totm nov 2023 SpaceX Discussion Thread

[Northstar1989]

On 9/30/2017 at 12:59 AM, mikegarrison said:

You know the last time this argument was used? With the Space Shuttle.

The Shuttle was expensive for all sort of reasons that had nothing to do with its size or reusability.  The Shuttle was expensive IN SPITE of its reusability, not bevause of it.  Among other problems:

(1) The SSME's were HIDEOUSLY complex as far as rocket engines go.  No other engine has ever come close to the complexity of the SSME's before, or since.  This made refurbishment of the engines INCREDIBLY expensive.

(2) The Shuttle was built for maximum performance rather than cost-effectiveness.  In the places where wide engineering-margins would have reduced the cost of reusability by requiring less frequent maintenance (*particularly* the engines) the margins were razor-thin.  If the Shuttle had been built with a lower payload-fraction to allow for wider engineering-margins in places such as the engines (which should have also been built for a bit lower target ISP to save on complexity) then the costs of refurbishment would have been much lower...

(3)  The contracts to build and supply parts for the Shuttle were often sweetheart deals with favored contractors (who donated lots of money to DC politicians).  So really, we didn't just get the Shuttle for the price taxpayers paid- we got the Shuttle and a whole lot of political ads and paid staff in election campaigns for that price.  Consider it a bundle deal- a bundle nobody but the politicians really wanted...

(4)  Besides being a moneypot, the Shuttle was also designed as a jobs-program.  Because the Shuttle was designed to employ as many people as possible, it required significantly more manpower than a design meant to be as affordable as possible.  Technically, jobs programs put idle labor to use (the US has considerable un/under-employment), but the people who struggle to find jobs typically aren't talented engineers...  A national street-cleaning program (to employ those with few skills), or expansion of the government's staff of historical site tour-guides (to put former history-majors now working as waiters to better use) would have been a better use of money, if the objective was really to reduce unemployment.

 

In short, comparing a commercial rocket designed to be as cost-effective as posdible and the Shuttle is a fool's errand.  And even if it weren't, the Shuttle could have been MORE affordable if it had traded payload-fraction for wider engineering-margins...

[Northstar1989]

22 hours ago, IncongruousGoat said:

Mine the regolith... for what, exactly? Ions need xenon, or cesium, or other heavy stuff you don't exactly find in lunar regolith. Hypergolics are even worse-they're nitrogen compounds, and you're definitely not going to find nitrogen in the lunar regolith. About the only thing that has any utility as a propellant that you'll find in the lunar regolith is oxygen trapped in oxides and silicates, and even that's no good without an appropriate fuel to burn.

Actually, in some areas the lunar regolith is extremely rich in Aluminum- which can be burned with Oxygen in a Hybrid Rocket (solid fuel+ liquid oxidizer).  Hybrid Rockets are throttleable, unlike SRB's, and can be turned on/off multiple times...

If you wanted to stick with Hypergolics, on the other hand, there's an enormous source of Nitrogen right in LEO- the edge of Earth's atmosphere.  It's entirely possible to collect Nitrogen with a Propulsive Fluid Accumulator, ship it to the Moon, and react it with Oxygen produced locally from regolith to produce N2O4 (the main oxidizer in most Hypergolics mixtures).

Of course, PFA's can also collect Oxygen- so the only real reason you'd have for making N2O4 on tbe Moon would be if you were going to burn it on/around the Moon: say for refueling supply ships before they head back to LEO...

Edited October 1, 2017 by Northstar1989

[Northstar1989]

22 hours ago, sevenperforce said:

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.

The right idea, basically, but there's no need for a tank that large if all you're doing is hauling small comsats to GEO and back- UNLESS you design the tug with super-large engineering margins so that it can remain in orbit for month or or years at a time without needing any refurbishment (such large engineering-margins would increase the dry mass by a lot- requiring larger tanks in order to achieve sufficient mass ratio).

Of course, if you're designing to minimize maintenance, why only equip the tug with one Raptor?  Two Raptors sufficiently close together that either can be thrust-factored through the Center of Mass (so only one engine needs be used at a time) would allow for alternating use of each engine to spread out the wear-and-tear and allow for less frequent servicing...

Not sure if you can do that with vacuum-specialized Raptors: but if your tug is really as oversized as you're suggesting you should still have plenty of Delta-V even with atmospheric variant Raptors...

