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Ah, I used this exact same strategy in KSP. It’s very convenient to lift a huge amount of fuel into space once using a disposable rocket, then top up smaller cr

Ah, I used this exact same strategy in KSP. It’s very convenient to lift a huge amount of fuel into space once using a disposable rocket, then top up smaller crafts before heading out of orbit. I assume they won’t be following my “dump extra fuel here before re-entering the atmosphere” strategy though.

zeusk 5y ago
But why would you dump extra fuel in KSP? couldn't you use it to reduce your surface velocity before atmospheric heating begins? or even use it as thermal mass and do a retrograde burn during landing like Falcon9?
- ashtonkem 5y ago
  Fuel costs money to lift, so by dumping it in the orbital fuel station I can delay an expensive and tedious refueling mission. Deceleration in KSP is easy because the aerodynamic and thermal models are exceptionally forgiving. Assuming you have literally any heat shield, you have to come in at interplanetary transfer speeds with a periapsis below airplane traffic before blowing up due to overheating is a concern. Most players will depend on drag to do the bulk of their deceleration because it’s cheap and easy, and even Falcon 9 style approaches can use parachutes and air brakes to do the heavy lifting. My falcon 9 equivalent rocket just uses a touch of throttle to prevent the landing legs from compressing so much that the rocket bell hits the ground. If I had a bit more clearance I probably wouldn’t need any fuel other than the deceleration burn.
sandworm101 5y ago
It works in KSP because your a launching from the equator into a zero-incline orbit. In the real world, or modded ksp, the advantages of on-orbit refuel are very difficult to realize. It is basically only practical for geostationary sats. Everything else, everything inclined, would cost more fuel to get to than simply launching a new refueler for every task.
- deleted 5y ago
  [deleted]
imtringued 5y ago
Why is it convenient? Why wouldn't you just put the fuel you get from the tanker into the rocket you actually want to launch and add a another stage? What I am getting at is that there is no difference between carrying the tank with you and sending it ahead of time and meeting up in orbit. It doesn't change the rocket equation. It just splits your rocket into two rockets.
- irjustin 5y ago
  You're assuming the launch vehicle can carry the weight of both the tank and the payload. Really if you want to maximize the tank size & weight you put into space, separating the two is the only way. To steel man your point, Starship could be looked at as one giant ass tank with a small payload?
- Cthulhu_ 5y ago
  You need a bigger launch vehicle to launch everything in one go. I mean why didn't they just launch the ISS in one go? If your launch vehicle can put 1 ton into orbit, you can either have a 500kg vehicle with 500kg fuel, or you can have a 1T vehicle with an 1T fuel thing to launch separately. In KSP you can just add more boosters and fuel to your launch vehicle, but in the real life you have to consider economics. IIRC for a possible Mars or even moon trip, SpaceX will need to refuel in orbit as well.
  - ashtonkem 5y ago
    KSP also allows for a lot of configurations that are mathematically solid, but impossibly expensive or complex IRL. This bends the economies of scale in ways that don’t happen with real rockets. My favorite example is “asparagus staging”. In a normal rocket “onion staging” is where multiple liquid rockets are wrapped around a center core, and the outside rockets are drained and expended first. This allows greater thrust at takeoff, while ditching weight once you don’t need it. In KSP these outer rockets can be trivially plumbed together to share fuel, so that only 2 rockets are drained at a time. At the end the center core is left fully fueled for continued flight, since only the exterior tanks were drained. This is mathematically correct and damn near impossible to engineer. The pumping pressures and volumes are just absurd, since every single tank would need external, severable fuel and oxidizer lines capable of feeding a single motor at 100% thrust. There’s a reason why this tends to show up in KSP and not at say, SpaceX.

zeusk 5y ago

But why would you dump extra fuel in KSP? couldn't you use it to reduce your surface velocity before atmospheric heating begins? or even use it as thermal mass and do a retrograde burn during landing like Falcon9?

ashtonkem 5y ago
Fuel costs money to lift, so by dumping it in the orbital fuel station I can delay an expensive and tedious refueling mission. Deceleration in KSP is easy because the aerodynamic and thermal models are exceptionally forgiving. Assuming you have literally any heat shield, you have to come in at interplanetary transfer speeds with a periapsis below airplane traffic before blowing up due to overheating is a concern. Most players will depend on drag to do the bulk of their deceleration because it’s cheap and easy, and even Falcon 9 style approaches can use parachutes and air brakes to do the heavy lifting. My falcon 9 equivalent rocket just uses a touch of throttle to prevent the landing legs from compressing so much that the rocket bell hits the ground. If I had a bit more clearance I probably wouldn’t need any fuel other than the deceleration burn.

sandworm101 5y ago

It works in KSP because your a launching from the equator into a zero-incline orbit. In the real world, or modded ksp, the advantages of on-orbit refuel are very difficult to realize. It is basically only practical for geostationary sats. Everything else, everything inclined, would cost more fuel to get to than simply launching a new refueler for every task.

deleted 5y ago
[deleted]

imtringued 5y ago

Why is it convenient? Why wouldn't you just put the fuel you get from the tanker into the rocket you actually want to launch and add a another stage? What I am getting at is that there is no difference between carrying the tank with you and sending it ahead of time and meeting up in orbit. It doesn't change the rocket equation. It just splits your rocket into two rockets.

irjustin 5y ago
You're assuming the launch vehicle can carry the weight of both the tank and the payload. Really if you want to maximize the tank size & weight you put into space, separating the two is the only way. To steel man your point, Starship could be looked at as one giant ass tank with a small payload?
Cthulhu_ 5y ago
You need a bigger launch vehicle to launch everything in one go. I mean why didn't they just launch the ISS in one go? If your launch vehicle can put 1 ton into orbit, you can either have a 500kg vehicle with 500kg fuel, or you can have a 1T vehicle with an 1T fuel thing to launch separately. In KSP you can just add more boosters and fuel to your launch vehicle, but in the real life you have to consider economics. IIRC for a possible Mars or even moon trip, SpaceX will need to refuel in orbit as well.
- ashtonkem 5y ago
  KSP also allows for a lot of configurations that are mathematically solid, but impossibly expensive or complex IRL. This bends the economies of scale in ways that don’t happen with real rockets. My favorite example is “asparagus staging”. In a normal rocket “onion staging” is where multiple liquid rockets are wrapped around a center core, and the outside rockets are drained and expended first. This allows greater thrust at takeoff, while ditching weight once you don’t need it. In KSP these outer rockets can be trivially plumbed together to share fuel, so that only 2 rockets are drained at a time. At the end the center core is left fully fueled for continued flight, since only the exterior tanks were drained. This is mathematically correct and damn near impossible to engineer. The pumping pressures and volumes are just absurd, since every single tank would need external, severable fuel and oxidizer lines capable of feeding a single motor at 100% thrust. There’s a reason why this tends to show up in KSP and not at say, SpaceX.