totm nov 2023 SpaceX Discussion Thread

[magnemoe]

46 minutes ago, Nothalogh said:

Well, an F-15 can break the sound barrier in a vertical climb, with a full weapon and fuel load.

So, I think it has that nailed.

The issue would be controllability in that retrograde descent.

Many moons ago, the USAF and USN played around with tailsitter VTOLs, and found them to be nightmarish creatures.

My suspicion is that BFR and SS are going to be a lot bigger by the time one gets sent to mars.

 

Tailsitters was nightmares for pilots to land. Its probably far easier for computers to handle and something who might be worth looking into again. 
One issue for carrier operations is however that you can take off with larger weight horizontal as your TWR can be below 1. 

[Nothalogh]

25 minutes ago, magnemoe said:

Tailsitters was nightmares for pilots to land. Its probably far easier for computers to handle and something who might be worth looking into again. 
One issue for carrier operations is however that you can take off with larger weight horizontal as your TWR can be below 1. 

Indeed

[tater]

47 minutes ago, magnemoe said:

So starship is a bit to large for an optimal satellite delivery rocket, yes as its designed for Mars and deep space. 

This is the problem I often see. Mass, diameter, volume? None of those matter even a little. All that matters is cost. For extant satellites, all that matters is retail cost the customer pays, and for larger payloads (or SpaceX's own projects) cost per kg.

If they can fly the thing for 5 M$, as long as it is price competitive for a given sat size, its excess capability doesn't matter at all. 

RocketLab charges 5M$ for 225kg (max). If SS can fly for 5M$, they could launch each Starlink with a single SS and it would be the same price as RocketLab. Maybe they make no money doing that, and bespoke launches are cheaper with smallsat providers for 225kg sized payloads. What if those customers could pay $20,000 for the same payload as Electron, but they have to be part of a rideshare? Do they pony up the 5 million, or take a less perfect orbit for 20k? How about they pay 30k, and add a kickstage?

[Nothalogh]

7 hours ago, mikegarrison said:

Zubrin has always been "visionary", which is usually another word for crazy.

The problem with Zubrin is that he's not crazy enough

[magnemoe]

4 hours ago, tater said:

This is the problem I often see. Mass, diameter, volume? None of those matter even a little. All that matters is cost. For extant satellites, all that matters is retail cost the customer pays, and for larger payloads (or SpaceX's own projects) cost per kg.

If they can fly the thing for 5 M$, as long as it is price competitive for a given sat size, its excess capability doesn't matter at all. 

RocketLab charges 5M$ for 225kg (max). If SS can fly for 5M$, they could launch each Starlink with a single SS and it would be the same price as RocketLab. Maybe they make no money doing that, and bespoke launches are cheaper with smallsat providers for 225kg sized payloads. What if those customers could pay $20,000 for the same payload as Electron, but they have to be part of a rideshare? Do they pony up the 5 million, or take a less perfect orbit for 20k? How about they pay 30k, and add a kickstage?

I agree with you size don't matter just cost. However an smaller starship would be cheaper to operate: cheaper to move, using less fuel and probably cheaper to maintain. 
But starship objective is Mars and deep space and size is important especially for refiling. 

[tater]

2 hours ago, magnemoe said:

I agree with you size don't matter just cost. However an smaller starship would be cheaper to operate: cheaper to move, using less fuel and probably cheaper to maintain. 

9m is substantially smaller than the initial concepts. I tend to think that the minimum possible size for full reuse is probably not all that much smaller than Starship, or at least that making things smaller might actually be substantially harder.

[Xd the great]

51 minutes ago, tater said:

9m is substantially smaller than the initial concepts. I tend to think that the minimum possible size for full reuse is probably not all that much smaller than Starship, or at least that making things smaller might actually be substantially harder.

Less mass to absorb the heat?

[StrandedonEarth]

25 minutes ago, Xd the great said:

Less mass to absorb the heat?

Less payload margin to use for recovery, coupled with square-cube law (tankage mass does not rise as quickly as propellant mass, and drag becomes less significant.)

Edited October 31, 2019 by StrandedonEarth

[sh1pman]

21 minutes ago, StrandedonEarth said:

Less payload margin to use for recovery, coupled with square-cube law (tankage mass does not rise as quickly as propellant mass, and drag becomes less significant.)

Unrelated: are there factors that are negatively affected by square-cube law when scaling up rockets? I can only think of one: TWR. A rocket that is scaled up by a factor of n in all dimensions will have its wet mass increased by ~n^3, but surface area available for mounting engines will only increase by a factor of n^2. So in order to maintain constant TWR you’ll need engines with more thrust/m^2 or some kind of widening base for mounting even more engines.

