Author Topic: BFS - without BFR eats the launch market.  (Read 76924 times)

Offline Elmar Moelzer

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Re: BFS - without BFR eats the launch market.
« Reply #180 on: 04/13/2018 03:28 am »
And here is another indicator for you, that my estimate is not too far off. The 2016 IAC presentation had a lot more detail about the various masses. In it, the BFS dry mass was 150 tons and the tanker dry mass 90 tons. So the tanker has exactly 60% of the dry mass of the BFS. Applying that to the current BFS with 85 tons dry, I get 51 tons dry mass, which is almost exactly what I had calculated for the cargo version earlier.

http://spaceflight101.com/spx/wp-content/uploads/sites/113/2016/09/ITS-022.jpg

Offline envy887

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Re: BFS - without BFR eats the launch market.
« Reply #181 on: 04/13/2018 04:02 am »
And here is another indicator for you, that my estimate is not too far off. The 2016 IAC presentation had a lot more detail about the various masses. In it, the BFS dry mass was 150 tons and the tanker dry mass 90 tons. So the tanker has exactly 60% of the dry mass of the BFS. Applying that to the current BFS with 85 tons dry, I get 51 tons dry mass, which is almost exactly what I had calculated for the cargo version earlier.

http://spaceflight101.com/spx/wp-content/uploads/sites/113/2016/09/ITS-022.jpg

Note that I made the same observation above... :D

A cargo vehicle would be slightly heavier since it adds a cargo door with associated structure and hardware, and a payload adapter. That should add less than 10% though.

Offline Elmar Moelzer

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Re: BFS - without BFR eats the launch market.
« Reply #182 on: 04/13/2018 05:20 am »
And here is another indicator for you, that my estimate is not too far off. The 2016 IAC presentation had a lot more detail about the various masses. In it, the BFS dry mass was 150 tons and the tanker dry mass 90 tons. So the tanker has exactly 60% of the dry mass of the BFS. Applying that to the current BFS with 85 tons dry, I get 51 tons dry mass, which is almost exactly what I had calculated for the cargo version earlier.

http://spaceflight101.com/spx/wp-content/uploads/sites/113/2016/09/ITS-022.jpg

Note that I made the same observation above... :D

A cargo vehicle would be slightly heavier since it adds a cargo door with associated structure and hardware, and a payload adapter. That should add less than 10% though.
Sorry, must have missed that :(
I would assume that the cargo bay on the SSTO would be relatively small, since it would only have a few tons of payload. Given that the entire leg assembly on F9 weighs 2.5 tons and the current F9 fairing weighs 1.7 tons. I would say it is safe to assume that the cargo section including door mechanism would be less than one ton. That is, if the cargo version and the tanker version in the original design were not already one and the same, which IIRC was not quite clear.
Either way, as you say, we are still well below 55 tons.
Also, not sure of the original tanker was meant to have legs or not. If it was, there would be about 2.5 tons worth of weight to shave off, if the SSTO could do a cradle landing.
« Last Edit: 04/13/2018 05:21 am by Elmar Moelzer »

Offline rakaydos

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Re: BFS - without BFR eats the launch market.
« Reply #183 on: 04/13/2018 04:29 pm »
So, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)

Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?

Offline Elmar Moelzer

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Re: BFS - without BFR eats the launch market.
« Reply #184 on: 04/13/2018 06:58 pm »
So, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)

Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?
I would assume that you can go straight to full throttle since there are many engines that can't throttle lower than 50%?

Offline envy887

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Re: BFS - without BFR eats the launch market.
« Reply #185 on: 04/13/2018 07:08 pm »
So, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)

Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?
I would assume that you can go straight to full throttle since there are many engines that can't throttle lower than 50%?

Even at maximum spin-up rate it will probably take 2-3 seconds to go from start to 100%.

However, the RVacs have full regen nozzles, which are more like the SSME nozzles and much stiffer than the superthin MVac radiatively cooled nozzles. I doubt they need much if any stiffening.