[tater]

Tug wise, I'm starting to really absorb the BFR kooaid. 

As long as marginal launch costs are low, just fly the beast to GEO with some comanifested sats. The sats can move around GEO a little as needed, they all have props loaded anyway.

If they feel it's useful to send stuff on a GTO from LEO, then perhaps they could make a tug that functions as an upper stage, but has enough dv to then come back to LEO. Put sat where it needs to be, then drop to the standard BFR orbit. Next time a BFR is in the area, it takes the tug home to be mated to another comsat.

[Northstar1989]

22 hours ago, sevenperforce said:

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.

Actually, the Moon has almost unlimited supplies of Oxygen (tied up in lunar aluminum-odide and silicate compounds) and exactly *ONE* useful fuel besides tiny amounts of Hydrogen in water-ice at the polar craters: Aluminum.

When electrolyzed or chemically-separated from the regolith, pure Aluminum will readily burn with Oxygen in a Hybrid Rocket (Oxygen is passed over solid Aluminum, basically, and the throttle is controlled by how quickly you add Oxygen).  It takes some effort to properly pack the Aluminum into the rocket in the first place though- I imagine It's the kind of thing you'd want to develop specialized machinery to do quickly and precisely...

If you can manage it, though, you get a rocket with ISP somewhere between SRB's and most liquid fuels, and fuel-density superior even to Hypergolics.  It's good enough to get you from the Moon to LEO and back, with a sufficiently large rocket- which means you can ship stuff mined and manufactured on the Moon to LEO space stations someday with nothing but propellant produced on the Moon...  (the spacecraft you need would be pretty large compared to liquid-propelled spacecraft, though- so building a Space Elevator or Mass Driver on the Moon someday would really help...)

Edited October 1, 2017 by Northstar1989

[tater]

Lunar ISRU might offset some of the landing costs with purpose built vehicles (these studies were done ages ago when I took a class on lunar bases, I can dig up the abstracts at some point and see what they said).

The bottom line IMO is that if you want to be spacefaring, then stay away from the bottom of gravity wells. Its easier to move something to a safe orbit, and mine it there. Lunar mining just aids access to the Moon, which is circular if nothing really useful can happen at the Moon. 

ISRU (seeing why it's underlined for me, ignore)

Edited October 1, 2017 by tater

[Northstar1989]

22 hours ago, IncongruousGoat said:

Aluminum is manufactured by the Hall-Heróult process, which involves electrolysis of an alumina-cryolite molten salt bath. Have fun pulling that off on the Moon, where power would be at a serious premium. Not to mention the effort required to extract oxygen from oxygen and silicates in the regolith.

That's how It's made on EARTH.  There are alternative processes involving chemical extractions that are less power-intensive (though this does add the complexity of needing to recycle certain chemical cofactors that undergo redox reactions in the process...)

Ultimately, though, *ANY* type of propellant-production ISRU boils down to leveraging electrical power to produce propellants in the end.  Some are more energy-hungry than others, but all that really means is that your propellant production-rate ends up being lower with a given amount of electrical power...

Higher energy demands just means larger solar arrays and battery-banks.  Good thing the BFR is supposed to be able to transport 150 metric tons of payload to the Moon in a single trip at a very reasonable price! (compared to any existing rocket)

22 hours ago, kerbiloid said:

(I know, that Al cannot be extracted chemically, I've mentioned the coke because lunar oxygen is contained in iron oxides, too.)

Actually, Al *can* be extracted chemically.  It requires some pretty harsh chemicals, though.  Fluorine compounds are one of the needed sets of chemicals if I remember correctly...

[tater]

I used to spend a lot of time with space nuclear power people, they always had a big reactor in a nearby crater (no LOS to Moon base) for such power-hungry activity.

Like ISRU. (as I typed ISRU it is not underlined, but it keeps posting that way (for me, anyway).

[Northstar1989]

22 hours ago, sevenperforce said:

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

Yup.  This is more interesting from a "what can a lunar outpost do that's useful" perspective.  The kind of machinery necessary to refuel Aluminum-Oxygen Hybrid Rockets might end up requiring human supervision and maintenance, at least in the early stages... (theoretically, a sufficiently sophisticated robot can do anything a human can- but we're not quite to that point in robotics yet...)

