Thrust to weight ratio of starship V2 with 6 vacuum engines on the second stage

[Sarigolepas]

Hello, I was wondering what the thrust to weight ratio at takeoff would be for a full stack with the second stage upgraded to 6 vacuum engines.
Most rockets lose a lot of weight as they fly which means their TWR gets higher and higher, but starship is almost as big as superheavy so the whole stack doesn't lose much weight during flight, which is why starship is designed with a TWR of 1.5 at takeoff.
But the new ship will be stretched by 10 meters and will be even heavier compared to the booster, which means that the whole stack will need an even higher TWR at takeoff to reduce gravity losses. What would you consider a good guess?
My guess is that they will use 300 tons engines so 9'900 tons of thrust for 5'500 tons so a TWR of 1.8 at takeoff.

[InterestedEngineer]

was analyzed here:

https://forum.nasaspaceflight.com/index.php?topic=49622.msg2475438#msg2475438


The problem is not TWR of the starship, it's of the booster to carry the additional starship mass plus additional propellant mass and tank.

Plus where to put the extra propellant in the booster,  There's no room.

[Sarigolepas]

was analyzed here:

https://forum.nasaspaceflight.com/index.php?topic=49622.msg2475438#msg2475438


The problem is not TWR of the starship, it's of the booster to carry the additional starship mass plus additional propellant mass and tank.

Plus where to put the extra propellant in the booster,  There's no room.

Yeah, you can't make superheavy bigger which means that making the ship bigger will reduce the mass ratio and velocity at MECO, which is why I'm betting that they will need to increase TWR at takeoff to maintain a good average TWR since the rocket won't get much lighter as it flies before stage separation. You need to start with an high TWR to be efficient.

[Keldor]

was analyzed here:

https://forum.nasaspaceflight.com/index.php?topic=49622.msg2475438#msg2475438


The problem is not TWR of the starship, it's of the booster to carry the additional starship mass plus additional propellant mass and tank.

Plus where to put the extra propellant in the booster,  There's no room.

Yeah, you can't make superheavy bigger which means that making the ship bigger will reduce the mass ratio and velocity at MECO, which is why I'm betting that they will need to increase TWR at takeoff to maintain a good average TWR since the rocket won't get much lighter as it flies before stage separation. You need to start with an high TWR to be efficient.

Around 90% of the mass of the rocket on the launch pad is fuel.  It gets *significantly* lighter toward stage separation.  Think of the difference between an empty soda can and a full one.  By the time stage separation comes around, they'll have burnt enough fuel that the TWR is in the 3-4 range, but after separation, it will drop way down since the empty booster stage is only maybe 200 tons, compared to the 1500 or 2000 ton fueled upper stage, and now there are many less engines.

Anyway, the idea behind slightly lengthening future boosters is that as SpaceX continues developing the engines, they're expecting to get more thrust out of them, which means they can carry more load, which is to say, a bit of extra payload mass and a lot of extra fuel to lift it, hence, they'll lengthen the tanks a bit to hold it.

Most rockets actually lift off with rather low TWR.  For example. I believe Saturn V had a TWR around 1.2 at liftoff.  Superheavy's TWR of 1.5 is actually on the high side, but I suspect that the reason has to do with doing a boostback burn to land the booster (something which most other rockets don't do, of course), which has the effect of making the dry (or almost dry) mass of the booster stage much more important.  Even though a low TWR rocket has high gravity losses near liftoff, the thing to remember is that in the world of rocketry, the fuel is cheap, and adding a couple tube sections to your tank is also pretty darn cheap, so you're not asking whether it's efficient at liftoff (it isn't), but rather, how much fuel can you put on board before adding a single extra drop will not only give you zero benefit, but will actually *reduce* your overall performance due to the extra tank weight.

[InterestedEngineer]

was analyzed here:

https://forum.nasaspaceflight.com/index.php?topic=49622.msg2475438#msg2475438


The problem is not TWR of the starship, it's of the booster to carry the additional starship mass plus additional propellant mass and tank.

Plus where to put the extra propellant in the booster,  There's no room.

Yeah, you can't make superheavy bigger which means that making the ship bigger will reduce the mass ratio and velocity at MECO, which is why I'm betting that they will need to increase TWR at takeoff to maintain a good average TWR since the rocket won't get much lighter as it flies before stage separation. You need to start with an high TWR to be efficient.

