Starship On-orbit refueling - Options and Discussion

[hkultala]

Only real way to increase tanker capacity is to shave dry weight, or you need bigger booster ....

yes.

(which MUST be larger diameter)

No, it does not have to, unless going to MUCH bigger.

The current specifications in SpaceX web site says "72 MN" for Superheavy thrust and 4600 tonnes total propellant + payload.

Assuming the spaceship + superheavy together have empty mass of  less than 400 tonnes, this means maximum of 5000 tonnes liftoff weight.

This is T/W ratio of 1.44. WHich is a lot

They can increase the first stage fuel tank size to something like 4000 tonnes, increasing the empty weight by only ~30-40 tonnes.

This would mean T/W ratio of 1.25, which is still enough. There would be more gravity losses, but payload would be greater.

There has also been talk about 250-tonne-thrust version of Raptor. The optimal nozzle size for that would be slightly bigger than the nozzle size of current raptor, but they can make it have the same nozzle and lose ~1% of isp, and fit the same number of those engines under the craft. And then they can increase the thrust from 7.2 MN to about 8.8 MN.

That would allow making the tanks much longer, adding considerable amount of propellant, while still keeping the T/W ratio very good.

[TheRadicalModerate]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

[Eer]

I look forward to the day when fuel is sourced from space itself - comets, asteroids, whatever. At that point, raising fuel from earth surface becomes unnecessary for most uses.

To begin with, though, “just in time” fuel supply can still benefit from “cross dock” delivery by filling an SS depot version (maybe never intended to land) so cruise SS with passengers can load and go just makes sense for many reasons (not least reducing suspense on part of passengers and crew whether all the tankers will arrive on schedule). That sort of risk reduction makes sense to me.

A small squadron of such depot SS versions can be deployed and filled in any number of orbits and gateway destinations, just like filling stations and recharging stations serve traffic where traffic regularly need services.

[xvel]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full.

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

[Twark_Main]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full?

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

This is all discussed in the thread, but briefly: the problem is that refueling in a higher orbit means multiple trips through the (edge of the) Van Allen belt. It also requires a greater number of rendezvous/docking events (ie more risk).

I'm also drawn to the Pusher Tanker architecture for the mission possibilities of an ultra-high delta-v "Starkicker." Starkicker only gets one chance for its escape burn, and Oberth says you want your engine firing to be as short a duration as possible. So there's no time to use re-fueling during the delta-v slam, and another approach (Pusher Tanker, or simply StarPusher (ie Pusher Tanker but with no propellant transfer to the departure stage)) is needed if you need/want more delta-v than Starkicker alone can deliver.

https://twitter.com/elonmusk/status/1111760133132947458

[xvel]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full?

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

Problem is (and this is all discussed in the thread), refueling in a higher orbit means multiple trips through the Van Allen belt. It also requires a greater number of rendezvous/docking events (ie more risk).

I'm also drawn to Pusher Tanker for the mission possibilities of an ultra-high delta-v "Starkicker." Starkicker only gets one chance for its escape burn, and Oberth says you want your engine firing to be as short duration as possible. So there's no time to use re-fueling during the delta-v slam, and another approach is needed if you need/want more delta-v than Starkicker alone can deliver.

https://twitter.com/elonmusk/status/1111760133132947458

We don't know how long it will take to refuel, and since both the starship and the tanker can enter a higher orbit at the same time, Van Allen belts may not be such a problem if refueling will be fast.

[Twark_Main]

... don't need a special tanker that can dock with its nose 

And as to the eye-rolling about docking with the nose, in this thread we seem to be all treating rockets like LEGO (lengthening, stacking two SH together, swapping SH interstage for a nose cone), so surely swapping SS's nose for SH's interstage adapter is fair game.

[Twark_Main]

[snip]

We don't know how long it will take to refuel, and since both the starship and the tanker can enter a higher orbit at the same time, Van Allen belts may not be such a problem if refueling will be fast.

Both the apogee-raise burn to HEEO and the departure burn are optimally performed at perigee, because Oberth. There are large performance losses (ie you can't reach the Moon) if you do it any other way.

