Hypothetical 1/3 Scale Starship

[Oersted]

Thank you all for you replies and constructive criticism.  It seems to be a consensus opinion that the second stage "Starship" would not scale down well and maintain it's planned functionality including EDL, That is certainly valid.  I perhaps should have chosen my terms better in that I was thinking more of the entire launch system including the booster which is also referred to as "Starship".  My hypothetical vehicle would have began as a 1/3 to 1/2 scale booster with an expendable (no aero surfaces / no thermal protection) upper stage.

With an expendable upper stage it would no longer make sense to refer to it as Starship. It is another launch vehicle then. Starship is by definition an attempt to overcome the huge challenge of designing a completely reusable launch system. That's the purpose baked into every facet of Starship as we know it.

[Lemurion]

Thank you all for you replies and constructive criticism.  It seems to be a consensus opinion that the second stage "Starship" would not scale down well and maintain it's planned functionality including EDL, That is certainly valid.  I perhaps should have chosen my terms better in that I was thinking more of the entire launch system including the booster which is also referred to as "Starship".  My hypothetical vehicle would have began as a 1/3 to 1/2 scale booster with an expendable (no aero surfaces / no thermal protection) upper stage.  I am imagining a booster that still is 9M or 8M diameter but much shorter (less than 40M).  Perhaps that could have provided growth opportunities by adding rings and engines to the booster later to facilitate later adding a re-usable upper stage vehicle similar to the current design.  Regardless, I understand completely that this path is moot as the "growth option" as I was describing eventually looks identical to the design as it stands today.  As another poster pointed out, if Terran R manages to reach operational status that might show if there was any validity to that approach.  Thanks Again - Bobby

Part of the problem with a further scaled down Starship is that a whole ton of issues around reusability get much harder to solve as you go smaller. You end up with less mass to play with on the upper stage so it’s less forgiving of mistakes. One of Starship’s advantages is that if they need an extra ton or two of hardware for something it’s not really a big deal. On a smaller ship it would be more of an issue as additional mass added up.

[FunBobby]

Thank you all for you replies and constructive criticism.  It seems to be a consensus opinion that the second stage "Starship" would not scale down well and maintain it's planned functionality including EDL, That is certainly valid.  I perhaps should have chosen my terms better in that I was thinking more of the entire launch system including the booster which is also referred to as "Starship".  My hypothetical vehicle would have began as a 1/3 to 1/2 scale booster with an expendable (no aero surfaces / no thermal protection) upper stage.

With an expendable upper stage it would no longer make sense to refer to it as Starship. It is another launch vehicle then. Starship is by definition an attempt to overcome the huge challenge of designing a completely reusable launch system. That's the purpose baked into every facet of Starship as we know it.

True - The name Starship would not have applied.  It would have just been a SPACEX Heavy Lift LV with a more incremental approach than was taken with today's Starship.  It still could have incorporated Rapidly Reusable Raptor engines on the booster, stainless steel construction, perhaps even landing via the chopsticks.  I do think that it's at least remotely possible (given the rate of progress on Starship), that such a vehicle could have already launched it's first revenue generating or Starlink deploying mission by now. 

[envy887]

Thank you all for you replies and constructive criticism.  It seems to be a consensus opinion that the second stage "Starship" would not scale down well and maintain it's planned functionality including EDL, That is certainly valid.  I perhaps should have chosen my terms better in that I was thinking more of the entire launch system including the booster which is also referred to as "Starship".  My hypothetical vehicle would have began as a 1/3 to 1/2 scale booster with an expendable (no aero surfaces / no thermal protection) upper stage.

With an expendable upper stage it would no longer make sense to refer to it as Starship. It is another launch vehicle then. Starship is by definition an attempt to overcome the huge challenge of designing a completely reusable launch system. That's the purpose baked into every facet of Starship as we know it.

There is a logical development sequence that goes through first making and flying an expendable upper stage that basically looks like SN5 or 6. However, the main advantage of that sequence is it spreads out the costs while still moving forward, so it only makes sense if cashflow constrained... which SpaceX is not.

[spacenut]

Well a full up starship loaded with propellants weighs close to 5000t and has a payload of around 100t. That's a mass fraction of around 2% even with the volumetric efficiencies. In this case larger is better as far as margins go.

Really thought I think that the system is sized for the smallest reasonable SSTO for the return trip from Mars to Earth.

The new Starship/Superheavy upgrade with 33 engines on the booster and 6 vacuum engines and 3 sea level engines on the Starship has an estimated payload capability of around 200 tons. 

[Twark_Main]

Amazing how closely this mirrors Elon Musk statements from over a decade ago.

(Elon said the same thing three times, all I did was arrange the thought process as one "supercut" with all the good bits)

NPC Luncheon with Elon Musk, 2011-09-29:

... the pivotal breakthrough that's necessary, that some company has got to come up with, to make life multi-planetary is a fully and rapidly reusable orbit class rocket. This is a very difficult thing to do because we live on a planet where that is just barely possible. If gravity were a little lower it'd be easier, but if it was a little higher it would be impossible.