Around 90% of the mass of the rocket on the launch pad is fuel.  It gets *significantly* lighter toward stage separation.  Think of the difference between an empty soda can and a full one.  By the time stage separation comes around, they'll have burnt enough fuel that the TWR is in the 3-4 range, but after separation, it will drop way down since the empty booster stage is only maybe 200 tons, compared to the 1500 or 2000 ton fueled upper stage, and now there are many less engines.

Anyway, the idea behind slightly lengthening future boosters is that as SpaceX continues developing the engines, they're expecting to get more thrust out of them, which means they can carry more load, which is to say, a bit of extra payload mass and a lot of extra fuel to lift it, hence, they'll lengthen the tanks a bit to hold it.

Most rockets actually lift off with rather low TWR.  For example. I believe Saturn V had a TWR around 1.2 at liftoff.  Superheavy's TWR of 1.5 is actually on the high side, but I suspect that the reason has to do with doing a boostback burn to land the booster (something which most other rockets don't do, of course), which has the effect of making the dry (or almost dry) mass of the booster stage much more important.  Even though a low TWR rocket has high gravity losses near liftoff, the thing to remember is that in the world of rocketry, the fuel is cheap, and adding a couple tube sections to your tank is also pretty darn cheap, so you're not asking whether it's efficient at liftoff (it isn't), but rather, how much fuel can you put on board before adding a single extra drop will not only give you zero benefit, but will actually *reduce* your overall performance due to the extra tank weight.

Reasonable principles, but do the math.  How do you get 200t to LEO, and still fit in the maximum height of the chopsticks?

There's about 10-12m you can add to the stack, that's all that's left w/ chopsticks still being able to stack Starship.   

You can use the 10-12m to add to the booster or the Starship.   How much for each?

[Robotbeat]

was analyzed here:

https://forum.nasaspaceflight.com/index.php?topic=49622.msg2475438#msg2475438


The problem is not TWR of the starship, it's of the booster to carry the additional starship mass plus additional propellant mass and tank.

Plus where to put the extra propellant in the booster,  There's no room.

Yeah, you can't make superheavy bigger which means that making the ship bigger will reduce the mass ratio and velocity at MECO, which is why I'm betting that they will need to increase TWR at takeoff to maintain a good average TWR since the rocket won't get much lighter as it flies before stage separation. You need to start with an high TWR to be efficient.

Around 90% of the mass of the rocket on the launch pad is fuel.  It gets *significantly* lighter toward stage separation.  Think of the difference between an empty soda can and a full one.  By the time stage separation comes around, they'll have burnt enough fuel that the TWR is in the 3-4 range, but after separation, it will drop way down since the empty booster stage is only maybe 200 tons, compared to the 1500 or 2000 ton fueled upper stage, and now there are many less engines.

Anyway, the idea behind slightly lengthening future boosters is that as SpaceX continues developing the engines, they're expecting to get more thrust out of them, which means they can carry more load, which is to say, a bit of extra payload mass and a lot of extra fuel to lift it, hence, they'll lengthen the tanks a bit to hold it.

Most rockets actually lift off with rather low TWR.  For example. I believe Saturn V had a TWR around 1.2 at liftoff.  Superheavy's TWR of 1.5 is actually on the high side, but I suspect that the reason has to do with doing a boostback burn to land the booster (something which most other rockets don't do, of course), which has the effect of making the dry (or almost dry) mass of the booster stage much more important.  Even though a low TWR rocket has high gravity losses near liftoff, the thing to remember is that in the world of rocketry, the fuel is cheap, and adding a couple tube sections to your tank is also pretty darn cheap, so you're not asking whether it's efficient at liftoff (it isn't), but rather, how much fuel can you put on board before adding a single extra drop will not only give you zero benefit, but will actually *reduce* your overall performance due to the extra tank weight.

largely agree. Caveat: it’s almost all oxygen, not fuel.

Secondly, if SpaceX succeeds at high flightrate, then the cost of propellant (specifically, fuel) starts to dominate. If a Starship stack costs $100m, has about 5000t of propellant that costs ~$250/tonne, then it takes just 80 flights for the propellant to cost more than the capital cost of the rocket stack.

SpaceX wants to fly the stack like 1000 times, at least for the booster. So launch costs might become dominated by propellant costs.