[xvel]

[snip]

We don't know how long it will take to refuel, and since both the starship and the tanker can enter a higher orbit at the same time, Van Allen belts may not be such a problem if refueling will be fast.

Both the apogee-raise burn to HEEO and the departure burn are optimally performed at perigee, because Oberth. There are large performance losses (ie you can't reach the Moon) if you do it any other way.

Yes, probably impossible to do faster than one full orbit

[TheRadicalModerate]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full.

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

Twark covered a lot of this, but:

1) Refueling in HEEO incurs an extra two passes through the VA belts, which is a big chunk of total radiation exposure, given average solar activity.  That won't quite double the total exposure for a lunar missions, but it'll come pretty close.

2) Whether this turns out to be "unnecessary complexity" depends in large part how much risk you incur with a docking/refueling operation.  P/T minimizes that.

An option to P/T that uses ordinary tankers (i.e., non-payload-carrying Starships) is to do the refueling rendezvous post-TLI.  That way, you don't incur the extra pair of VA belt transits.  The tanker then does a free return and reenters at TEI speeds.

[Twark_Main]

...

There has also been talk about 250-tonne-thrust version of Raptor. The optimal nozzle size for that would be slightly bigger than the nozzle size of current raptor, but they can make it have the same nozzle and lose ~1% of isp, and fit the same number of those engines under the craft. And then they can increase the thrust from 7.2 MN to about 8.8 MN.

I believe the 250 tonne version is simply a change of injectors. No throttling, but higher thrust.

In another thread I proposed a slightly more extreme version of this: cram the Raptor engines together in a honeycomb pattern as tight as the combustion chamber / plumbing will allow, and simply truncate the nozzle to a hexagonal profile (imagine taking a hexagon cookie cutter to the existing nozzle, without otherwise changing its shape). Bonus: the cut off nozzles will have a wicked "sawtooth" pattern when viewed from the side.

this means

Darn, they got to him... 

[mikegi]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

I'm definitely no rocket scientist (understatement of the year!) but ... why not make the Pusher more of a large, reusable service module (LSM) for LEO->Moon->LEO? You attach a simple capsule and lunar lander to the front. The whole thing enters LLO, the lander does its thing, then the LSM returns them to Earth. The capsule detaches and reenters as the LSM passes by Earth. With no people onboard, the LSM can take as long as necessary to get back to a stable LEO (e.g. maybe multiple short passes through the atmosphere, make burns when optimal, etc.).

[Paul451]

I on the other hand think an orbital platform could be the best idea. What happens if one of the four launches required to refuel fails? Or weather suddenly makes it impossible to launch. You could have to scrub the mission or have a starship and crew hang out for days/weeks/months waiting for fueling. Having a platform that is constantly stocked up means you can launch knowing the required fuel is there and get the mission under way.

You use one of the tankers as a pseudo-depot, you launch the mission ship itself last. If weather delays a fuel launch (or it fails), your crew are still on the ground. You only launch once the tanker-cum-depot has enough fuel for the mission. Then the tanker-depot lands and goes back into ordinary service.

That latter (ordinary service) is also why (IMO) the tankers wouldn't be modified from the standard cargo vehicles for a long time, even if in practice it meant launching 5 cargo-ships-as-fuel-tankers instead of 4 mass-optimised tankers. The additional cost of that one extra launch is not worth building a whole extra type of Starship that has such limited use.

[Aside: The same reasoning says that the first "passenger" Starships would just be cargo ships with a passenger pods. Different pods for different missions. Lets you do Shuttle-like missions with mixed cargo and personnel. Such as satellite recovery missions, service missions, mixed science missions, etc. And the latter lets researchers use Starships as single-purpose specialised space-stations, by swapping out mission pods.]

And the ease of using a cargo-ship-cum-tanker as a single-mission orbital depot is why you don't need an actual permanent depot for a long time.


Of course, I intend to contradict myself in my very next post:

[Paul451]

If a plane change requires more than a mass ratio of 1- that is, if the mass of fuel needed for a useful "next plane" change of orbit is more than the (already reduced, by the removal of reentry mass) dry mass of the tanker, without fuel, it's easier to launch an entirely new tanker in the new orbit than move the old one.

