Tech Tuesday 06/18/19 – “Is Plastic-Man Biodegradable?” Edition

George Turner in reply to DensityDuck says:

The analysis stemmed from a post about the likely early configuration of Starship/Superheavy, which will likely only have about 150% of the payload capability of Falcon 9 Heavy. Musk could get a bigger increase (163%, by his calculation) by just adding two more side boosters to F9H.

So that brought up a discussion that the Superheavy architecture was designed for in-orbit refueling. Well, given the early guesses as to the design, the early architecture is supposed to have a LEO payload capacity of 100,000 kg, and the ship has a dry weight of 87,000 kg. Landing fuel requirements probably set the minimum orbit mass at 103,000 kg.

So the assumption was that a Starship, fully fueled in LEO, could fly a lunar mission. I ran the numbers and found it could deliver -7,400 kg to the lunar surface, which is slightly less than nothing. But someone pointed out that Musk said it might have to be refueled in a high elliptical orbit for such a mission. That made sense, so I ran the numbers for a common elliptical orbit, a GTO (which is the insertion half of putting a geostationary satellite in position, before the satellite does a circularization burn).

The Starship’s Raptor engines have a specific impulse is 380 seconds, and that means the required mass ratio to insert into a geosynchronus transfer orbit is about two, which is nearly the same as the attainable mass ratio of Starship in the simple, straight-from-launch, low-earth orbit configuration. Oddly enough, despite its truly massive size, its high dry mass and return and landing requirements gives the Starship less GTO payload capacity than a Falcon 9. It would be almost useless for launching geostationary satellites.

That also means that Starship/Superheavy can’t refuel a Starship in GTO because they can’t deliver any useful amount of fuel (50 to 100 refueling flights just to top up one ship).

The only way around that fundamental problem (caused by hitting LEO with a high dry weight and low mass ratio), is to use a much bigger booster. The simplest way to do that is to use three instead of one, which is something SpaceX already does with Falcon Heavy.

This leads me to believe that the early configuration of Starship is basically a bigger LEO Space Shuttle (Four times more payload capacity), and the deep space capability will be added to the booster (Super Heavy), not to Starship.

Strangely enough, although landing Starship on the moon on its tail is pretty daft, the surface payload capability is almost exactly the same as if it parked in low lunar orbit and discharged a huge, one-way hypergolic lander with an ISP of 310 seconds and a cargo to landed-weight ratio of 75%.

There are of course some serious stumbling blocks to refueling in GTO. For one, nobody has really ever refueled anything in space yet, much less multiple times. For another, sending the crewed Starship to GTO first would mean the astronauts would be zipping all the way through the Van Allen Belt every five hours while waiting for multiple refueling rendezvous that might be days or weeks apart. Flying through the Van Allen belt twice exposed Apollo crews to a pretty big dose of radiation, so much that they saw spots as their optic nerves fired off. So it’s certainly not a mission plan, just a potential capability.

But what it points out is how obsolete NASA Artemis architecture is. It would take nine to twelve Superheavies (three boosters in each launch) to launch three or four Starships to execute one lunar mission that delivers 255,000 kg of cargo to the lunar surface. That mission would deliver more payload than about twenty SLS Orion missions, so each Superheavy booster is accomplishing about twice the useful work (useful pounds to the moon) as the SLS. More importantly, none of those boosters, nor a Starship, nor anything else gets thrown away. They can all be used over and over again, so the only thing being expended is fuel.

In contrast, each SLS mission throws away the incredibly expensive core booster that takes 4.5 years to build, two solid rocket boosters, an upper stage that isn’t even flying yet, but which will likely use four expensive RL10 engines, and of course an Orion (which isn’t reusable) and at least a lander descent stage. The cost estimate for launching just a basic SLS Orion mission is somewhere between $1.5 and $2.5 billion dollars. A lunar landing mission might hit $3 to $6 billion, and those are recurring costs for each mission.

The Starship approach might have a 50 or 100 to 1 cost advantage per pound delivered.

One approach might conceivably lead to commercial lunar development, and even permanent settlement. The other might put twelve more people on the moon before being abandoned due to high cost and low returns.

But Bridenstein is boxed in by all the existing bureaucratic and technical structures of NASA human space flight program, including the Congressional mandates, and there’s really no good way to break out of it. If Starship or Bezo’s New Glenn and New Armstrong start flying, we’d be much better off with NASA attaching themselves to those two programs, providing massive expertise, personnel, and mission support than trying to run a program that by its nature can’t really get past an exploration phase.Report