[Sarigolepas]

Around 90% of the mass of the rocket on the launch pad is fuel.  It gets *significantly* lighter toward stage separation.  Think of the difference between an empty soda can and a full one.  By the time stage separation comes around, they'll have burnt enough fuel that the TWR is in the 3-4 range, but after separation, it will drop way down since the empty booster stage is only maybe 200 tons, compared to the 1500 or 2000 ton fueled upper stage, and now there are many less engines.

Anyway, the idea behind slightly lengthening future boosters is that as SpaceX continues developing the engines, they're expecting to get more thrust out of them, which means they can carry more load, which is to say, a bit of extra payload mass and a lot of extra fuel to lift it, hence, they'll lengthen the tanks a bit to hold it.

Most rockets actually lift off with rather low TWR.

It's not just an empty soda can, it still carries a 1'500 tons ship and in this case a 2'000 tons ship. Most rockets will have a TWR of 4 before stage separation because they throttle down but for starship this will be peak TWR and the average TWR during the first stage of the flight will be rather low, which is why they have to start high.

And yes the ship will get lighter as it flies, but if we expect the ship to do all the work then the booster is useless.

[Sarigolepas]

You can use the 10-12m to add to the booster or the Starship.   How much for each?

Everything would go to the ship, a bigger booster can't fit in the megabay.
Starship is designed to be refilled in orbit so it makes sense to make the ship bigger since it's part of the payload.

[Eka]

You can use the 10-12m to add to the booster or the Starship.   How much for each?

Everything would go to the ship, a bigger booster can't fit in the megabay.
Starship is designed to be refilled in orbit so it makes sense to make the ship bigger since it's part of the payload.

The current magabay. We don't know how tall the new one will be. It could be much taller. Also the launch tower can be increased in height, or a new taller one built.

[Anguy]

Launch tower HAS to be taller. Much taller in fact... A lot of problems with SH damaging Stage zero would be solved by increasing the distance between engines and ground...

[InterestedEngineer]

You can use the 10-12m to add to the booster or the Starship.   How much for each?

Everything would go to the ship, a bigger booster can't fit in the megabay.
Starship is designed to be refilled in orbit so it makes sense to make the ship bigger since it's part of the payload.

One can't add 500t to the Starship (Fuel + cargo + rings) and not add fuel to the booster, the rocket equation doesn't work like that.

The current deltaV of the booster is about 3.5km/sec.   That's a mass ratio (M_r) of 2.7

If you add 500t of payload to the booster (aka fuel and payload and rings for Starship), to get the same deltaV, the booster needs 850t of fuel -- (M_r-1) times payload increase

There's no place for that fuel to go without adding rings.  Each ring adds 100t of fuel capacity.   So ~8 more rings.

[wannamoonbase]

I have been wondering if a stretched 9 engine Starship would most likely be a tanker or another form of large volume same mass vehicle.

Seems both a 6 and 9 engine configuration could fly from the same Stage 0 infrastructure.

It’s a shame they stuck with the 9 meter diameter after moving the project out of Hawthorne, a 10 meter vehicle could change a lot of this debate.

[Eka]

A stretched 9 engine SS has been mentioned for getting humans to Mars faster. It can also be used for bulkier loads.

[spacenut]

With the stainless steel construction, they could have gone with a 12m diameter rocket.  More engine room and/or more engines on the booster.  A wider Starship and shorter, might have made landing easier as well as given room for not only engines but longer wider legs. 

9m was chosen when they were looking at using composite.  Don't know why they stayed with 9m. 

[InterestedEngineer]

You can use the 10-12m to add to the booster or the Starship.   How much for each?

Everything would go to the ship, a bigger booster can't fit in the megabay.
Starship is designed to be refilled in orbit so it makes sense to make the ship bigger since it's part of the payload.

One can't add 500t to the Starship (Fuel + cargo + rings) and not add fuel to the booster, the rocket equation doesn't work like that.

The current deltaV of the booster is about 3.5km/sec.   That's a mass ratio (M_r) of 2.7

If you add 500t of payload to the booster (aka fuel and payload and rings for Starship), to get the same deltaV, the booster needs 850t of fuel -- (M_r-1) times payload increase

There's no place for that fuel to go without adding rings.  Each ring adds 100t of fuel capacity.   So ~8 more rings.

