sevenperforce

Seems very feasible. Aerocapture no, but aerobraking yes. The more I think about it, the utility of dedicated Raptor-based tugs seems paramount. You could have multi-manifested missions, not just with respect to multiple comsats, but multiple destinations. Send crew and cargo up toward the ISS, pause to meet with a tug to hand off a 9-tonne comsat, then keep going. Or you could send supplies to a commercial/tourist station on the tug, or send something to lunar orbit. One problem is that the demand for comsats may not be able to keep up with the launch supply.

In KSP, you can basically circularize virtually anything by successive aerobraking passes if you're patient enough, just by dipping only barely into the top of the atmosphere with each pass. Even if you have a bunch of solar panels. Does anyone know if it's worse or better IRL? Would there be any significant wear on an unprotected tug by sending it through the atmosphere with a perigee of 95 km or so a few dozen times?

In order to maximize profits, though, you do want to service as many paying customers as possible per launch. A GTO injection costs about 2.1 km/s. Looking at the dV/payload curve that Musk debuted in his presentation, a reusable non-refueled BFR can take 150 tonnes to LEO, but only around 25-30 tonnes if you need to reserve 2+ km/s in dV for the GTO injection. However, if it executes a rendezvous with a waiting tug, transfers out the fuel, and allows the tug to take the payload out to GTO, it could likely loft 50-90 tonnes to GTO in a single launch. Don't have time to do the math right now (based on estimated Raptor dry mass and the tankage ratio of composites) but it's pretty straightforward. That 85 tonnes of BFR dry mass is a killer if you're trying to go beyond LEO. Or you could put an arm on the tug itself, both to make payload deployment easier, and to aid in this sort of thing. The tug could easily grapple defunct satellites, put itself on an aerobraking trajectory, jettison the sats, and raise its perigee.

Indeed. It would definitely be ironic if some little startup bought space on the BFR, landed a set of autonomous processing systems, and in a few years had a massive lunar ISRU enterprise competing with BFR for BLEO payloads. Of course the whole BFR itself can fly to GTO or GEO and return, easily enough. But it is much more efficient to use a separate tug, since you aren't lofting all that unnecessary dry mass. A tug would be most efficient if it could recircularize using high-atmospheric aerobraking passes. The reason for a large tank is flexibility. In most cases, you can transfer excess fuel directly from the launch BFR to the tug...precisely as much as you need for that particular mission. However, with larger tanks (which don't hurt you as far as dry mass is concerned; tanks are pretty lightweight), you can build up excess propellant in your tug and occasionally take one REALLY large payload much farther than the BFR can. You know, like a ginormous space telescope, or something. You could also use it to take a smaller payload into lunar orbit and return with enough margin for a braking burn. Finally, the tug is a de facto propellant depot and can top off BLEO BFR missions when you need a little extra propellant but not so much as to require an entire additional tanker launch. Speaking of the tanker, Musk didn't seem to have any renders of the inside of the tanker. Does it have larger tanks? Is the whole top just an empty aeroshell? Does the cargo variant, sent up without payload, double as a tanker? Well, if you want to be able to use either engine, you can design them already angled inward toward the CoM...though changes in CoM depending on fuel load and payload mass/size might make it challenging. But the idea was to save on dry mass, so better to have multiple tugs in LEO than to put an extra engine on each tug. Plus, the Raptor Vac is a bit large to fit two of them like that.

The acceleration is needed because pressing with nitrogen or any other gas doesn't necessarily force the liquid in the desired direction in microgravity.

