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Per Wikipedia, SpaceX has landed boosters successfully 79 out of 90 times. Yet Starships seem to be reliably exploding upon landing.
What is so different about the two that Starships are so much more difficult to land? Is it something to do with the multiple engines? Or just that it's a larger scale? It seems like the starships are trying to accomplish something which they've done successfully many times already.
There are a few reasons:
The most obvious but easier to overcome is that the Starship is new. They haven't had the time to perfect everything. For example, the Raptor engines use two separate turbopumps, one for liquid oxygen and one for methane. This improves efficiency but makes the engines far more complex. The Raptor is the first engine of its kind to ever fly, so its current track record is surprisingly good considering everything it has to do.
The main and most difficult issue is the way Starship is landing. Unlike the Falcon 9 which falls engines first, the Starship falls on its side to maximize drag. While this improves efficiency because the rocket is not going as fast and allows for safer re-entry, it creates a problem in that the engines are pointing sideways. While a Falcon 9 just has to light one engine at the right time, a Starship has to light three engines, gimbal them to flip it vertically, then shut off two and land. It is much more difficult and complicated. Combined with how untested the rocket is, it's a marvel that SN10 actually landed without immediately exploding.
The experience of landing a booster for SpaceX is actually more relevant directly to the Super Heavy booster, which has yet to fly. (BN1 was built as a pathfinder, then disassembled. BN2 is under construction as of Apr 2021).
Falcon 9 lands its first stage, which is analogous to Super Heavy. However, it does not land its upper stage, which is more similar to Starship.
The landing approaches are different. It is worth noting they did land SN10, though a Raptor underperformed leading to a rough landing that caused an explosion after landing. Getting it that close on their 3rd try demonstrates that the past Falcon 9 experience very clearly resolved a lot of risk.
The engines are very different and start differently. (Starts: TEA-TEB hypergolic 'lighter fluid' for Merlin, and igniters for Raptor). It also uses a completely new type of design with two turbopumps, adding complexity but increasing efficiency. The Raptor engine is still very young in its design life. As of Apr 2021 we have seen serial numbers as high as 66 which means they have built a large number of them (For rocket engines... In the car world, this would be 1 hours output of a factory). There has already been a major revision change (as seen in tank watcher photos) at the SN54 mark (which is mounted on Starship SN15).
The landing profile of the two vehicles is different with Starship, putatively reentering from orbital speeds, needing to burn off more velocity, so using aerodynamic friction to reduce speed more than a Falcon 9 first stage. Though if you watch a Falcon 9 when they show the speed/altitude numbers on the live cast (Like on the last NRO mission, where they were not allowed to show the upper stage for secrecy reasons) it is quite amazing how much velocity is lost just by aerodynamic friction. My recollection was almost 1500-2000 km/hr was lost just through friction, before the landing burn scrubbed off the final velocity.
This means the flip is needed. This means header tanks are needed to avoid sloshing fuel and ingesting bubbles.
That means reigniting a new engine design, while on its side as opposed to the usual upright. (Which is interesting since they use horizontal test stands, and often the worry is that vertical usage might differ).
All these things are new, and they got very very close with the first 4 landing attempts.
Each time they fail, they learn something new, and clearly since the vehicles are cheap enough, and are being built fast enough it is worth trying them out as soon as they are able.
