Here is a video of a flight of a Soviet N1 rocket hot staging. [likely animated]
Flight History of N1 Rocket
N1 1L – full scale dynamic test model, each stage was individually dynamically tested; the full N1 stack was only tested at 1/4 scale.
N1 2L (1M1) – Facilities Systems Logistic Test and Training Vehicle (FSLT & TV); two first stages painted gray, third stage gray-white and L3 white.
N1 3L – first launch attempt, engine fire, exploded at 12 km.
N1 4L – Block A LOX tank developed cracks; never launched, parts from Block A used for other launchers; rest of airframe structure scrapped.
N1 5L – partially painted gray; first night launch; launch failure destroyed pad 110 East.
N1 6L – launched from the second pad 110 West, deficient roll control, destroyed after 51s.
N1 7L – all white, last launch attempt; engine cutoff at 40 kilometres (22 nmi) caused propellant line hammering, rupturing the fuel system.
N1 8L and 9L – flight ready N1Fs with improved NK-33 engines in Block A, scrapped when the program was canceled.
N1 10L – uncompleted, scrapped along with 8L and 9L
It is interesting that the SpaceX Super Heavy Starship has similar numbers of engines compared to the N1 rocket. Both the N1 and the SpaceX Super Heavy Starship will use hot stage separation.
The first stage, Block A, was powered by 30 NK-15 engines arranged in two rings, the main ring of 24 at the outer edge of the booster and the core propulsion system consisting of the inner 6 engines at about half diameter.
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Don’t understand why hot staging is so useful. Igniting the second stage 5 seconds after the first stage shut down loses only 160 ft/sec velocity (5g), orbital velocity is about 25,000 ft/sec.
Can somebody explain?
Fuel consumption is exponential with delta V, not linear.
When a falcon 9 second stage separates it is essentially in a zero G environment. So the fuel and oxidizer are floating in the tanks If a gas bubble formes in the engine fuel lines the engine may fail at startup.
The US typically uses small cold gas thrusters to keep the fuel at the bottom the tank near the engine intakes. Russia however uses hot stageging on most of their rockets (including their manned rockets.. While the first stage is still accelerating the rocket the second stage starts its engines just before stage separation. For russia it has been a reliable way to stage rockets. And it avoids the need for cold gas trusters or small rocket engines.
SpaceX wants to keep unneeded mass to a minimum. They believe with hot saging they won’t need as much fuel for the cold gas thrusters which reduces mass.
Cool video from Hazegrayart, worthy to check his channel.
The more I think of it, the more it makes sense to have hot staging, despite the N1 reminiscences.
Hot staging increases payload, but the most crucial aspect, it’s the ability to do abort at any stage of the launch.
If anything goes wrong with Superheavy, they can trigger the hot staging immediately and have Starship go on its merry way.
In most cases that will be towards a more controlled return and landing, but also as we saw in the first SS/SH test, it might be to complete a mangled first stage launch.
That sounds like an odd capability to tout, but let’s recall the first goal is to deliver the payload to orbit. Starship can be ultimately, expendable.
This is an animated video, no N1 ever made it far enough to ignite the second stage. There are lots of clues in the video that it isn’t real (in addition to wikipedia telling us when each N1 met its maker). Specifically the exhaust plum of the second stage expands too far outward from the stage which would indicate that they were using rocket bells for the first stage on the second stage engine. The N1 second stage had a vacuum version of the first stage engine and the plume wouldn’t expand so far outward once the second stage was underway.
Soviets used expendable first stage and SpaceX does not. They need to shield it appropriately and even if they replace heat shield that is not all. There are strong forces from main Starship engines, which can damage upper part of first stage.
I’m pretty sure that they have thought this through. SpaceX has a great deal of experience recovering F9 first stages that have had a vacuum Merlin ignite close to the tanks.
Of course they had, but they hadn’t done hot staging in actual flight yet. Starship engines are a powerful. If they want to reuse 1st stage, it has to be in good condition.
I think they decided 10 % is worth the risk and they can at least try and get the data.
It is a test flight.
Hot staging strikes me as a significantly more tractable problem than reuse. 10% payload to LEO is a serious improvement and merits all sorts of workarounds to make this happen.
It’s up to 10%, not 10%. It will also increase mass, complexity and failure points. It might work or it might not; nothing is sure at this point and there’s no reason to assume that elon won’t just review the test data of the next launch and decide it’s not worth it.