Edited October 31, 2019 by sh1pman

[tater]

8 minutes ago, sh1pman said:

Unrelated: are there factors that are negatively affected by square-cube law when scaling up rockets? I can only think of one: TWR. A rocket that is scaled up by a factor of n in all dimensions will have its wet mass increased by ~n^3, but surface area available for mounting engines will only increase by a factor of n^2. So in order to maintain constant TWR you’ll need engines with more thrust/m^2 or some kind of widening base for mounting even more engines.

 

[Elthy]

Isnt even Starship/Superheavy already limited by this? Raptor has quite a high thrust/area number and even there the engines are sticking out below the booster a bit (6 beneath the legs). It will propably be hard to build a methalox rocket taller than 150m without dropable sideboosters.

[sh1pman]

8 minutes ago, Elthy said:

Isnt even Starship/Superheavy already limited by this? Raptor has quite a high thrust/area number and even there the engines are sticking out below the booster a bit (6 beneath the legs). It will propably be hard to build a methalox rocket taller than 150m without dropable sideboosters.

Elon mentioned that the next gen rocket after Starship will be 18m wide. I wonder how they’re going to solve the TWR problem. 

[tater]

It's interesting that the huge SSTO (some have drop tanks, though) VTVL concepts from the 60s/70s look like this:

Spoiler

 

 

[magnemoe]

12 minutes ago, Elthy said:

Isnt even Starship/Superheavy already limited by this? Raptor has quite a high thrust/area number and even there the engines are sticking out below the booster a bit (6 beneath the legs). It will propably be hard to build a methalox rocket taller than 150m without dropable sideboosters.

It is, add that an reusable first stage want high TWR, you want to get second stage up to speed fast so don't go so far downrange. 
On an disposable rocket you want an pretty low TWR as fuel and tanks are cheaper than engines 

You could make the first stage cone shaped however, or just make the thing thicker. 

[sh1pman]

8 minutes ago, magnemoe said:

or just make the thing thicker. 

thicc

[Rakaydos]

Note that packing efficiency (the proportion of engine thrust to area to rocket thrust to area) improves with larger engine clusters. A wider rocket can also be slightly taller, just from reducing inefficiency.

[kerbiloid]

Spoiler

1 hour ago, sh1pman said:

Elon mentioned that the next gen rocket after Starship will be 18m wide. I wonder how they’re going to solve the TWR problem. 

They use TweakScale.

 

[Codraroll]

3 hours ago, sh1pman said:

Elon mentioned that the next gen rocket after Starship will be 18m wide. I wonder how they’re going to solve the TWR problem. 

Presumably, by making the rocket shorter.

[CatastrophicFailure]

So like... are they gonna stack and finish it first or test fire as is? There’s a certain logic to that, actually... they’ve never lit off three Raptors at once AFIAK...

[Xd the great]

1 hour ago, CatastrophicFailure said:

So like... are they gonna stack and finish it first or test fire as is? There’s a certain logic to that, actually... they’ve never lit off three Raptors at once AFIAK...

Maybe. To see if the raptors have some sort of syncing oscillation which will tear the booster apart.

[Nothalogh]

11 hours ago, StrandedonEarth said:

square-cube law (tankage mass does not rise as quickly as propellant mass, and drag becomes less significant.)

This is the most underestimated part about this whole design, and the main reason why I see the design getting a lot bigger by the time development is complete

[kerbiloid]

The air drag is just about 1 km/s of 10. And it won't disappear, just will take, say, 800 m/s instead of 1000.

On 10/31/2019 at 6:15 PM, sh1pman said:

thicc

Starship XL

[CatastrophicFailure]

I tell you, I... canard-ly wait for them to finish! 

[Xd the great]

2 hours ago, CatastrophicFailure said:

I tell you, I... canard-ly wait for them to finish! 

Looking foward to recreate said BOOM!

[satnet]

On 10/31/2019 at 9:37 AM, sh1pman said:

Unrelated: are there factors that are negatively affected by square-cube law when scaling up rockets? I can only think of one: TWR. A rocket that is scaled up by a factor of n in all dimensions will have its wet mass increased by ~n^3, but surface area available for mounting engines will only increase by a factor of n^2. So in order to maintain constant TWR you’ll need engines with more thrust/m^2 or some kind of widening base for mounting even more engines.

Another negative impact of the square-cube law is that expander cycle rockets are limited to ~300 kN of thrust. This is because the surface area over which you can extract heat grows slower than the volume you need to fill with fuel and at some point you can't extract enough to run the pumps to fill the volume.