[sevenperforce]

38 minutes ago, Northstar1989 said:

Actually, the Moon has almost unlimited supplies of Oxygen (tied up in lunar aluminum-odide and silicate compounds) and exactly *ONE* useful fuel besides tiny amounts of Hydrogen in water-ice at the polar craters: Aluminum.

When electrolyzed or chemically-separated from the regolith, pure Aluminum will readily burn with Oxygen in a Hybrid Rocket (Oxygen is passed over solid Aluminum, basically, and the throttle is controlled by how quickly you add Oxygen).  It takes some effort to properly pack the Aluminum into the rocket in the first place though- I imagine It's the kind of thing you'd want to develop specialized machinery to do quickly and precisely...

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.

43 minutes ago, tater said:

Tug wise, I'm starting to really absorb the BFR kooaid. 

As long as marginal launch costs are low, just fly the beast to GEO with some comanifested sats. The sats can move around GEO a little as needed, they all have props loaded anyway.

If they feel it's useful to send stuff on a GTO from LEO, then perhaps they could make a tug that functions as an upper stage, but has enough dv to then come back to LEO. Put sat where it needs to be, then drop to the standard BFR orbit. Next time a BFR is in the area, it takes the tug home to be mated to another comsat.

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.

50 minutes ago, Northstar1989 said:

The right idea, basically, but there's no need for a tank that large if all you're doing is hauling small comsats to GEO and back- UNLESS you design the tug with super-large engineering margins so that it can remain in orbit for month or or years at a time without needing any refurbishment (such large engineering-margins would increase the dry mass by a lot- requiring larger tanks in order to achieve sufficient mass ratio).

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? 

Quote

Of course, if you're designing to minimize maintenance, why only equip the tug with one Raptor?  Two Raptors sufficiently close together that either can be thrust-factored through the Center of Mass (so only one engine needs be used at a time) would allow for alternating use of each engine to spread out the wear-and-tear and allow for less frequent servicing...

Not sure if you can do that with vacuum-specialized Raptors: but if your tug is really as oversized as you're suggesting you should still have plenty of Delta-V even with atmospheric variant Raptors...

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.

Edited October 1, 2017 by sevenperforce

[Northstar1989]

16 hours ago, tater said:

Haven't seen tankage numbers on the booster, but propellant costs are, in the world of spacecraft, virtually free.

Now if only SQUAD would get that memo!  I'm sick and tired of designing 100% reusable spaceplanes in stock, sometimes to find out they are actually MORE expensive than expendable rockets (with SRB first stages and optimized ascent-curves) simply because of fuel-costs...

Yes, I know that RealFuels fixes the problem- but it also introduces realistic ISP's and TWR's that are much too powerful for stock, and then I end up using RSS 64K, and then re-entry heating becomes a major problem and all my launches take longer (which has become a real problem lately on my failing laptop...)

Edited October 1, 2017 by Northstar1989

[tater]

1 minute ago, sevenperforce said:

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.

That's the thing, though, the cost of tanker is so small (per launch), that the BFS itself is in effect a tug that can use aerobraking.

 

[Green Baron]

This is funny. Since Elon Musk has started to utter his dream of dying on Mars we have been through the same arguments over and over again, maybe with slight variations of the theme.

But i don't want to participate (yet). I'll do so if there is a rocket to get people there and, what's more important, a somewhat developed idea of how to make a living there.

Question: what's the latest info on FH ? In early June Musk said he wanted to have boosters at the cape "in 2 to three months" and be ready for a launch "a month later". That'll be now. How are things, does anyone know ?

Cheers :-)

[tater]

Another use Musk mentioned, could really be a thing.

BFS cargo goes to GEO with a new sat or sats. Inside, they put an arm, and some system to grapple a few defunct sats inside the bay safely.

Concern would obviously be reentry, right? Hard to really bolt them down. So you don't.

BFS does a burn to drop perigee deep into the atmosphere in prep for aerobraking/reentry from GEO. Open (if it was even closed) cargo bay. Dump defunct sats. Close cargo bay, raise perigee to one appropriate for entry and landing. You've just disposed of a bunch of space junk. They could do this routinely---wonder if anyone (governments) would pay for this? Perhaps space powers pitch in to a fund that pays a bounty for junk deorbited?