Online eriblo

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Re: BFS - without BFR eats the launch market.
« Reply #186 on: 04/14/2018 12:46 pm »
So, the Vac Raptors have an expansion ratio low enough to be run at about half throttle without flow separation. (250 bar / 120:1 aspect ratio= a bit over 2 bar)

Which is great for them, but you cant instantly go from zero to full throttle. Is it possible to have disposable stiffeners to dampen flow separation instabilities for throttleup (just a few seconds), like you see fall off the MerlinVac just after stage separation?
Just a correction: scaling the pressure by the area ratio ignores everything about how rocket nozzles work. Think about it - for an ideal gas it would require expansion without a change in velocity or temperature (so that the pressure scales as the volume which scales as the cross sectional area...).
In reality the velocity increases (the whole point of the nozzle) which increases the volume of a "gas package" beyond the area increase and the temperature drops (providing the work), both of which will decrease the pressure.

Others have done more detailed analysis but I just tested the trial version of RPA for the first time with the simplest possible input to provide a number. If I did it right:
Methalox at 3.6 O/F, 250 bar Pc and 120 ER gives an vacuum ISP of 376, a nozzle diameter of 2.42 m at 1900 kN and an exit pressure of 0.16 atm. RPA suggest this is just below the flow separation limit but I guess it would depend on the detailed nozzle design.

Offline RobLynn

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Re: BFS - without BFR eats the launch market.
« Reply #187 on: 05/04/2018 06:33 am »
If they are serious about the point to point market then a gas-turbine based booster stage may start to look tempting.  Can probably boost to Mach 1-2 at 20000m, allow high Isp engines on the BFS, save perhaps up to $100k of fuel per flight, and use engines that are quieter and potentially Ok for many 1000's of flights.  Downsides: developments costs, high dynamic pressure staging, and probably a billion dollars worth of Jet Engines to launch the BFS.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline speedevil

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Re: BFS - without BFR eats the launch market.
« Reply #188 on: 05/04/2018 10:07 am »
If they are serious about the point to point market then a gas-turbine based booster stage may start to look tempting.  Can probably boost to Mach 1-2 at 20000m, allow high Isp engines on the BFS, save perhaps up to $100k of fuel per flight, and use engines that are quieter and potentially Ok for many 1000's of flights.  Downsides: developments costs, high dynamic pressure staging, and probably a billion dollars worth of Jet Engines to launch the BFS.
A billion dollars that break even at 10000 flights may not look very good.
It's still going to be quite loud - though perhaps not as loud.

You can possibly shave some off your costs by using non premium, perhaps even used jet engines, reworked with large afterburners, rather than Boeings latest wonder, especially as you really, really want>M1, which those engines won't do.

Offline RonM

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Re: BFS - without BFR eats the launch market.
« Reply #189 on: 05/04/2018 01:42 pm »
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.

Offline Lars-J

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Re: BFS - without BFR eats the launch market.
« Reply #190 on: 05/04/2018 04:32 pm »
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.

And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.

Offline RobLynn

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Re: BFS - without BFR eats the launch market.
« Reply #191 on: 05/05/2018 02:25 am »
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.

And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.

F135-PW-100 engine has afterburning T/W of about 11, expected to increase about 10% soon.  For a low deltaV booster that is more than good enough.  Thrust can be (relatively easily) augmented with water/LOX mass injection MIPCC, which can maintain thrust and good Isp for up to about Mach 4 and 30km altitude with Isp in 1500-2000s range http://www.ssdl.gatech.edu/sites/default/files/papers/mastersProjects/YoungD-8900.pdf

I don't expect SpaceX to persue this.  But it does have more potential for a high flight rate application like point-to-point ballistic transport than it ever did for low flight rate space-launch.
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline RonM

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Re: BFS - without BFR eats the launch market.
« Reply #192 on: 05/05/2018 04:54 am »
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.

And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.

F135-PW-100 engine has afterburning T/W of about 11, expected to increase about 10% soon.  For a low deltaV booster that is more than good enough.  Thrust can be (relatively easily) augmented with water/LOX mass injection MIPCC, which can maintain thrust and good Isp for up to about Mach 4 and 30km altitude with Isp in 1500-2000s range http://www.ssdl.gatech.edu/sites/default/files/papers/mastersProjects/YoungD-8900.pdf

I don't expect SpaceX to persue this.  But it does have more potential for a high flight rate application like point-to-point ballistic transport than it ever did for low flight rate space-launch.

MECO for F9 is at 162 seconds and about 75 km altitude. As you noted, the F135-PW-100 with MIPCC is good up to 30 km. Either it would have to be a separate booster or it would be dead weight after 30 km.

