happyjack27 wrote:to get a transport up to orbit, you need more engines on it, and that means you're going to need more fuel. by the time you've done all that, with enough extra thrust to carry a payload, what you have is a tanker, that can only get to leo, which is just as well because its poorly optimized for interplanetary - its got way too many engines.
I think you were misunderstanding me. I was not talking about using the proposed SSTO for interplanetary transport. Let me try to explain again:
The full BFR- ITS stack (of booster and ITS) would be overkill for anything but colonization or large scale BEO missions. You rarely need to launch hundreds of tons into a single LEO and that mass to the moon only makes sense if you plan to have a colony there. The ITS in its current configuration really is only good to transport 100 people or the equivalent in cargo (most of it pressurized too) or as a self transporting outpost. If you landed it on the moon, you would have an instant moon village there. But the technology could be adapted to have a launch vehicle with less payload, but less maintenance and a higher launch cadence.
The tanker can launch on its own in its current configuration
all the way to orbit, but without any payload. Not even in this configuration would you need to add extra engines
for it to work as an SSTO.
But in its configuration as a second stage it is badly optimized to be an SSTO because most of its engines are for vacuum use only. Replace them with engines that are half way between LEO and vac and thus can be used right from takeoff and you have a much more capable LV. The smaller engine bells will probably safe some weight too which would directly translate into more payload. Then add a payload shroud instead of the tanker tank and the result is an extremely cheap (less than 2.4 million per launch), low maintenance SSTO that can have a payload of roughly Falcon 1.1 (or more, some estimates go up to 40 tonnes) to LEO.
For anything beyond LEO, you would probably assemble a dedicated spacecraft in orbit or do on orbit refueling for some mission
profiles like to GEO or maybe even to lunar orbit. The point is that you now have options.
okay it looks to me like we are pretty much in agreement. my point was that SSTO is only viable for LEO. which you seem to agree on.
however, as you mention, a transport, in it's current form can only carry itself to LEO - no payload. and that's after modifying its thrusters to high-thrust sea-level optimized, which is going to reduce their isp substantially. (from 382 to 334), thus reducing your total delta-v to 334/382 or about 87%. This is assuming that gives you enough thrust without having to add more engines, thus hurting you delta-v even more. at the end of the day, the transport will NOT be able to reach LEO, even with no payload. the original delta-v was only theoretically -- it was at vacuum pressure and no air resistance.
might be better off starting with a booster and scaling it down (scaling down engines, dry mass, and fuel at the same rate) - except for its lack of heat shield. in any case you still have no payload, which makes the mission moot.
if you want payload, a fully loaded transport (450 ton payload) barely makes it to LEO with the help of a booster.
fully loading it maximizes the return per launch, so you always want to fully load it when you can.
which means you always want to use the booster.your best option is still two stages.
A viable SSTO would need a dramatic improvement in ISP. this means air-breathing engines. Such as SABRE. https://en.wikipedia.org/wiki/SABRE_(rocket_engine
) A sabre engine gives you about 10x the ISP in air-breathing mode. That means 10x the delta-v for the same mass ratio.
And bear in mind, once it's out of atmosphere, you don't need high thrust anymore - only enough to reach orbit before reaching the periapsis. (and preferably before the apiopsis). so you can dramatically cut down on the engine mass you need to carry.
Add this to the fact that your rocket engines don't add any more mass at all! They are your jet engines! Now on a two stage it may have made sense to drop you jet engines at this point to save mass. But now they _are_the second stage engines. Your jet engines were never carrying the mass of your rocket engines - they _were_ your rocket engines.
so you got an optimized craft up to the height of where the second stage would normally start (vaccum thrusters, low thrust), without having to drop anything. indeed, all you had to do was carry a little more kerosene (as in like 1/10th the amount you would normally need) (or methane, if you made methalox version), and have a slightly larger tank. That's WAY cheaper than a booster, even a fully reusable one.
but that small cost isn't done being amortized - the air-breathing mode of sabre engines that you had no good reason to drop will come in handy again on re-entry. you can use these to fly to any landing site.