2 minutes ago, Green Baron said:

This is funny. Since Elon Musk has started to utter his dream of dying on Mars we have been through the same arguments over and over again, maybe with slight variations of the theme.

But i don't want to participate (yet). I'll do so if there is a rocket to get people there and, what's more important, a somewhat developed idea of how to make a living there.

Question: what's the latest info on FH ? In early June Musk said he wanted to have boosters at the cape "in 2 to three months" and be ready for a launch "a month later". That'll be now. How are things, does anyone know ?

Cheers :-)

They moved another launch on the manifest to 39A, which means SLC-40 is not expected to be done by that launch, pushing FH out a little.

My gut is that while they want to fly FH, they'd be better off delaying it until after Crew Dragon, since any pad failure of FH would set them back about a year on commercial crew.

If I were SpaceX, I'd think about making SLC-40 crew capable before risking 39A.

[Nibb31]

5 minutes ago, Green Baron said:

Question: what's the latest info on FH ? In early June Musk said he wanted to have boosters at the cape "in 2 to three months" and be ready for a launch "a month later". That'll be now. How are things, does anyone know ?

Cheers :-)

Boosters are at the cape. The launch is scheduled for december. Payload unknown.

Edited October 1, 2017 by Nibb31

[Green Baron]

That doesn't sound too determined :-)

There is a payload ? I thought expectations were low that it reaches orbit. Or was that a joke ?

Edited October 1, 2017 by Green Baron

[Nibb31]

3 minutes ago, Green Baron said:

That doesn't sound too determined :-)

There is a payload ? I thought expectations were low that it reaches orbit. Or was that a joke ?

There has to be at least ballast, otherwise your trajectory will be screwed up.

Musk has said several times that he wasn't too confident and that FH has proven much more complicated than they expected. I took that as admission that it was pretty much a dead end.

[tater]

FH requires 39A to be finished for FH (it apparently needs ~2 months work). 39A cannot be worked on until SLC-40 is back in business, so that they can still launch F9 and work out their backlog. So they have to finish SLC-40, then start on finishing 39A.

39A is also the only pad for crew, however. 

 

1 minute ago, Northstar1989 said:

Mere survival is a powerful motivating factor to work.  Most people would rather work hard than die...

Working hard doesn't matter even a little on Mars. You either brought what you needed to live at least until the next resupply (with dire needs expressed to Earth some months before the next window), or you die.

In New England in 1630, yeah, you just work hard. You can build houses, cut firewood, hunt for food, etc. On Mars, none of that happens. If habitats are 3d printed, then there is no "hard work," the printer builds it and you wait. 

 

1 minute ago, Northstar1989 said:

Mars will produce considerably *less* IP than Earth- it's a smaller planet and the ratio of IP to population will be far lower (as a MUCH larger fraction of the population must be dedicated to producing basic necessities).

However even a medium-sized country like Germany supports MILLIONS of scientists and engineers here on Earth.  If the entire PLANET of Mars can only support as many scientists and engineers as the mid-sized NATION of Germany once Mars is self-sufficiency and fully developed, it will still be a worthwhile investment for humanity to colonize Mars in terms of IP production...

Put another way, if you could spend 100-200 Billion USD to add another Germany sized nation to Earth, it would be a worthwhile investment, wouldn't it?

No. You could spend far less money and educate people in areas that are already on Earth, and are not at all like Germany, and try to make them like Germany. Mars is a pipe dream, and any notion of it somehow having any reason aside from "because it's cool to have a multi-planet species" is not realistic, IMO. Musk's goal is fine, as long as there are people willing to go, and other people's money to throw at it, but I don't pretend that the economic math works out.

[sevenperforce]

15 minutes ago, tater said:

That's the thing, though, the cost of tanker is so small (per launch), that the BFS itself is in effect a tug that can use aerobraking.

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.

25 minutes ago, tater said:

Another use Musk mentioned, could really be a thing.

BFS cargo goes to GEO with a new sat or sats. Inside, they put an arm, and some system to grapple a few defunct sats inside the bay safely.

Concern would obviously be reentry, right? Hard to really bolt them down. So you don't.