If there's room for 6 more rings (11 meters for the entire stack), then that's 600t of fuel.   378t of fuel for the Booster and 222t of fuel for Starship.   So 2 rings added to Starship and 4 to the Booster.

With Starship have an M_r of 6 that means added cargo of a mere 37t.

[dodageka]

You can use the 10-12m to add to the booster or the Starship.   How much for each?

Everything would go to the ship, a bigger booster can't fit in the megabay.
Starship is designed to be refilled in orbit so it makes sense to make the ship bigger since it's part of the payload.

One can't add 500t to the Starship (Fuel + cargo + rings) and not add fuel to the booster, the rocket equation doesn't work like that.

The current deltaV of the booster is about 3.5km/sec.   That's a mass ratio (M_r) of 2.7

If you add 500t of payload to the booster (aka fuel and payload and rings for Starship), to get the same deltaV, the booster needs 850t of fuel -- (M_r-1) times payload increase

There's no place for that fuel to go without adding rings.  Each ring adds 100t of fuel capacity.   So ~8 more rings.

If there's room for 6 more rings (11 meters for the entire stack), then that's 600t of fuel.   378t of fuel for the Booster and 222t of fuel for Starship.   So 2 rings added to Starship and 4 to the Booster.

With Starship have an M_r of 6 that means added cargo of a mere 37t.

That’s an additional 25% (assuming ~150t in the current configuration) which I wouldn’t describe as “mere”. Has it ever been confirmed though which version of SH/SS the performance figures quoted by SpaceX refer to? The current iteration (most certainly not), a somehow matured version of the vehicle in the current dimensions, or a potential future (already extended) one?

[wannamoonbase]

A stretched 9 engine SS has been mentioned for getting humans to Mars faster. It can also be used for bulkier loads.

I recall the origin of the 9 meter diameter was that was the maximum diameter that the Hawthorne facility could handle.  Then they moved to the port of LA with some of the carbon fiber tooling.

Not that 9 Meters or the current vehicle is 'small' by any means.

[Sarigolepas]

One can't add 500t to the Starship (Fuel + cargo + rings) and not add fuel to the booster, the rocket equation doesn't work like that.

The current deltaV of the booster is about 3.5km/sec.   That's a mass ratio (M_r) of 2.7

If you add 500t of payload to the booster (aka fuel and payload and rings for Starship), to get the same deltaV, the booster needs 850t of fuel -- (M_r-1) times payload increase

There's no place for that fuel to go without adding rings.  Each ring adds 100t of fuel capacity.   So ~8 more rings.

You don't need to get the same deltaV, you can just have stage separation earlier. So the ship would do most of the work to reach orbit.
What I'm saying is that if stage separation happends earlier you will need more TWR at takeoff because the rocket won't lose much weight before stage separation.

[Sarigolepas]

I have been wondering if a stretched 9 engine Starship would most likely be a tanker or another form of large volume same mass vehicle.

Seems both a 6 and 9 engine configuration could fly from the same Stage 0 infrastructure.

It’s a shame they stuck with the 9 meter diameter after moving the project out of Hawthorne, a 10 meter vehicle could change a lot of this debate.

It will probably be heavier but not 50% heavier.
Since stage separation will be at a lower velocity they also need to increase the TWR on the ship. And in the case of the ship they won't lose specific impulse by doing so because there is room left for more engines so they don't need to make the combustion chamber bigger.

It could be a fuel depot, mothership or space station, anything where the ship is part of the payload and not just what carries it.

[launchwatcher]

One can't add 500t to the Starship (Fuel + cargo + rings) and not add fuel to the booster, the rocket equation doesn't work like that.

The current deltaV of the booster is about 3.5km/sec.   That's a mass ratio (M_r) of 2.7

If you add 500t of payload to the booster (aka fuel and payload and rings for Starship), to get the same deltaV, the booster needs 850t of fuel -- (M_r-1) times payload increase

There's no place for that fuel to go without adding rings.  Each ring adds 100t of fuel capacity.   So ~8 more rings.

You don't need to get the same deltaV, you can just have stage separation earlier. So the ship would do most of the work to reach orbit.

earlier stage separation also means that, for a RTLS flight path, the boostback burn requires less deltaV, which reduces the booster propellant reserves needed at stage separation.   This is likely only a small benefit, though.