Even for an expendable launch vehicle, where you don't have to have any recovery, after a lot of smart people have done their best to optimize the weight of the vehicle and efficiency of the engines and the guidance systems and everything, you get maybe 2 to 3% of your liftoff weight to orbit. That's not a lot of room for error. If your rocket ends up being just a little bit heavier, you get nothing to orbit, and this is why only a few countries have ever reached orbit.

Elon Musk lecture at the Royal Aeronautics Society, 2012-11-16:

So okay, well what if you want to add in the reusable bits? Adding the reusability tends to take another 2 to 3%. So then you end up with zero or negative, and there's not much point sending a rocket to orbit with nothing on it.

In the past, things have been cancelled when it looked like success was not one of the possible outcomes. In fact, usually they've been cancelled after it was clear that success was not one of the possible outcomes. [laugh]

So, the trick then, is to make a rocket that is so mass efficient that it gets close to 4% of its payload to orbit in an expendable configuration, and then improve the weight of the reusability bits, push that down to around 2% and you get a net of four minus two - so, on the order of 2% of your payload to orbit in a fully reusable scenario.

That requires paying incredibly close attention to every aspect of the rocket's design. The efficiency of the engine, the weight of the engine, the weight of the tanks, the legs, even the secondary structure, the wiring, the plumbing, and the electronics, making sure your guidance system is extremely precise, and just pulling all sorts of tricks—every trick in the book, and then coming up with some new ones—in order to achieve that level of mass efficiency.

Elon Musk Mars Pioneer Acceptance Speech, 2012-08-09:

You have to really get straight A across the board in all elements of the rocket design. Every little tiny thing. The engine efficiency, thrust to weight, the engine, the tank mass, the pressurant mass, the secondary structure, the wiring, the weight of the computers, everything matters immensely.

But if you do all those things right, then it is possible to make this work, and this is what has given me hope recently in the last few years. Because I wasn't sure whether it was possible, but in the last few years I've become convinced that it is possible.

Of course, just because something is possible doesn't mean it will occur, but I think it can occur. Success being one of the possible outcomes is very important.

That's the breakthrough that SpaceX is really trying to achieve.

[xvel]

Amazing how closely this mirrors Elon Musk statements from over a decade ago.

The laws of physics haven't changed much in the last ten years, so...

[Khadgars]

Interesting quotes from Musk there.  I dont' think SpaceX has even started mass efficiency yet on any part of Starship. 

It may end that reaching orbit was the easy part.

[volker2020]

I think at one point, Elon did mention, that the size of starship matters. Because if it was smaller, to keep the same effectiveness, the walls had to be thinner.

Just think about it. Making it shorter, means to keep the same performance ratios, you need thinner steal ...
At one point, it won't work at all. So I really doubt, that shrinking the design without having a closer look at the production, really makes sense.

At least I am quite optimistic, that they can make it bigger.

[Robotbeat]

Interesting quotes from Musk there.  I dont' think SpaceX has even started mass efficiency yet on any part of Starship. 

It may end that reaching orbit was the easy part.

Starship doesn’t need much design efficiency to be viable for initial uses because it can be refueled.

Starship, like Falcon 9, will be continually optimized over time.

I’d say orbit and recovery (and turnaround for relaunch) are the hard parts because once that is accomplished, you have a viable vehicle for Starlink and HLS.

You need 5 launches vs 15 launches per HLS mission depending on mass efficiency… but it can still work.

Getting it to work is the hardest part. Optimizing it will happen over time.

[Robotbeat]

I think at one point, Elon did mention, that the size of starship matters. Because if it was smaller, to keep the same effectiveness, the walls had to be thinner.

Just think about it. Making it shorter, means to keep the same performance ratios, you need thinner steal ...
At one point, it won't work at all. So I really doubt, that shrinking the design without having a closer look at the production, really makes sense.

At least I am quite optimistic, that they can make it bigger.

Steel can be extremely thin. Atlas Centaur has stainless steel tank thickness of jus 0.5mm, versus 3-4mm for Starship.

[Okie_Steve]

In the past Musk made comments that some interpreted as a larger future 18 meter rocket was under consideration. He also made comments that maybe a smaller 7 meter rocket would have been an easier choice than 9 meters. Neither of which matters at this point of course since, as he observed when discussing stretching things, longer is relatively straight forward while changing the diameter is incredibly hard.

So, there might be a case for a slightly smaller reusable rocket but larger apparently has a stronger case.

[sebk]

I think at one point, Elon did mention, that the size of starship matters. Because if it was smaller, to keep the same effectiveness, the walls had to be thinner.

Just think about it. Making it shorter, means to keep the same performance ratios, you need thinner steal ...
At one point, it won't work at all. So I really doubt, that shrinking the design without having a closer look at the production, really makes sense.