If anyone's curious, assuming 380s Isp and 350nm LEO, that's a roughly 20 degrees plane change. (Maths below. You might be able to improve that with a Low/High/Low plane change, but I haven't worked that out.)

However, for the Moon or Mars, there's virtually no inclination requirement (Figures below). Hell, you can boost from polar orbit if you want, with barely a hit. So any LEO depot works for BEO missions. LEO-to-GEO is more restrictive, but low-inclination-LEO to equatorial-GEO hides most sins. So a general LEO depot for equatorial boost is likely to see a reasonable amount of use, IMO.

If there's enough traffic beyond GEO, a large general-purpose depot definitely make sense. And if there's enough traffic to a specific inclination of GEO (such as equatorial), then a depot for that makes sense.


Rocket equation: delta_V = Isp * 9.8 * ln(wet_mass/dry_mass)
Target is fuel_mass = dry_mass, so 2n/n, so 2.
dV = 380* 9.8 * ln(2) = ~2.5km/s.

Maximum plane change we can get from a 2.5km/s burn in a 7.5km/s orbit:
delta_V = 2*sin(phi/2)*Vc
:: sin(phi/2) = 2500 / (2*7500)
:: phi = ~19 degrees


For Mars/Moon/Eq_GEO, the effective plane-change is reduced by the distance of the target. Starting at, say 27 degree LEO, Equatorial GEO is around a 4.5 degree effective plane change. The Moon is a sub-half degree plane change. And Mars is so close to zero as to not be worth mentioning. (Mars' solar inclination relative to Earth's is vastly greater, and nothing done in Earth orbit changes that.)

[TrevorMonty]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full.

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

Twark covered a lot of this, but:

1) Refueling in HEEO incurs an extra two passes through the VA belts, which is a big chunk of total radiation exposure, given average solar activity.  That won't quite double the total exposure for a lunar missions, but it'll come pretty close.

2) Whether this turns out to be "unnecessary complexity" depends in large part how much risk you incur with a docking/refueling operation.  P/T minimizes that.

An option to P/T that uses ordinary tankers (i.e., non-payload-carrying Starships) is to do the refueling rendezvous post-TLI.  That way, you don't incur the extra pair of VA belt transits.  The tanker then does a free return and reenters at TEI speeds.

Does the SS need to pass through VA belt twice?. If it is refuelled close earth before orbit hits belt, it can then do its earth departure burn, only passing through belt once.

[Paul451]

We shouldn't be too hysterical about the Van Allen belts. Apollo 14 (which came through near the thickest parts of both VA belts, both out and back) gave the crew a total dose of about 10 milligreys. (A CT scan is about 8mgr.)

The particles also aren't overly penetrating, compared to cosmic radiation, and secondary (shotgun) radiation from the skin isn't an issue. So combining the skin of Starship with a radiation shelter made from your water storage should protect the passengers for a good month of exposure, if they wanted to.

The VA belts are extremely dangerous for an unprotected human. But an "unprotected human" in space died before the airlock finished depressurising.

[Twark_Main]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

This is similar to an idea that Twark_Main and I discussed (thread is here).  I'd pretty much assumed that you just stretched the SS with a few extra tank rings and cannibalized some of the payload bay space, but you could probably start with a set of SH tanks as well.

The most demanding mission is getting a heavy cargo + crew to the lunar surface and back without lunar surface refueling.  You have to avoid refueling in HEEO to keep the crew out of the Van Allen Belts during refueling, and you have to carry all your prop with you for the return.  The "pusher/tanker" described in the link gets launched almost empty, uses regular tankers (whatever those are--likely just no-payload SSes) to acquire however much prop is needed, then docks nose-to-tail with the payload SS.  It transfers some of the prop to the payload SS, but then just acts as a first stage for most of the TLI delta-v.  Once in TLI, it either does a direct or free return to LEO, either aerobraking or propulsively inserting into whatever orbit the next mission needs.

The big deal here is risk reduction for crewed missions.  You only need one rendezvous/docking operation in the relatively low-radiation environment of LEO, and one jettison of the pusher/tanker.  Pusher/tanker isn't quite as prop-efficient as a series of refuelings in LEO and one refueling in HEEO, but prop efficiency probably isn't the most important criterion for a crewed mission.

