Should Starship have a third stage for single-launch high-energy trajectories?

[Pipcard]

One of the criticisms of Starship I've seen in a certain spaceflight discussion community is that the second stage has a relatively high dry mass, and multiple launches and refuelings would be required for high-energy trajectories. If you only wanted to launch a small probe, it is said that it would add too much operational risk and not be cheaper than just adding a small third stage for a single launch mission architecture. Would that be a good idea or not (it doesn't have to be hydrolox either)?

Below is an estimate of BEO performance by Neo_EimajOzear for a single-launch Starship, a single-launch Starship with a Centaur or F9 second stage, and a Starship with 8 refuelings:
https://twitter.com/Neo_EimajOzear/status/1382820071626657798/

[Scintillant]

8 refuelings, at say $4m per refueling, costs $32m. Can you buy a Centaur/F9 upper/Castor/etc, redesign it to be launched from SS, install new GSE for the new fuel type, and do all the other things for $32m? I doubt it. Plus, even if you do that, you get worse performance (more money spent on mass-shaving the probe) and slower transit time (more money spent on scientist salaries).

Not worth it IMO.

See below for previous discussions of this.
https://forum.nasaspaceflight.com/index.php?topic=47776.0
https://forum.nasaspaceflight.com/index.php?topic=52877.msg2240114#msg2240114 (your post!)

[Pipcard]

8 refuelings, at say $4m per refueling, costs $32m. Can you buy a Centaur/F9 upper/Castor/etc, redesign it to be launched from SS, install new GSE for the new fuel type, and do all the other things for $32m? I doubt it. Plus, even if you do that, you get worse performance (more money spent on mass-shaving the probe) and slower transit time (more money spent on scientist salaries).

Not worth it IMO.

How are you sure that Starship will cost that low? What if Starship doesn't end up costing $4 million, but instead $40 million?
(I want to be optimistic as you, too, but I would also like some solid evidence as well)

https://forum.nasaspaceflight.com/index.php?topic=52877.msg2240114#msg2240114 (your post!)

That was for hydrolox specifically, this is about the general idea of a third kick stage for Starship.

[TrueBlueWitt]

8 refuelings, at say $4m per refueling, costs $32m. Can you buy a Centaur/F9 upper/Castor/etc, redesign it to be launched from SS, install new GSE for the new fuel type, and do all the other things for $32m? I doubt it. Plus, even if you do that, you get worse performance (more money spent on mass-shaving the probe) and slower transit time (more money spent on scientist salaries).

Not worth it IMO.

How are you sure that Starship will cost that low? What if Starship doesn't end up costing $4 million, but instead $40 million?
(I want to be optimistic as you, too, but I would also like some solid evidence as well)

https://forum.nasaspaceflight.com/index.php?topic=52877.msg2240114#msg2240114 (your post!)

That was for hydrolox specifically, this is about the general idea of a third kick stage for Starship.

However, if you're going for disposable High C3 Starship, the flaps,legs and heat tiles can all be discarded making it cheaper and lighter(better performance), right?   They might even be able to delete 2 of the SL engines (at the cost of redundancy) to cut weight. Just enough power and control to get to orbit.

[Scintillant]

8 refuelings, at say $4m per refueling, costs $32m. Can you buy a Centaur/F9 upper/Castor/etc, redesign it to be launched from SS, install new GSE for the new fuel type, and do all the other things for $32m? I doubt it. Plus, even if you do that, you get worse performance (more money spent on mass-shaving the probe) and slower transit time (more money spent on scientist salaries).

Not worth it IMO.

How are you sure that Starship will cost that low? What if Starship doesn't end up costing $4 million, but instead $40 million?
(I want to be optimistic as you, too, but I would also like some solid evidence as well)

All the information regarding Starship cost we've gotten from Elon has been trending towards the cheaper end as time goes on. Raptor cost is projected to be well below $1m, and ideally >$250k (2019 tweet). Marginal launch costs estimates have gone from $2m to $1.5m to less than $1m in the past 2 years. Steel is cheap, labor costs are (fairly) low for mass produced items like Starship, and fixed/R&D costs will be distributed across the many, many flights.

The usefulness of a third kicker stage is questionable at a per-refueling cost of $4m. The current estimate is less than a quarter of that. I just don't see how the economics work out.

[Pipcard]

All the information regarding Starship cost we've gotten from Elon has been trending towards the cheaper end as time goes on.

But for a significant amount of people, that aspirational cost estimate is hard to believe and thus they "take Elon's words with a grain of salt" (not necessarily dismissing it entirely, and I personally don't think Starship is a deliberate scam or snake oil or anything like that).

They're probably just inferring from the precedent of the Shuttle's expected operating costs vs the expensive reality, so they expect the "realistic" costs of Starship to be at least 10 times more.

