughly in line with projections for SpaceX Starship's percentage etc.

From general principles, a hydrogen/oxygen rocket launching from Earth will require ca. 5/6ths of its' initial mass to be expended as fuel (and oxidiser). The fuel fraction vs dry weight should thus roughly be 5x dry mass one one (up). The 2k3 formula is explicitly two way - lift to orbit and return.

The Fix

Without giving derivations, the following formulae are approximately correct:

(3)

(4)

Tonnage should probably be dry mass.

Time to orbit should be 8-10 minutes for an Earth sized planet. Any quicker and the accelerations are liable to injure anyone onboard. Slower and the vehicle incurs additional gravity drag, and will have to expend even more fuel.

Both rockets and rocket planes are listed as taking 1 h to reach orbit, whilst scramjets take 2 h and so do catapults. Since the stutterwarp zone is about 10,000 km above the surface of Earth, once you've spent ca. 10 minutes getting to LEO, the rest is likely the orbital transfer.

Spaceplanes with airbreathing engines can use these engines in the lower atmosphere, but they rapidly get too high to get any benefit from airbreathing or lift surfaces. An option is for a shallower ascent than straight up, but this increases aerodynamic drag losses (remember, that lift is diverted acceleration) and gravity drag losses. A typical spaceplane might get upto about 25-30 km altitude and Mach 5 (1.7 kps, ca. one Mongoose Traveller "burn") before having to engage the rocket engine. From that point the fuel consumption etc. is similar.

The equation is for a round trip, although the majority of the fuel (> 90%) is burnt in the ascent.

Oddly, in terms of time, the spaceplane should be the one that takes longer to reach orbit, by both the table and common sense. Ergo, reverse the lift values for shuttles and spaceplanes. The advantage of a spaceplane is that there is no 3+ G acceleration, and the ride is acceptable for untrained non-astronauts. As described, most cargo goes to orbit in shuttles, whilst people take spaceplanes. Spaceplanes with air breathing engines can likely get away without external boosters, even if they have an external fuel tank.

The Effect

Yes, going up and down the gravity well is more difficult, or rather it is as difficult as described in the main rules.

No starship is going to be SSTO. Lifting a starship requires stacking on boosters (if the engine doesn't generate enough thrust, i.e. time to orbit isn't 10 mins for a rocket). Visiting a planet almost certainly means using landing craft. The gut check is whether the craft has 1 MW of MHD thruster per 120 tons of dry mass. Most of the fighters make this (the Bufer being the exception), as does the Aconit. The Yinma Lander doesn't, nor does the Hayabusa, LC-20 etc.

Most landers will require at least an external fuel tank to reach orbit. A Yinma, Hayabusa, LC-20, Thorez etc. will need to be stacked on a booster.

This gives problems for the likes of Bayern, but where are all the long runways on virgin planets for a spaceplane or shuttle to land on? Where are the cleared, level landing sites for thrust landings? Landing on a virgin planet, without infrastructure already in place, is a major issue.

Stutterwarps in Orbit

"Stutterwarp vessels can also leave orbit without using conventional drives."

2300AD Director's Guide, p. 62

It is continuously stated in the GDW books that stutterwarp does work inside the dead zone, and does provide enough movement for orbital maneouvres, but not enough for landings. It is explicit that stutterwarp equipped ships don't need rockets to leave orbit.

From the point of view of a starship, there is a certain truth to this. When a starship inserts itself into an orbit with stutterwarp, and makes no reaction burn, the orbit must either crash into the planet (or enter the atmosphere and land), or leave the dead zone at some point. Without a reaction drive, a starship simply cannot actually achieve a circular orbit below the threshold altitude. Thus it would never need a reaction drive to break orbit. If necessary, it could insert into an orbit with a very low perigee to launch landing craft.

For landing on the surface, one can devise deadglide strategies what obviate the need for a reaction drive to land, but for taking off, the reaction drive is necessary, and probably it's needed to reach the stutterwarp threshold.

Hence, why older players get a little confused. The reaction drive is simply unnecessary.

 

* Note: this uses GDW Livres. Mongoose Livres are far less valuable, and pegged to the Imperial Credit.