Offline hkultala

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Re: BFS - without BFR eats the launch market.
« Reply #193 on: 05/05/2018 06:18 am »
There's a good reason why you don't see jet engines on boosters. Jet engines will flame out long before MECO and become dead weight. Doesn't work for vertical takeoff. Rockets don't spend enough time in the lower atmosphere to make jet engines worthwhile.

And their thrust to weight ratio is anemic. Putting jet engines as boosters on a VTOL rocket makes as much sense as using propellers as additional thrust to help a 747 lift off.

F135-PW-100 engine has afterburning T/W of about 11, expected to increase about 10% soon.
  For a low deltaV booster that is more than good enough.

It's still anemic. And very expensive.

Lets compare to F9, on a LEO mission.

As the staging happens earlier, the second stage has to be bigger.
F9SS + 20-tonne payload is about 130 tonnes. Lets say we need 28 tonnes more fuel and 1 tonne more tank mass in the second stage due to the earlier staging, and another 1-tonne engine to counter the much worse gravity losses caused by the earlier staging and more fuel(*). Now we have a 160-tonne second stage+payload.

(*) I was originally considering just only extending the tankage, not engine count, but then I understood that for with some 0.7 km/s earlier staging, the gravity losses would become so terrible, that there would not any any tank size where the craft would reach orbit with some 20-tonne payload.


Stupid, way too optimistic calculation:

Lets say we have 10 tonnes of structural and tank mass in our first stage, and assume we need T/W of 1.1 for safe liftoff. Lets ignore the fuel weight for now.
To have this T/W of at least 1.1 , we need 11 F135 engines.
With 11 engines we have thrust of 210 tonnes and engine weight of ~19 tonnes, for total weight of 189 tonnes.
These 11 engines cost total of 143 millions.

With those "upgraded" versions of F135, we still need 10 engines, total weight of 177 tonnes, cost of 130 millions.

F9 first stage is quite many times cheaper than this 143 or 130 millions, and allows using much cheaper second stage.

Better, more correct calculation:

But actually, the situation is MUCH WORSE. Because the craft would have to reach that staging speed before rising to too high altitude and flameouting. We need much higher T/W than 1.1.

45 degrees might be close to a good staging attitude. Lets assume staging velocity of 3000 km/s, or 833 m/s. The vertical component of this is 583 m/s.

Lets make a simple calculation assuming horizontal acceleration is free, and concentrate only in the vertical part. T/W of 1.55 gives 0.55g acceleration. This 0.55g acceleration means that in 105 seconds, velocity is 577 m/s and
altitude is 30km. Close enought. But during the same time we also need to gain same ~580 m/s of horizontal acceleration. which increases the required T/W slightly, mayby to about 1.65

This means that instead of 11 engines, we need about 17 engines (or 16 of those upgraded models).

Those 16 or 17 F135 engines cost total of 208 or 221 millions. This is starting to get VERY expensive.

And with 4000km/h instead of 3000km/h staging velocity, much higher acceleration would be required, increasing the required thrust and engine count considerably

Quote
Thrust can be (relatively easily) augmented with water/LOX mass injection MIPCC, which can maintain thrust and good Isp for up to about Mach 4 and 30km altitude with Isp in 1500-2000s range http://www.ssdl.gatech.edu/sites/default/files/papers/mastersProjects/YoungD-8900.pdf

Calling things that have never been done "relatively easy" is not very good argument.

Possible, yes. Easy - we do not know all the practical problems when actually trying to build and fly these things and speeds no jet has ever flown.

And at least atmospheric drag/thermal protection in the ascent phase is one practical problem needing solutions that again may increase the cost and weight.

Quote
I don't expect SpaceX to persue this.  But it does have more potential for a high flight rate application like point-to-point ballistic transport than it ever did for low flight rate space-launch.

No, there is no point in this because rocket engines (and tanks for their fuel) in the booster are much cheaper than using big and expensive jet engines.

