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Note: if you have not heard of the Interplanetary Transport System or ITS, it was a 2016 concept for a fully reusable rocket which could deliver 300 tonnes of paylaod into low earth orbit. It didn't die, but it was downscaled instead in 2017. The only differences between it and the current evolution (the ITS was one of many different rocket concepts by Elon Musk that evolved over the years, but it was the largest of them) is that the ITS was to be it being made of carbon fibre composite and its larger size. It is now Starship, with 150 tonnes of payload to low earth orbit.

Why was the ITS downscaled? It would have a lower cost per kg than Starship and would be suitablw for interplanetary missions once its tanks had been filled completely just once in low earth orbit. I'll cover some reasons that have been given and might be given:

  • Doesnt fit in SpaceX's existing facilities. (Too wide)

SpaceX built a whole new seperate spaceport specifically to accommodate the size of the rocket, and work had started in 2014. As for their other facilities, they could upgrade those later when the project is not such a financial strain and SpaceX is making more money.

  • Too bold/expensive

No. The current Starship is 9m wide, uses the same Raptor engines, and is also fully reusable. So it wouldn't be that much bigger an undertaking than they are doing already. The ITS was only different in that it was 12 metres wide (the current design and the ITS are the same height) and to be made of carbon composite (but SoaceX should stick with the stainless steel they were using for Starship if they return to an ITS sized rocket).

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    $\begingroup$ as a guess: they've already had difficulty with handling the power output of the Booster during launch. It's not actually guaranteed that they'll be able to adapt the Boca Chica launch site to work as an operational orbital launch site for Starship (though I personally think they can still make it work). Those problems, and other launch-power related issues like how large of a zone to evacuate, would increase roughly in proportion to the launch mass. $\endgroup$
    – Erin Anne
    Sep 4 at 20:16
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    $\begingroup$ How large of a zone to evacuate? What does this mean? Then again you are just guessing, so I guess I will just wait until someone answers $\endgroup$ Sep 5 at 6:32
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    $\begingroup$ Boca Chica Village has been evacuated during Starship tests; the keep-out-zone for the first orbital test wasn't quite as large. The size of the keep out zone will scale based on how powerful the rocket is. $\endgroup$
    – Erin Anne
    Sep 5 at 7:20
  • $\begingroup$ Would the payload penalty from not using an equatorial launch site (so the large keep out zone would not be an issue) offset the increased payload capacity of ITS? $\endgroup$ Sep 5 at 11:58
  • $\begingroup$ Welcome to Stack Exchange! I mean the ITS concept is a bit strange anyway. $\endgroup$ Sep 7 at 0:05

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This is my take, only speculation so take with a grain of salt.

The ITS had quite a few problems, to start its 300t payload capacity. Let's say we are using Raptor 2 engines. How many would we need?

According to this article, the ITS would have a launch mass of 10,500t. Each raptor 2 generates 250t of thrust (I belive this is slightly generous). We'd need MINIMUM 42 engines to get a thrust to weight ratio of 1 (that is, the vehicle would hover). And hey, that's the amount of engines that were planned. Obviously my maths isn't super accurate but the point is that this thing would just manage to lift off with the planned number of engines. And with starship we've seen that so far raptors haven't been totally reliable, so all in all I'd say in reality you'd want about 45-46 engines. That is a lot of engines that give more chance for failure (best part is no part etc). And on top of that, it would be expensive to have this many engines.

The ITS would be very expensive to develop, test, and run its first few launches. One problem: it launches 2 things and 2 things only, being the tanker and the ship. The only return money you get from this is people wanting to go to mars. Realistically, everyday people don't really want to be among the first on mars, it would be an awful experience. So not only would you struggle to getpeople to pay for this trip, you get no income from launching satellites, which is how every orbital launch company (not Blue Origin or Virgin Galactic, they are suborbital) makes their money, and there's a reason for that: it works. So this project is already a horrendous cash burner. The cost of the oxidiser alone for the booster alone would cost 844,000 US dollars which is shockingly close to the $900,000 total fuel cost for Starship + Superheavy. Absurd. I dont know how anyone at Spacex thought these launches could be cheap. Oh, and to enter the satellite market you need cheap costs which don't come from a vehicle this big.

TLDR: The project would be expensive and potentially unreliable. Its only function was to send people to mars, so SpaceX had to create a vehicle that could launch any satellite. The high price would make it undesirable for potential customers, therefore the project was downscaled.

This is my first answer so I hope I helped, tell me if something is unclear and sorry if I said anything stupid :) Also I know I could have cited more sources but I was too far in when I realised that.

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    $\begingroup$ Your statement about Blue Origin is not fully correct. Blue Origin is developing the New Glenn rocket which is orbital. Even though only New Shepard has flown and it is a suborbital rocket, I wouldn't say that Blue Origin is suborbital. $\endgroup$ Sep 5 at 21:24
  • $\begingroup$ It would need a lot of engines, be expensive, and might not be reliable also perfectly describes Starship and Booster. $\endgroup$
    – Erin Anne
    Sep 5 at 22:18
  • $\begingroup$ I'm aware of New Glenn, I just didn't count them as orbital because as of right now their main source of income is space trips and suborbital launches, not satellites. $\endgroup$
    – pelican't
    Sep 6 at 8:20
  • $\begingroup$ @ErinAnne whilst these issues do apply to starship too, ITS being larger makes these issues larger too $\endgroup$ Sep 6 at 8:57

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