Space Booster

[philaj]

I was wondering if SpaceX had considered building a space-based variant of the Super Heavy Booster.  One of the issues of going to Mars is the time spent in space and the effect on the Astronauts.  A vacuum tuned Super Heavy Booster (eg. call it a Space Booster), fully fueled in orbit, would join a fully fueled Starship in the same way earth bound Super Heavy Booster joins to the Starship.  The vacuum tuned Super Heavy Booster would then accelerate the Starship, at a moderate rate over an extended period, to reach a higher speed that just the Starship itself. The 2 ships would separate, and the vacuum tuned Super Heavy Booster would return to Earth orbit.  The Starship would be travelling at a higher speed and arrive at Mars quicker and for the most part fully fueled.  So, the Starship would have the fuel to slow down and do whatever was required at Mars.

[JaimeZX]

Yes, variations on this theme have been discussed in a few threads.

[Alvian@IDN]

The problem is that you can't return to Earth in case the Mars EDL is no go. For that you need not more nor less than 6 months of trip time (which is exactly what's quoted on SpaceX Mars page)

And that's why Elon said recently that Starship doesn't need to be fully fueled in orbit in order to go to Mars

[Twark_Main]

The problem is that you can't return to Earth in case the Mars EDL is no go. For that you need not more nor less than 6 months of trip time (which is exactly what's quoted on SpaceX Mars page)

And that's why Elon said recently that Starship doesn't need to be fully fueled in orbit in order to go to Mars

Presumably you could devote the extra performance to landing more payload, rather than decreasing the trip time.

[kkattula]

The problem is that you can't return to Earth in case the Mars EDL is no go. For that you need not more nor less than 6 months of trip time (which is exactly what's quoted on SpaceX Mars page)

And that's why Elon said recently that Starship doesn't need to be fully fueled in orbit in order to go to Mars

Presumably you could devote the extra performance to landing more payload, rather than decreasing the trip time.

There would be a limit to how much mass the Starship can have for Mars EDL, but they could use extra propellant for a faster transit and some propulsive deceleration, with some added risk.

Of course if Mars EDL is a no go, I'm not sure that Earth free return that takes 1.5 to 2 years is all that helpful, since you would need to use the same systems for Earth EDL you couldn't use at Mars.  Maybe if there's a bad dust storm. Or if you could expect rescue approaching Earth.

[Twark_Main]

The problem is that you can't return to Earth in case the Mars EDL is no go. For that you need not more nor less than 6 months of trip time (which is exactly what's quoted on SpaceX Mars page)

And that's why Elon said recently that Starship doesn't need to be fully fueled in orbit in order to go to Mars

Presumably you could devote the extra performance to landing more payload, rather than decreasing the trip time.

There would be a limit to how much mass the Starship can have for Mars EDL, but they could use extra propellant for a faster transit and some propulsive deceleration, with some added risk.

No-one's denying that there's a limit. But if Starship goes in this direction architecturally (a big "if" btw), I do expect we'd see the vehicle design re-balanced to increase the EDL cargo mass.

To the zeroth approximation we can run the usual napkin math (possibly considering stretched/shrunk Starship variations), but there's no reason to expect Starship would just-so-happen to be optimally designed for such a radically different architecture. Anyone running numbers using Starship assumptions should recognize that sandbagging.

[Blackjax]

Apologies if this takes things too far off topic but I've been wondering if an entirely different kind of in space booster might make sense, but not for mars.  If Starship ever gets used as a platform for missions to the outer planets or beyond, might it make sense to have en electric propulsion tug that docks with the nose, which will take over from the on-orbit-refueled main engines of a starship once they have expended whatever amount of fuel makes sense for the mission profile?  Perhaps this might allow more distant missions to be carried out with a much more abbreviated timeline than we've seen in the past.

The challenge would obviously be that you'd be building EP on an epic scale by comparision to anything that has flown before and there is a big question of where all the power would come from.

[Elmar Moelzer]

I thought trip times were supposed to be 3 to 5 months? Has that changed recently?
The longer the trip time, the more difficult this gets. I am all for making trip times as short as possible. Solves a lot of problems.