Shocked no one has done this yet...but glad, because I wanted to be the one to do it. I'm sure most of us have seen Elon Musk's 2017 IAC presentation, which raises the bar (even though that "bar" is somewhat smaller) tremendously over the 2016 version of BFR/ITS/MCT. If anyone hasn't had a chance to see it, I highly recommend checking it out: What's the challenge? Build the 2017 version of the BFR, of course. Specifically: Build a large two-stage fully-reusable launch system capable of propulsive landings on Duna, Kerbin, and the Mun. It must be capable of in-orbit propellant transfer and needs to be able to deliver substantial payload (at least 20 kerbals, or a fully-functioning autonomous ISRU unit) to Duna. No nukes, ions, jets, rolling landings, or parachutes. Tweakscale is the only allowed part mod for engines and tanks. Once you've demonstrated proof of concept with a single launch, subsequent launches (for refueling, etc.) may use the debug menu to get into orbit. You can also use Unbreakable Joints and No Crash Damage as long as your landings are under 5 m/s. The scoring system is designed so that the closer you make your version to the one revealed by Musk, the more points you get. I may add additional ways of earning points as the challenge progresses, but I'll try to keep it balanced. Scoring: Basic challenge (two-stage, reusable, propulsive landings on Duna, Kerbin, and the Mun): 5,000 points Booster executes RTLS: 200 points Dedicated propellant transfer ports in tail: 50 points Single-stage to Mun and back to Kerbin after Kerbin orbit refueling: 600 points Demonstrate single-stage return from Duna to Kerbin after Duna ISRU: 800 points Six engines on second stage: 60 points Thirty-one engines on first stage: 31 points No reaction wheels: 200 points No monoprop (Vernor engine RCS only): 175 points Booster lands in launch cradle: 650 points Delta wings on second stage: 85 points Dedicated crew vehicle, cargo vehicle, tanker: 400 points Ties are broken by total upper stage dry mass, the lighter, the better.

Presumably there is redundancy on the refilling pipes (four are shown), so it could merely dock to one side or the other. Definitely not a full BFR load to GTO. More like 40-50 tonnes to GTO or 10-20 tonnes to GEO. It wouldn't need a tanker, either; it could refuel from the same cargo BFR that brought up the comsats.

Not undoable, perhaps, but definitely outside the realm of autonomous, readily-reusable liquid-fueled rockets with regular propellant transfer.

Really no fuel on the moon. Much better to do a dedicated single-engine tug than to redesign the cargo BFR as a tug. You REALLY don't need that much thrust for sending comsats to GTO. Being able to choose whether you put comsats in GTO or GEO is big, too. Pay a little extra for the extra tug-fuel to circularize your big comsat in GEO, and you don't have to worry about self-circularizing. The tug burns retrograde, aerobrakes down to LEO, and is none the worse for wear.

Absolutely. I would say to make the tug quite large -- just large enough that it can fit inside the BFR on a nominal launch. No need for ions; just give it a single Raptor Vacuum and large propellant tanks. Send it up ahead of time. Instead of selling single launches and sometimes dual-manifesting or having secondary payloads, SpaceX would begin to sell "slots" on group GTO launches. Schedule well in advance. Cargo BFR would take several (up to 10 or 15, perhaps) large comsats up to LEO at once, then rendezvous with the tug. The tug takes the comsats up individually or one-at-a-time, depending on mission requirements, then uses upper-atmosphere aerobraking passes to come back to LEO, either to take the next comsat up, or to wait for the next mission. The tug can refuel from the cargo BFR each time they rendezvous, since the BFR will virtually always have excess propellant for comsat missions. After several dozen missions, the tug can simply rendezvous with the BFR one last time, dock inside the payload bay, and return to Earth for servicing and refurbishment. It works because a big tank and a single Vac Raptor can take comsats MUCH further with a given amount of fuel than the BFR cargo can, simply because it doesn't have nearly as much dry mass to contend with.

On the subject of landing on the Moon -- recall that one of the first things Elon talked about was engine redundancy on landing. The two landing engines are close enough together that each can gimbal its thrust through the CoM, enabling single-engine landings if the other engine fails. A single SL raptor, downthrottled all the way, should be able to hover on the Moon with around 30-50% propellant loading. I'm sure some people will feel that way and some people won't.

The economics of the Apollo program and Sputnik (and current space exploration) worked because the idea was worth something to the people who paid for it.