And for BLEO, things are MUCH worse for this idea because the early staging starts to hurt MUCH more.
« Last Edit: 05/05/2018 07:29 am by hkultala »

Offline RobLynn

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Re: BFS - without BFR eats the launch market.
« Reply #194 on: 05/05/2018 08:12 am »
This thread is about the BFS, which may be marginally SSTO capable, not the F9 upper stage.
BFS does not require much saving in delta V or boosted Isp to give it a lot more payload capacity.  Eg for 80tonnes in LEO the BFS as an SSTO with mission 'average' Isp of 3500m/s and 9300m/s deltaV you need about 1150tonnes GTOW.  But for Mach2-3 and 20-30km altitude staging from a gas turbine booster you can get more like 3700m/s Isp and assuming 8300m/s deltaV need more like 750 tonnes at staging - a 300 tonne rocket fuel saving (assuming 100 tonnes in jet engines) or a 40tonne payload boost - possibly 200 paying passengers.  Which could be increasing revenue by 0.5-1 million dollars per flight.

10x 100 reuse booster raptors that cost $3million each would be $300k per flight.
60x $10million afterburning jet engines that last 3000 flights would be $200k per flight, and should be saving a lot of fuel expense as well.

Again I am not really advocating this.  I am simply pointing out that the economics of gas turbine based boosters are more attractive for this use case than they have ever been for other space-launchers due to the potentially high flight rate, and desire for operating near to major cities.

The glass is neither half full nor half empty, it's just twice as big as it needs to be.

Offline hkultala

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Re: BFS - without BFR eats the launch market.
« Reply #195 on: 05/05/2018 09:28 am »
This thread is about the BFS, which may be marginally SSTO capable, not the F9 upper stage.
BFS does not require much saving in delta V or boosted Isp to give it a lot more payload capacity.  Eg for 80tonnes in LEO the BFS as an SSTO with mission 'average' Isp of 3500m/s and 9300m/s deltaV you need about 1150tonnes GTOW.  But for Mach2-3 and 20-30km altitude staging from a gas turbine booster you can get more like 3700m/s Isp and assuming 8300m/s deltaV need more like 750 tonnes at staging - a 300 tonne rocket fuel saving (assuming 100 tonnes in jet engines) or a 40tonne payload boost - possibly 200 paying passengers.  Which could be increasing revenue by 0.5-1 million dollars per flight.

So, you are using a stupid craft configuration (BFS-SSTO) as a baseline and then making ANOTHER stupid craft (which completely nullifiest of the original points of the original stupid craft(saving in R&D cost, saving 1st stage mfg cost, saving on staging)) and saying that "look, I got a better result!"

The full BFR is definetly MUCH better than what you are proposing and probably also cheaper.

But if we want to have a more straight comparison with your hypothetic jet stage, lets compare it to a small undersized rocket-powered 1st stage.

In order to lift that 1150 tonne craft to mach 3, 30km staging you need about 1900 tonnes of thrust. This means 100 F-135 engines.
This is ignoring the weight of your first stage itself. The engines itself weight about 170 tonnes more, and the rest of the stage and the fuel some more.

Practically at least something like 120 of those engines is needed. Cost of those engines is over 1.5 billion dollars, for ONE very low-impulse first stage.

Also good luck trying to physically fit those 120 engines to any reasonably-shaped craft, and also having the air intakes etc so that they work, with all the required speed ranges.

Oh - that reminds me. I did not take into account the weight of the intakes. The movable intake ramps for intakes that work from 0 to mach 3 are big and heavy. Your T/W of 11 does not take into account them.

Or, if you plan to do parallel staging, and use the SL raptors of the BFS, then (ignoring the booster mass) you need "only" about 1400 tonnes of thrust. So "only" 73 engines. Practically about 90 due to the booster mass. Still cost of over billion dollars.


The ordinary BFR probably stages at somewhere around 6000 km/s like F9. To equal your 3000km/h, the booster it could be made MUCH smaller. Something like total stage mass 1050 tonnes, total liftoff mass 2200 tonnes, total thrust at liftoff 3300 tonnes to have T/W of 1.5 at liftoff, total raptor engine count 20.

This 20-engine mini-BFB would be MUCH cheaper than your jet-based first stage.

So, even with that very suboptimal mach 3 staging rocket engines make much more sense than jet engines.
Jet engines simply have way too low thrust to cost -ratio


Quote
10x 100 reuse booster raptors that cost $3million each would be $300k per flight.
60x $10million afterburning jet engines that last 3000 flights would be $200k per flight, and should be saving a lot of fuel expense as well.

You are making stupid reliability/reuse assumptions. You assume you can only use raptors 100 times but your "overcharged" jet engines (which in reality are designed for long duration while NOT using afterburner, and only use afterburner for very short bursts) could survive 30 times more.