[Twark_Main]

I thought trip times were supposed to be 3 to 5 months? Has that changed recently?
The longer the trip time, the more difficult this gets. I am all for making trip times as short as possible. Solves a lot of problems.

As Alvian@IDN pointed out upthread, the current Mars page on SpaceX.com lists the travel time as 6 months. This is in contrast to Musk's statement at IAC 2017. Looks like the current page first appeared on May 22, 2020.

https://www.spacex.com/human-spaceflight/mars/

[Mandella]

I thought trip times were supposed to be 3 to 5 months? Has that changed recently?
The longer the trip time, the more difficult this gets. I am all for making trip times as short as possible. Solves a lot of problems.

As Alvian@IDN pointed out upthread, the current Mars page on SpaceX.com lists the travel time as 6 months. This is in contrast to Musk's statement at IAC 2017. Looks like the current page first appeared on May 22, 2020.

https://www.spacex.com/human-spaceflight/mars/

Huh. Thanks to you both for linking that, since I've been using the 3-4 month trip quote myself. I'd like to think that the SpaceX page is referring to the cargo Starships, but all the art is showing the windowed version, thus crewed.

Be a good thing to ask Elon about in the upcoming presentation, if he doesn't cover it himself.

[warp99]

I thought trip times were supposed to be 3 to 5 months? Has that changed recently?
The longer the trip time, the more difficult this gets. I am all for making trip times as short as possible. Solves a lot of problems.

As Alvian@IDN pointed out upthread, the current Mars page on SpaceX.com lists the travel time as 6 months. This is in contrast to Musk's statement at IAC 2017. Looks like the current page first appeared on May 22, 2020.

https://www.spacex.com/human-spaceflight/mars/

Huh. Thanks to you both for linking that, since I've been using the 3-4 month trip quote myself. I'd like to think that the SpaceX page is referring to the cargo Starships, but all the art is showing the windowed version, thus crewed.

Be a good thing to ask Elon about in the upcoming presentation, if he doesn't cover it himself.

The Mars entry velocity is shown as 7.5 km/s which is consistent with a 6 months transit time but not with 3-4 months which is more like 9 km/s. 

It is also the same as LEO entry velocity so they can use the same TPS system.  This has the advantage of not requiring a multiple pass entry profile and means the TPS will get a lot of relevant testing with tanker and Earth orbit cargo ships before attempting a Mars landing.

[sebk]

The Mars entry velocity is shown as 7.5 km/s which is consistent with a 6 months transit time but not with 3-4 months which is more like 9 km/s. 

It is also the same as LEO entry velocity so they can use the same TPS system.  This has the advantage of not requiring a multiple pass entry profile and means the TPS will get a lot of relevant testing with tanker and Earth orbit cargo ships before attempting a Mars landing.

Actually SpaceX entry profile has much higher g-load than the Earth which is expected as Mars is two times smaller, has 2.5 times less surface gravity and there is big atmospheric variability on top of that. With roughly 2-2.5x g-load you also get 2-2.5x heat flux at the same velocity.

So, to actually have similar heat shield requirements you must enter at about 0.7 of Earth entry speed.

Thus two steps, i.e. capture and then EDL are actually prescribed (And would be in-line with relatively recent Elon's tweets about 2 passes being a good option and single pass being marginal)

[philaj]

My initial thinking was to come up with a way to reduce the time to Mars.  Spacex is already building Super Heavy Boosters.  To design/build a space tuned version of a Super Heavy Booster (Space Booster) wouldn't seem to me a big leap. There is already a mechanism to attach the Booster to to the Starship.  The Starship wouldn't use any fuel to leave Earth orbit. The Starship could use some of it's fuel to accelerate to a higher velocity than it would on its own.  Thus getting to Mars quicker. Also it could leave some of its fuel to slow down on arrival at Mars.

[Slarty1080]

My initial thinking was to come up with a way to reduce the time to Mars.  Spacex is already building Super Heavy Boosters.  To design/build a space tuned version of a Super Heavy Booster (Space Booster) wouldn't seem to me a big leap. There is already a mechanism to attach the Booster to to the Starship.  The Starship wouldn't use any fuel to leave Earth orbit. The Starship could use some of it's fuel to accelerate to a higher velocity than it would on its own.  Thus getting to Mars quicker. Also it could leave some of its fuel to slow down on arrival at Mars.