Which is why they think "Starship is only good for LEO and needs a third stage to be good beyond LEO."

[M.E.T.]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Edit

Meaning no need for new stages or major infrastructure. Just chuck the fully fuelled F9 upper stage out of the payload bay and there’s your third stage ready to go.

[RotoSequence]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Please correct me if I'm wrong, but as far as I know, for a third stage packing Starship, wouldn't the best maneuver involve refueling the ship in LEO, burning to a highly eccentric orbit, and dropping the third stage off for a massive Oberth burn while the Starship returns to Earth?

[hkultala]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

From a rocket equation calculator:

114 tonnes to 14 tonnes , 3420 m/s exhaust gives  about 7.2km/s delta-v, which is quite a lot.

But F9SS would not be very optimal orbital tug stage for Starships, mostly because it uses kerosene.

1) Low isp. Though most of this comes from the low  gg cycle, not from kerosene. 3800 m/s (staged conbustion methane) gives the same delta-v with about 20% smaller propellant mass.

2) Need for third infrastructure at the launch site.

Moving the bulkheads to make the oxygen tank smaller and fuel tank bigger, and swapping the merlin 1dvac for raptor vac would make much more optimal orbital tug for large payloads, but the engine might be too powerful for small payloads, raptor might not be able to throttle deep enough to keep g-forces reasonable.

[Pipcard]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Please correct me if I'm wrong, but as far as I know, for a third stage packing Starship, wouldn't the best maneuver involve refueling the ship in LEO, burning to a highly eccentric orbit, and dropping the third stage off for a massive Oberth burn while the Starship returns to Earth?

The Starship skeptics I've been talking to are claiming that the dependency on refueling large amounts of cryogenic propellant is too risky (economically or technically) and is the main weakness of the architecture.

"A dozen refuelings to go to the Moon" is considered a "problem" by those people.

[M.E.T.]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Please correct me if I'm wrong, but as far as I know, for a third stage packing Starship, wouldn't the best maneuver involve refueling the ship in LEO, burning to a highly eccentric orbit, and dropping the third stage off for a massive Oberth burn while the Starship returns to Earth?

Not an expert in the field, so don’t know. But my point is, wherever you want to release the 3rd stage, you can have it in the Starship cargo bay in the form of an already existing F9 upper stage, rather than designing a whole new Starship variant for it.

[RotoSequence]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Please correct me if I'm wrong, but as far as I know, for a third stage packing Starship, wouldn't the best maneuver involve refueling the ship in LEO, burning to a highly eccentric orbit, and dropping the third stage off for a massive Oberth burn while the Starship returns to Earth?

Not an expert in the field, so don’t know. But my point is, wherever you want to release the 3rd stage, you can have it in the Starship cargo bay in the form of an already existing F9 upper stage, rather than designing a whole new Starship variant for it.

If the Falcon 9 second stage weighs ~84 tons fully fueled (does anyone have better numbers?), Starship should be good for a 16 ton probe on top of that.

[M.E.T.]

Could a fully fuelled F9 2nd stage (without the need for fairings) not fit inside the Starship cargo bay? What trajectories can you achieve with say a 10 ton payload if you have a fully fuelled F9 2nd stage in LEO?

Please correct me if I'm wrong, but as far as I know, for a third stage packing Starship, wouldn't the best maneuver involve refueling the ship in LEO, burning to a highly eccentric orbit, and dropping the third stage off for a massive Oberth burn while the Starship returns to Earth?

Not an expert in the field, so don’t know. But my point is, wherever you want to release the 3rd stage, you can have it in the Starship cargo bay in the form of an already existing F9 upper stage, rather than designing a whole new Starship variant for it.

If the Falcon 9 second stage weighs ~84 tons fully fueled (does anyone have better numbers?), Starship should be good for a 16 ton probe on top of that.

Well, Elon’s talking a 150 ton Starship payload, so…

Point is, an expendable F9 2nd stage costs around $10m. This one would have no need for fairings, being inside the Starship cargo bay for the ride to orbit.

So let’s say a Starship launch costs $10m. Add the $10m F9 upper stage expendable cost, and you have a 10-20 ton outer space probe launched for a cost of $20m.

That’s a steal.

[RotoSequence]

Well, Elon’s talking a 150 ton Starship payload, so…

I can't say I've seen any of that, but suffice to say, Starship has an over-specced payload bay by weight capacity for any existing second stage being used as a kick stage. 

[joek]

...
They're probably just inferring from the precedent of the Shuttle's expected operating costs vs the expensive reality.
...

Comparisons with Shuttle can be dismissed IMO. We have an existence proof to the contrary with F9 reusability.  In other words...

We should be careful to get out of an experience only the wisdom that is in it and stop there lest we be like the cat that sits down on a hot stove lid. She will never sit down on a hot stove lid again and that is well but also she will never sit down on a cold one anymore.