And you are using a both way too low engine count, AND also too low engine price for your engines. You need 90 or 120 engines, worth 13 million each, not 60 worth 10 millions each.

So, your numbers don't make any sense at all.

Quote
Again I am not really advocating this.  I am simply pointing out that the economics of gas turbine based boosters are more attractive for this use case than they have ever been for other space-launchers due to the potentially high flight rate, and desire for operating near to major cities.

No, they are not because you are having a very stupid comparison and using totally bogus numbers for your reuse counts.

Again, rocket-powered first stage gives all this for much cheaper
« Last Edit: 05/05/2018 01:21 pm by hkultala »

Offline philw1776

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Re: BFS - without BFR eats the launch market.
« Reply #196 on: 05/05/2018 07:29 pm »
What would be the point of that? If SSTO is not viable, then just use the BFR first stage with it and it's all good. I think that strap on F9 boosters would be a bad idea for so many reasons. Two different fuels to handle for starters, different launch pad needed and infrastructure needed, etc, etc.

Say BFS exists.  A cheaper to build, gentle flight profile engine depopulated BFB Lite dropping one ring of 12 Raptors for a total of 19 Raptors instead of 31 would lift even an 85 tonne BFS with 65+ tonne payloads to LEO and enable Starlink launches.
Depopulated BFB Lite only reaches 1.1+Km/sec max and is gentler on its flight wear & tear and on the launch pad with "only" 32 Mega Newtons or 7.3 million pounds thrust.
Perhaps it could be built of AlLi tanks to save cost (and add tonnes of mass) only dropping payload to 50 tonnes.
A scenario where Raptor engines are expensive & time consuming to build (at first) yet something completely re-useable, more capable and  less expensive than F9s is needed ASAP say for Starlink.

« Last Edit: 05/05/2018 09:05 pm by philw1776 »
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Offline alang

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Re: BFS - without BFR eats the launch market.
« Reply #197 on: 05/20/2018 03:20 pm »
If SSTO BFS is possible but marginal then is it possible that SpaceX might experiment with on orbit refuelling from another vehicle (but not the tanker variant) before BFR and the tanker variant become available?
This would be just to prove the concept and would likely mean a bespoke falcon or falcon heavy payload. It just seems to me that on orbit refuelling is a very difficult thing to get right and a big risk that it would be good to 'retire' sooner rather than later.

Offline speedevil

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Re: BFS - without BFR eats the launch market.
« Reply #198 on: 05/20/2018 04:06 pm »
If SSTO BFS is possible but marginal then is it possible that SpaceX might experiment with on orbit refuelling from another vehicle (but not the tanker variant) before BFR and the tanker variant become available?
If you've got two, you don't actually need to go all the way to orbit.

A ballistic trajectory in which you experiment is just fine.

A couple of kilometers a second downwards retroburn of extra fuel (a hundred tonsish) buys you six minutes, even with very partial fuel loads on both.
That's plenty to play with gradually working up (probably on several missions) rapid approach to 100m, followed by docking tests.

Perhaps even first try doing it with a beachball.

If F9B5 is in fact very refurbishable, that might be another interesting option.

Offline RobLynn

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Re: BFS - without BFR eats the launch market.
« Reply #199 on: 06/02/2018 03:03 am »
I recall comments from Shotwell? that SpaceX intend to do far larger follow-ons to BFS/BFR.  Does this significantly improve the payload mass fractions possible for a follow-on bigger Huge FS (HFS), making it more viable as an SSTO?  My thinking is that it will help a lot as:
-Less additional structure required for stiffness due to increased tank wall thicknesses (stiffness proportional to thickness³) but little or no increase in overall height reducing need for stringers to stop buckling in cylindrical tank walls and load spreading structural overhead from individual engine mounts.
-Increased thermal inertia of thicker walls inside TPS reduces TPS thickness needed.
-Increased insulation of thicker walls for cryogens likely helps in some small way, reduces liner/coating mass overheads and may eliminate any minimum gauge limited areas (but may not be significant)
-TPS mass proportionally decreased as well as increased areal loading on reentry doesn't increase the TPS mass-per-area much, and probably yields a proportional reduction in TPS mass (I am less certain about this).
-Reduced servicing/maintenance of proportionally smaller surface area TPS.

Downsides: longer landing legs, other?
The glass is neither half full nor half empty, it's just twice as big as it needs to be.

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