Trying to fill an orbiting Superheavy booster with 100tonne loads would require a very large number of retanking missions. It seems very wasteful to boost so many engines and so much tankage from LEO. Should they go down this route (unlikely IMO) it would make much more sense to build a dedicated tug that could boost out of LEO and return for retanking.

It could be a greatly scaled down version of Superheavy - same diameter but much shorter using far fewer vacuum raptor engines.

[philaj]

Sounds reasonable (a cut down version) given the cost of getting fuel to LEO.

[philaj]

According to the SpaceX website we have:
"Starship will be the world’s most powerful launch vehicle ever developed, with the ability to carry in excess of 100 metric tonnes to Earth orbit"
So a single launch could lift 100 metric tonnes of fuel in one trip.  So maybe a Space Booster with RVac engines is possible...

[Twark_Main]

Should they go down this route (unlikely IMO)

I agree it's unlikely, but as far as future evolutions go I find it far more conservative than other proposals (nuclear, SEP, Aldrin cyclers, hollowed out asteroids, etc). It almost entirely leverages technologies that SpaceX has already developed:

 •  the in-space docking port and rendezvous-and-docking procedure borrows from Starship in-space refueling

 •  the engines and thrust structure borrows from Super Heavy, probably just deleting some engines

Other than that... that's it. There's no new "hardcore" R&D required from SpaceX.

it would make much more sense to build a dedicated tug that could boost out of LEO and return for retanking.

It could be a greatly scaled down version of Superheavy - same diameter but much shorter using far fewer vacuum raptor engines.

I think you do it like Zubrin's proposal: the StarPusher boosts the Starship to near-escape and then separates, with Starship engines completing the TMI burn at/near perigee for maximum Oberth. Meanwhile the StarPusher retro-boosts (if necessary) to keep within Earth's SOI, and aerobrakes back to LEO, ready to be refueled for another flight.

This lets you achieve full reuse, but the time to re-cycle the system for the next TMI burn is long, which isn't ideal given the relatively short Mars window. So this plan works better if you pair it with a propellant depot and/or a large fleet of pre-fueled tankers staged in LEO.

Note that you can get an easier performance boost simply by refueling Starship in an elliptical orbit. No R&D, just operational changes. So I'm assuming you'd only roll out StarPusher if you're already refueling in an elliptical orbit, and StarPusher is intended to provide extra performance on top of that.

[ChrML]

Presumably you could devote the extra performance to landing more payload, rather than decreasing the trip time.

Makes no sense at all according to first principles. SpaceX focus on rapid production of Starships. Why not just send more of them to increase payload to Mars, rather than developing a completely new vehicle/concept?

[Keldor]

How would you perform maintainence on a vehicle that can never return to Earth?  It's not a lot of good if you have to retire it after 5 missions because of wear to the engines.  Also, it would not be able to use the atmosphere to stop at either end of a flight, which will significantly increase fuel needed.  There might not acutally even be any extra performance here if you want to recover the space booster.

I guess it could be useful for deep space missions that really could use the extra delta-V.  The booster and Starship would be expended in this case, but it might make some missions feasible that otherwise would take too long or even be outright impossible.  Sedna mission, anyone?  Entering orbit of Neptune or Pluto?

[GregTheGrumpy]

How would you perform maintainence on a vehicle that can never return to Earth?  It's not a lot of good if you have to retire it after 5 missions because of wear to the engines.  Also, it would not be able to use the atmosphere to stop at either end of a flight, which will significantly increase fuel needed.  There might not acutally even be any extra performance here if you want to recover the space booster.

I guess it could be useful for deep space missions that really could use the extra delta-V.  The booster and Starship would be expended in this case, but it might make some missions feasible that otherwise would take too long or even be outright impossible.  Sedna mission, anyone?  Entering orbit of Neptune or Pluto?

If it's cheap enough to build, throwing it away is good economic sense