Unnecessary complexity, such manoeuvres can be done with a simple starship tanker, fael097 calculated that the bulkhead shift alone will give a capacity of 2100t, so if it sends such a tanker with 1300t of fuel on the LEO will arrive with 100t, then it will be 20x refuelled to full. consumption of 700t gives dV 1.4km/s and in the tanker is 1200t to refuel the starship and 200t to return to the LEO (there would still be 100t left). when the starship enters the LEO it will be once refueled with another tanker which will give it dV 1.4km/s then both can meet in a higher orbit and the starship can be refueled to the full.

Instead of refuelling and docking, you have two refuellings and don't need a special tanker that can dock with its nose 

Twark covered a lot of this, but:

1) Refueling in HEEO incurs an extra two passes through the VA belts, which is a big chunk of total radiation exposure, given average solar activity.  That won't quite double the total exposure for a lunar missions, but it'll come pretty close.

2) Whether this turns out to be "unnecessary complexity" depends in large part how much risk you incur with a docking/refueling operation.  P/T minimizes that.

An option to P/T that uses ordinary tankers (i.e., non-payload-carrying Starships) is to do the refueling rendezvous post-TLI.  That way, you don't incur the extra pair of VA belt transits.  The tanker then does a free return and reenters at TEI speeds.

Does the SS need to pass through VA belt twice?. If it is refuelled close earth before orbit hits belt, it can then do its earth departure burn, only passing through belt once.

Sure, but that wouldn't be "refueling in HEEO," it would be refueling in LEO. And without HEEO refueling, Starship doesn't have the performance go to the Moon and back.

[joek]

...
Musk has other ideas
...

Right.  That article is what I was referring to--and as I noted in a previous post in another thread at that time)--Musk wants to stay out of that fray.

Clearly Musk sees on-orbit refueling as a requirement (obvious from Starship conops).  However, Musk likely wants to stay out of the kerfuffle which would potentially put him at odds with certain major NASA programs-stakeholders (as articulated in Eric Berger's article):

But we were banned from even saying the 'd' word out loud.
...
Sowers' suggestion that "depots" should not be uttered in public is consistent with observations at the time that a US Senator from Alabama, Richard Shelby, had told NASA to stop talking about propellant depots. The NASA spaceflight center that manages the SLS rocket's development, Marshall Space Flight Center, is based in Alabama.

The operative question is: How to best achieve on-orbit refueling?  Musk's opinion is clearly "tankers"--at least in the near-term and for the missions SpaceX is considering.  Not to mention avoids the 'd' word.

[MATTBLAK]

We shouldn't be too hysterical about the Van Allen belts. Apollo 14 (which came through near the thickest parts of both VA belts, both out and back) gave the crew a total dose of about 10 milligreys. (A CT scan is about 8mgr.)

The particles also aren't overly penetrating, compared to cosmic radiation, and secondary (shotgun) radiation from the skin isn't an issue. So combining the skin of Starship with a radiation shelter made from your water storage should protect the passengers for a good month of exposure, if they wanted to.

The VA belts are extremely dangerous for an unprotected human. But an "unprotected human" in space died before the airlock finished depressurising.

Didn't the Apollo 14 crew receive the highest overall dose of radiation of any Apollo crew? I'm sure I read that somewhere. EDIT: found table of dosage (see attachment) from bottom of this page - https://history.nasa.gov/SP-368/s2ch3.htm

[Paul451]

We shouldn't be too hysterical about the Van Allen belts. Apollo 14 (which came through near the thickest parts of both VA belts, both out and back) gave the crew a total dose of about 10 milligreys. (A CT scan is about 8mgr.)

Didn't the Apollo 14 crew receive the highest overall dose of radiation of any Apollo crew?

That's why I picked them as my example. (1rad is 10milligrays. Symptoms start in the low single digit grays, or low hundreds of rads.) They received the highest dose because of their trajectory, passing through the middle of the VA belts. The other missions were able to avoid most of the inner belt.