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As of now, SpaceX is preparing for Starship Flight 4, and once starship is mature enough and achieves Elon Musk's goal, will they make a Heavy version like with Falcon. Even Before Integrated flight test 1 of Starship, YouTuber piolet1549 published a video of a Starship Heavy concept (name of video: Starship Heavy concept[Fully Reusable] Stock KSP). It showed 3 super heavy cores strapped together to form a starship that can land on the Mun (the equivalent of the moon in KSP) and come back without refueling.

That got me wondering: Is it possible? If yes, how?

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  • $\begingroup$ At the moment this is likely to attract votes as opinion based, noting starship is still pretty early in development. Suggest editing to 'What performance would a super heavy variant of starship get', which can be answered with math rather than needing to know internal decision making of a very private company. $\endgroup$ Apr 17 at 9:16
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    $\begingroup$ KSP broadly overincentivizes horizontally mating identical booster cores. In reality I can only think of two times it's happened: Delta IV and Falcon 9. $\endgroup$
    – Erin Anne
    Apr 17 at 22:39
  • $\begingroup$ You ask if they will create a Heavy version of a Super Heavy ship. It is not clear to me if you're asking if they will create a larger variant, or a smaller one. I would interpret it as the latter, but @Dragongeek interpreted it as the former in his answer. $\endgroup$
    – Opifex
    Apr 18 at 9:37
  • $\begingroup$ Please limit question posts to just one question! $\endgroup$
    – Rory Alsop
    Apr 18 at 11:31

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Lotta questions here.

As of writing, SpaceX has not released any plans at building a larger Starship in the near future.

That said, there have been larger SpaceX concepts historically, such as previous iterations of what became Starship like the "ITS" (Interplanetary Transport System) which had a larger diameter and was taller than the current Starship before it was scaled down and redesigned.

Additionally, the current size of the Starship upper stage is not fixed. It's already been lengthened once, and there have been rumblings/Elon tweets that the different variants (HLS, orbital tanker, propellant depot) might have significantly different final lengths according to their mission purpose.

SpaceX is extremely unlikely to build a three-core "heavy" Starship

Elon has gone on record, multiple times, where he basically states that Falcon Heavy was a mistake in retrospective. Originally, the plan was "just slap two boosters on the side" but in the end, they ended up basically needing to redesign the entire center core from scratch due to the much higher loads on it. The center core, despite externally looking like a standard B5 Falcon 9, is very different and not just a regular booster with two mounting brackets on the sides.

Knowing what SpaceX knows now, if they want a larger rocket in the future, they'll likely just build a larger rocket rather than kludging together something in the way it wasn't designed.

Orbital refueling will become and likely will remain a cornerstone of beyond-Earth space exploration

You ask:

will starship be able to finally travel to Mars or the moon without in orbit refueling

and the answer is, pretty solidly, no. Orbital refueling isn't just some "hack" that they're trying to implement until they can do without, it is fundamentally instrumental to the entire system and architecture. "Get rid of orbital refueling" is not a goal that SpaceX has (or will have) because from a rocket-equation perspective, it is just unbelievably advantageous and Moon or Mars missions without it at the scale they're planning is just completely unfeasible.

Yes, orbital refueling adds some "complexity" to the system, but ultimately, SpaceX are wagering that it will be worth it.

For example, could you build a car that can drive from New York to California and back (6000 miles) without refueling a single time? Sure, it wouldn't be easy, but if you put a couple engineers on the task and construct a car that's basically entirely fuel-tank by mass, it could be done. Still, it would be hard, expensive, and should only be done if there are no other options (this is current lunar and mars exploration). If we can refuel on the way, well, then why not do that?

Do we even need a larger launch vehicle?

Starship is big, and the amount of payload it can put into orbit is enormous should it operate at the parameters that SpaceX is targeting.

In fact, it is so big, that it has no current purpose. There are a lot of people dreaming about what could be done if we had one, but the current space launch market is already served quite well. In fact, the only major near-term "customer" of Starship launches is SpaceX themselves for their Starlink business, and beyond that, the only other real customers are either only purchasing a couple rides (NASA / HLS) or doing one-off missions (Dear Moon)--neither of which are going to get anywhere near the aspirational capacity of the Starship launch system.

A hypothetical "Starship Heavy" just wouldn't have any payloads to lift, and even if someone wanted to put something into orbit that a single Starship couldn't do, it would likely be easier to simply assemble whatever you are building on-orbit in modules or whatever.