BFS does a burn to drop perigee deep into the atmosphere in prep for aerobraking/reentry from GEO. Open (if it was even closed) cargo bay. Dump defunct sats. Close cargo bay, raise perigee to one appropriate for entry and landing. You've just disposed of a bunch of space junk. They could do this routinely---wonder if anyone (governments) would pay for this? Perhaps space powers pitch in to a fund that pays a bounty for junk deorbited?

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.

[kerbiloid]

(Many letters, sorry)

Electrolysis.
The electrolysis process widely used today, according to the wiki, requires per 1000 kg of raw aluminium:

• 1920 kg of alumina
• 65 kg of cryoilte
• 35 kg of aluminium fluoride
• 600 kg of graphite electrodes
• 61 GJ of energy

sqrt(61 GJ / 1000 kg * 2) ~= 11000 m/s

So, the aluminium extraction from the oxide requires as much energy as throwing this aluminium from the Earth.

Ok, let's forget about cryolite and fluoride, let them be refurbished for free.
Alumina is available everywhere on the Moon, let's presume it's pure.
But 600 kg of graphite electrodes should be restored partially from carbon oxides, partially from the polluted remains of the electrodes.
So, probably we should add some energy to electrolyzie 600 * 44/12 = 2000 kg of carbon dioxide (say, we just burn the remains of the electrodes and recycle CO₂).

Also, alumina should be mined, separated and crushed before melting. Additional MJs of energy.

Looks like melting out the aluminium from the regolith requires as many energy as delivering it from the Earth.
So, aluminium rocket on the Moon makes no sense in terms of energy.

Alternative:
Wiki provides a link to the alternative aluminum extraction process description: carbothermic technology.
Though, this process (conversion of the aluminium oxide into aluminium carbide and heating the latter up to 2000°C) also requires a lot of energy.

 

Oxygen can be extracted from the lunar iron oxide, too.
As far i can read, this requires 2000 KW*h/t of iron. So, 2*10⁶ * 3600 / 1000 ~= 7 MW / kg of iron.
With gathering/crushing/separation and also with carbon electrodes recycling we get that it's probably cheaper to deliver the oxygen from the Earth, too.

 

So, the regolith disintegration (and oxygen/metals production) makes sense in 2 cases:

• Far from the Earth (say, Mars or Phobos) and wanting a lot of metals (so, just H₂O/CO₂ splitting is not an option).
• Big Dumb Reactor giving a lot of pure dumb energy, when nobody cares about it.

Say, lunar regolith contains up to 1 g of ³He per 100 t of regolith.
According to wiki, 1 g of ³He (with 0.67 g of D) gives about 15*10⁶ * 10³ * 40*10⁶ * 10^-6 = 6*10¹¹ J.
< 6 * 10¹¹ / 60*10⁹ = 600 t of aluminium can be extracted with this energy.
(Much less than 600 t for the reasons listed above).
This means that ³He from 100 t of regolith can extract oxygen and metals from ~1000 t of regolith.
So, if you have a Big Dumb Harvester/Powerplant/Melter, all the famous ³He will be spent to extract oxygen and metals from the regolith gathered to extract that helium.
(Btw nothing is left to export to the Earth, forget it.)

This makes me think that main lunar colony will be forever named Donation City.

Edited October 1, 2017 by kerbiloid

[sevenperforce]

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?

[regex]

2 minutes ago, sevenperforce said:

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?

It's been done on a few missions IRL.

https://en.m.wikipedia.org/wiki/Aerobraking

Aerocapture is another thing entirely but if you're at orbital velocities, and thus aerobraking, then it's a viable strategy for many missions.

[sevenperforce]

10 minutes ago, regex said:

It's been done on a few missions IRL.

https://en.m.wikipedia.org/wiki/Aerobraking

Aerocapture is another thing entirely but if you're at orbital velocities, and thus aerobraking, then it's a viable strategy for many missions.

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.

Edited October 1, 2017 by sevenperforce

[magnemoe]

Back to more "reasonable" stuff, one issue with BFR is that its serious overkill for most satellite launches.
Well why don't use the passenger version, fill it with tourists, launch it deploy the satellite, if high orbit use an tug to take it to designated orbit and return to BFR for reuse, you would want to give the tourists some days in orbit anyway. An polar orbit would be more tourist friendly. 
Or you could set it up as an lab for various experiments as long as they don't take a very long time for that you want an station. 

[sevenperforce]

I see tugs functioning similarly to the ASDSes....