... but don't want to turn this into another reusability-whatever discussion.
I'll take Musk's word that SpaceX will get there. Maybe not the <$2M marginal cost/launch, but within spitting distance.

Which is why they think "Starship is only good for LEO and needs a third stage to be good beyond LEO."

That statement requires qualification. If Mars or Luna is not BLEO, not sure what is. Neither of those require SS/SH third stage to deliver significant payload to cislunar or Mars destinations.

This thread's title states "...high-energy trajectories". I take that to mean beyond cislunar or Mars. That said, at some point the C3-payload potential becomes somewhat academic. Fast transits may be great, but unless you are only interested in fast flyby (with consequently short observation window), you also need to slow down at your destination (which of course adds mass for the requisite propellant).

[soyuzu]

8 refuelings, at say $4m per refueling, costs $32m. Can you buy a Centaur/F9 upper/Castor/etc, redesign it to be launched from SS, install new GSE for the new fuel type, and do all the other things for $32m? I doubt it. Plus, even if you do that, you get worse performance (more money spent on mass-shaving the probe) and slower transit time (more money spent on scientist salaries).

Not worth it IMO.

How are you sure that Starship will cost that low? What if Starship doesn't end up costing $4 million, but instead $40 million?
(I want to be optimistic as you, too, but I would also like some solid evidence as well)

All the information regarding Starship cost we've gotten from Elon has been trending towards the cheaper end as time goes on. Raptor cost is projected to be well below $1m, and ideally >$250k (2019 tweet). Marginal launch costs estimates have gone from $2m to $1.5m to less than $1m in the past 2 years. Steel is cheap, labor costs are (fairly) low for mass produced items like Starship, and fixed/R&D costs will be distributed across the many, many flights.

The usefulness of a third kicker stage is questionable at a per-refueling cost of $4m. The current estimate is less than a quarter of that. I just don't see how the economics work out.

Frankly speaking, although I trust SpaceX in general, these “proof” have no more credibility than what is used to back a scam. In F9’s case, at least a great quantity of source from other SpaceX staffs, government agencies and commercial contracts are available.

Judging by the other smaller and simpler engine they are producing at Hawthorne at similar quantity cost ~$1M, I can hardly believe Raptors’ cost can be reduced to much below the confirmed $2M cost in near term, and 38 Raptors can single-hardly exceed the cost of a complete F9.

And I believe you are intentionally misinterpreting a long term goal as one that can be achieved within the first years of its commercial operations.

Also, Starship suffer from high empty mass, even a stripped down expendable version will weights 40t, which is not very efficient when the payload mass is on the order of several tons. Even adding a simple solid kick stage like Castor30XL can add 3-4km/a delta-v for the same payload and number of refueling.

[Pipcard]

Comparisons with Shuttle can be dismissed IMO. We have an existence proof to the contrary with F9 reusability. 

That's why they think the minimum reasonable cost for Starship will be as much as the F9, but not below that.

[joek]

...
Also, Starship suffered from high empty mass, even a stripped down expendable version will weights 40t, which is not very efficient when the payload mass is on the order of several tons. Even adding a simple solid kick stage like Castor30XL can add 3-4km/a delta-v for the same payload and number of refueling.

Who cares? I cringe every time I see % comparisons. In absolute terms, if SS has the capability to deliver X tonnes for $Y, who cares what the dry mass, wet mass, PMF, or whatever %'s are? Those metrics may be of historical interest, but are meaningless without a reference to $ in any discussion that includes the word "efficiency".

[soyuzu]

...
Also, Starship suffered from high empty mass, even a stripped down expendable version will weights 40t, which is not very efficient when the payload mass is on the order of several tons. Even adding a simple solid kick stage like Castor30XL can add 3-4km/a delta-v for the same payload and number of refueling.

Who cares? I cringe every time I see % comparisons. In absolute terms, if SS has the capability to deliver X tonnes for $Y, who cares what the dry mass, wet mass, PMF, or whatever %'s are? Those metrics may be of historical interest, but are meaningless without a reference to $ in any discussion that includes the word "efficiency".

The problem is even a fully fueled, stripped down version of Starship CANNOT give a 5 ton payload more than 12km/s of delta-v, while adding a solid kick stage can easily give it more than 15km/s of delta-v, and adding a Centaur V can give it 18km/s of delta-v, which should be extremely useful as APL’s interstellar probe has ditched Solar Orbeth maneuver and went for a traditional approach.

There is a limit for every rocket, Starship included, and adding a third stage can break that limit.

[joek]

...
There is a limit for every rocket, Starship included, and adding a third stage can break that limit.

Agree. But that is not an SS issue, it is a payload issue. As payload owner, you have 100-150t delivered to LEO to play with. Knock yourself out (and stop throwing shade at SS because it does not satisfy all payload owner desires).