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    $\begingroup$ Your first sentence is about a week out of date. At his most recent talk, Elon revealed plans to stretch Superheavy slightly, and Starship significantly. arstechnica.com/space/2024/04/… $\endgroup$ Apr 17 at 19:26
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    $\begingroup$ Your car example doesn't work out too well. Assuming a 30% loss of fuel economy from being heavily loaded, my car could go from New York to Los Angeles and back on about 1100 pounds of gasoline, amounting to a 45% increase over its empty weight. $\endgroup$
    – Mark
    Apr 17 at 22:29
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    $\begingroup$ @Mark now redesign your car to incorporate about a cubic meter of fuel tank to hold that gasoline without compromising its other functions. The example might be a little exaggerated, but only a little. $\endgroup$ Apr 18 at 12:35
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    $\begingroup$ @MSalters not without using a significant fraction of its cargo capacity and severely impacting safety, you can't. And that truck's nearly twice the mass of the car Mark referenced. $\endgroup$ Apr 18 at 16:03
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    $\begingroup$ @MSalters ...this car specifics debate isn't really the point of my example, but no. Assume the F150 gets 18 mpg base, and you load it with 460 gallons of fuel. Assuming a reduction of efficiency of 1% per 100lbs extra load, this would result in a final value of about 13mpg and those 460 gallons of fuel would mass north of 2700 lbs. This is above the max carrying capacity of an F150, so no, you can't just slap a large fuel tank onto a F150 specifically $\endgroup$
    – Dragongeek
    Apr 18 at 16:18
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Given the structural considerations, Strapping additional cores to the main starship stack would be a huge engineering undertaking and likely not make sense. The additional mass that would have to be added to make the center core strong enough to handle the stress of side booster thrust would be significant. It makes more sense to add performance by making the stack taller and adding vacuum performance as has already been announced.

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  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
    – Community Bot
    Apr 19 at 6:57
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There are a number of options for increasing the lift capacity of Starship, once they basic design is flying re-usably.

Firstly they will reduce the margins in some areas. Propellant margins will be reduce to the minimum safe level and some structural elements will be adjusted, reduced or removed to save weight when they know exactly how Starship behaves. After that they will look at stretching Starship and Super heavy as well as adding another 3 engines to Starship.

All of that should produce a superheavy lift vehicle with a very low £/tonne figure.

After that it is doubtful that they will make many changes. The goal is to send humans to Mars it’s not to build the biggest rocket conceivable. As rocket size increases any potential benefits from scaling have to be compared to the costs.

Firstly, safety there might well be difficulties in obtaining permission for launching such a super jumbo three core monster from a noise perspective and also the explosive damage that might result when it took off or perish the thought if anything went wrong.

Then there is the fact that they would already have a perfectly useable superheavy rocket and might be loath to build another one, because any such three core monster would be a different beast with different aerodynamics and very different stresses and strains on the central booster in particular and a very different vibrational environment for the payload this could easily swallow vast amounts of money in development.

Any three booster Starship would also require three catching towers, a super three bay deluge system and a much bigger tank farm.

All of these costs would have to be set against the option of simply launching two standard Starships. It is unlikely that there would be any need for the launch of a monolithic mass in excess of 200tonnes as there are no monolithic 100tonne payloads as it is.

In the unlikely event that they really did need to build a bigger one (they won’t) they would probably start from scratch with a larger diameter rocket.

Is it a theoretical possibility? Yes it probably is given enough resources, but will it happen? No, it’s no going to happen because there are better ways.

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  • $\begingroup$ "it is doubtful that they will make many changes" - I actually expect there will be many changes by the time they go to Mars. Starship has only just started flying, already there have been major changes like carbon fiber to stainless steel, and hot staging. Besides stretching, Elon has more than once talked about wider diameter, although apparently not in versions 2 and 3 based on his recent pep talk. Not disagreeing with your points about three core, but in 20 years when they send humans to Mars, Starship will possibly be different in many ways to these early prototypes. $\endgroup$ Apr 22 at 12:57
  • $\begingroup$ @Steve Pemberton Yes, no doubt Starship will be different in 20 years but many due to early development, followed by stretching and more engines. Once they have it working well I suspect the changes will be a lot more minor - as is now the case with Falcon 9. No doubt any Mars or Moon bound Starship will need a lot of work as they will be very different versions with different tasks. $\endgroup$
    – Slarty
    Apr 22 at 19:30

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