I'm still not sold that the booster is required. You only need about 4000-5000 m/s for most suborbital spaceflight hops. Surely the upper stage alone can manage that if you swap out the vacuum raptors for SL ones. Potential trouble is that the optimal trajectory has an apogee of around 500 km...

Earth is an oddball because of the collision with Theia that formed our moon. The distribution of heavy metals and crust and core is all wonky. Mars lost its magnetic field because convection in its core died. Our core is vastly heavier and will continue to do the dynamo for many millions of years. Well, it DOES have to be economically feasible, even if the economics just means selling an idea.

Agreed. We do not currently see people settling in large numbers in Antarctica. Antarctica is a much more reachable and hospitable place than Mars, and it has far more resources.

The whole point of his example was that you can't buy used rockets. A new 172 will run you $250-300K, but if you actually want to be able to have meaningful range, you'd probably want to pick up a Citation or a Citation II. A new Citation will run you around $35M. For that price, you can charter any airliner for a round-trip between any two airports in the world. Much better than buying a new Cessna Citation and crashing it into the tarmac at the end of the trip.

Cost/ton wouldnt see the Falcon 1 on the top of the chart, though. Precisely. Falcon 9 beats the heck out of Falcon 1 for cost/ton. Falcon Heavy moreso. Chartering a 777 across the Pacific for a single passenger is cheaper than buying a Cessna, flying it to the end of its range, and crashing it nose-first into the tarmac.

Still wanna do it; just haven't had a chance.

To get Elon Style, you just have to get the ship back to Kerbin's surface. Don't have to go back to orbit again. Elon Style is definitely compatible with Brute Force, even though I didn't originally expect anyone to pull that off. @Kergarin managed it. Consistency requires the same LV design, not the same physical vehicle.

This is precisely the mission architecture I had wanted to see. The actual docking and propellant transfer could take place near perigee but the idea is to be almost all the way to the moon first, then do the fuel transfer. The tanker can come back and aerocapture without expending any more propellant. If they don't have good enough margins for this, they can put both the tanker and the mission vehicle in a lunar free-return, then transfer propellant. The tanker can loop around the moon, come back, and aerocapture; the mission vehicle will have enough to land and return. Obviously that's what I meant. I don't understand why they would keep the booster at all. Swap out the four Vac raptors for SL ones and you could do the suborbital hop with the Spaceship alone. I have some concern about the cargo version. What about servicing smaller launches and GTO stuff? Are we going to see an orbital tug? It's a huge waste to put a BFR into orbit just to send up a 6-tonne GTO comsat.

If we assume that we have a bunch of space stations and orbiting colonies and propellant depots and transatmospheric ferries (i.e., commercial transport services to get passengers, cargo, or both down to the surface of a body and back again), then sailing from body to body might be a realizable leisure activity. Might even have some pretty nifty designs...like, a high-thrust engine for initial transfer injection, a high-impulse engine to provide constant low acceleration to decrease transit times, and a heat shield for aerocapture at the destination. Sporty.

It wouldn't be at the perigee. At MECO, the Shuttle stack was climbing in a technically-complete orbit with a fairly high apogee and a perigee of about 0 km, plus or minus 15 km. OMS circularization happened close to apogee less than half an orbit later.

What can I say, I like challenges!

Never. Atlas V variants that have flown, in order of the number of launches, are 401, 551, 501, 421, 541, 411, 531, 431, and 521. In theory, they offer a two-engine Centaur for all the other configurations, but none have flown. The "N22" intended for the Starliner is essentially a 422 variant but with the capsule and LES instead of a four-meter fairing.

If I recall correctly, the circularization burn with the OMS system was quite some time after the ET jettison. Using the SSMEs for full orbital insertion and circularization would have required a different flight profile. What was the minimum throttle range of the SSMEs? Since they couldn't be restarted in-flight, you'd have to figure out a way to keep them burning all the way through circularization...but cutting off two of the engines and dropping the remaining one to minimum throttle might still have burned through all the propellant before the proper circularization height was reached.