On 04/04/14 17:01, Keith Henson wrote:
On Thu, Apr 3, 2014 at 9:09 PM, Peter Fairbrother <zenadsl6186@xxxxxxxxx> wrote: snipMore, even if Skylon-plus-lasers just works somehow (and is somehow politically acceptable, big laser weapons being involved), that isn't even close to the cheapest or fastest way to get 500,000 tons per year to orbit.Please go into details. I am not welded to Skylon and lasers. I am well aware of the problems involved. If you have better ideas, I will gladly adopt them.
I rather thought that I had, but: [] The Mission:Looking at 500,000 tons per annum to GEO, in the form of powersat parts, they will have to be assembled in space. Assembly in space is much cheaper and easier when done in LEO rather than in GEO, so we will assemble the parts in LEO.
That means a stopoff in LEO rather than going straight to GTO, which is useful for all sorts of other reasons, like exactly where the assembly takes place (see below), stage recovery, and so on.
Supposing a payload of 7.5 tons to LEO (and 5 tons to GEO), that's 100,000 launches per year or 12 per hour. We need three launch sites each launching every 15 minutes, and six locations in LEO where assembly takes place.
We need to get the payloads to the assembly areas on first orbit, otherwise we will have a huge mess. That means the assembly areas pretty much have to be in equatorial LEO, and the launch sites located on the equator.
[] Hardware:Two stages to LEO is much more sensible than one, or than three or more. One stage is too expensive and technically challenging, with more than 2 stages recovery for reuse becomes very difficult. So, we use two stages.
[] The first stage booster:The first stage doesn't do very much in terms of delta-V - one reason for this is fast turnaround. If the first stage can land where it took off from it can turn around much faster than if it lands elsewhere. If the first stage adds a lot of delta-V then it is going to land several thousand miles away from the launch site, so instead it basically just goes up and down.
Another reason for the performance of the first stage being low is reliability and robustness - we can build it a lot stronger if the dry mass is larger and the required performance is lower. Compared to Skylon, the dry mass of the first stage will be somewhere between 60% and 80% higher.
Yet another reason why the first stage does much less than half the LEO delta-V is that it operates in atmosphere, where engines are less efficient and Isp is lower. The first stage main engines burn LOX and kero, not LOX and LH2 - much cheaper.
The first stage has wings to increase cross-range so it can add a bit of sideways delta-V as well as going up and down while still landing at the place it took off from. This extra delta_V makes the job of the second stage easier and increases the payload.
With wings comes wheels and horizontal takeoff and landing. We add some jet engines so we can do go-arounds, land at alternative airports, and so on, increasing safety and reliability - also the jet engines increase cross range, and give a bit of extra height and delta-V before main rocket engine burn.
The first stage's main rocket engines now start at 30,000 feet, and can be optimised more towards vacuum, making them more efficient.
Summing up, the first stage booster is basically a jet aircraft with rocket engines stuck on it. That means we can have pilots on board too. It has a take-off mass of about 400-460 tons, somewhere between a Boeing 747-400 and an Airbus 380, and a second stage payload of 50 to 60 tons, with a LEO payload of 7.5 to 10 tons.
At nominal flight rate and two-hour turnaround there are eight booster stages operating from each of three airfields, making 24 operating boosters in total. A fleet of around 60 boosters would be needed to sustain this.
[] The second stage:The second stage is powered by a LOX/LH2 engine. It is carried inside the first stage until that reaches 60 km high, so aerodynamic loads are very low.
It consists of two LH2 tanks, an engine/electronics/rcs module and an optional cargo module all attached to a central semi-structural LOX tank. Payload can also be strapped directly to the LOX tank supports without using the cargo module.
One version masses 50 tons when fuelled, has a dry mass of 5 tons and a LEO payload of 7.5 tons at an Isp of 445s. The disassembled stage without cargo module fits into 3 standard shipping containers.
The tanks are reused in orbit or deorbited; the engine modules are returned in batches under a heatshield and parachute for reuse.
Depending on turnaround time, a total of about 5,000 second stage engine modules will be needed.
[] Assembly personnel:There is also an alternative second stage for passengers, which re-enters and lands at a runway. It is launched from the same piloted booster.
[] LEO to GEO:There are several possibilities. If we have power sat parts which can supply power for an electric drive we can do something with them, so that the amount of mass needed to get from LEO to GEO is reduced. Perhaps a small booster stage (or refuel and reuse one of the second stages) to get some initial velocity to clear any near-earth debris cloud and decrease transit time, then use the electric drive, or something like that. I won't mention tethers.. :)
[] Conclusion:If done in a newspace SpaceX-type manner it should cost somewhere around $28 billion in capital over 4 years for hardware, and about $9 billion per annum operating expenses.
Somewhere around $20 per lb. Probably $80 per lb if done in an oldspace manner.
That's about as cheap as I can conceive, for a requirement of 500,000 tons per annum to LEO, with little new technology or risk of failure. I don't think there is any potential new tech which would be cheaper, even if higher-risk.
-- Peter Fairbrother