Steel can be thinner. But heatshield tiles can't. And they do not weigh nothing, far from it. The shield must be very roughly 10cm thick, and at 144kg/m^3 this means 14kg per m^2. With 500-600m^2 of shield this is 8 tonnes. It's about 7% of payload mass. If you'd go with 1/3 mass SSH your shield mass would not decrease 3x but about 2x.

[zubenelgenubi: edit for legibility]

[volker2020]

I think at one point, Elon did mention, that the size of starship matters. Because if it was smaller, to keep the same effectiveness, the walls had to be thinner.

Just think about it. Making it shorter, means to keep the same performance ratios, you need thinner steal ...
At one point, it won't work at all. So I really doubt, that shrinking the design without having a closer look at the production, really makes sense.

At least I am quite optimistic, that they can make it bigger.

Steel can be extremely thin. Atlas Centaur has stainless steel tank thickness of jus 0.5mm, versus 3-4mm for Starship.

Yes Steel can be extremely thin, but it only works, when under pressure. Not having that pressure at production, means you need extensive skeletons to keep the structures in places, which is a hazard for production. Everything becomes more complicated, starting with welding. I did not say, that it was impossible to build it, but that you need to have a look at the production costs.

[Keldor]

I think at one point, Elon did mention, that the size of starship matters. Because if it was smaller, to keep the same effectiveness, the walls had to be thinner.

Just think about it. Making it shorter, means to keep the same performance ratios, you need thinner steal ...
At one point, it won't work at all. So I really doubt, that shrinking the design without having a closer look at the production, really makes sense.

At least I am quite optimistic, that they can make it bigger.

Steel can be extremely thin. Atlas Centaur has stainless steel tank thickness of jus 0.5mm, versus 3-4mm for Starship.

Yes Steel can be extremely thin, but it only works, when under pressure. Not having that pressure at production, means you need extensive skeletons to keep the structures in places, which is a hazard for production. Everything becomes more complicated, starting with welding. I did not say, that it was impossible to build it, but that you need to have a look at the production costs.

The thickness they need increases proportionally to radius (as the curvature decreases, tension increases at a constant pressure), as well as height (a taller fuel stack means more pressure at the bottom).  Starship is 3 times wider and the fuel tank is also 2-3 times as tall, so if we were to just scale Atlas-Centaur up, naively we'd need walls 6-8 times thicker, which just happens to be 3-4mm.

[Nomadd]

Steel can be thinner. But heatshield tiles can't. And they do not weigh nothing, far from it. The shield must be very roughly 10cm thick, and at 144kg/m^3 this means 14kg per m^2. With 500-600m^2 of shield this is 8 tonnes. It's about 7% of payload mass. If you'd go with 1/3 mass SSH your shield mass would not decrease 3x but about 2x.

Where in the world are you getting 10cm for the heat shield?

[sebk]

Steel can be thinner. But heatshield tiles can't. And they do not weigh nothing, far from it. The shield must be very roughly 10cm thick, and at 144kg/m^3 this means 14kg per m^2. With 500-600m^2 of shield this is 8 tonnes. It's about 7% of payload mass. If you'd go with 1/3 mass SSH your shield mass would not decrease 3x but about 2x.

Where in the world are you getting 10cm for the heat shield?

From different thread (about Starship heatshield). To ensure acceptable skin under the heatshield temperature you need about 11cm thick Li-900 heatshield for aluminum structures, but about 8-9cm for stainless steel.

 Note that in the case of Starship you have not only just the tiles but also the "mineral felt" backing material. Looking at photos and stuff it seems that both materials together are in the order of 3 inches thick. Am I wrong?

[livingjw]

I put together a quick scaling model. You start with an existing vehicle's mass breakdown. Different items scale at different rates.

Note: Weight spread sheet available here:
 https://forum.nasaspaceflight.com/index.php?topic=50049.msg2149131#msg2149131
I don't know if it has been updated. We should ask equiserre.

John

[Nomadd]

Steel can be thinner. But heatshield tiles can't. And they do not weigh nothing, far from it. The shield must be very roughly 10cm thick, and at 144kg/m^3 this means 14kg per m^2. With 500-600m^2 of shield this is 8 tonnes. It's about 7% of payload mass. If you'd go with 1/3 mass SSH your shield mass would not decrease 3x but about 2x.

Where in the world are you getting 10cm for the heat shield?

From different thread (about Starship heatshield). To ensure acceptable skin under the heatshield temperature you need about 11cm thick Li-900 heatshield for aluminum structures, but about 8-9cm for stainless steel.

 Note that in the case of Starship you have not only just the tiles but also the "mineral felt" backing material. Looking at photos and stuff it seems that both materials together are in the order of 3 inches thick. Am I wrong?

The underlayment varies, but the tiles are only about an inch thick at the edges and where the brackets are, and only about 5/8 inch for the rest. That's from pre SN15 samples. (Most of which were used up in an attempt to build a heat shield for a Harley exhaust pipe)

[FunBobby]

So, the scaling guide posted above is quite interesting.  If I am reading it correctly, one of the big advantages of really large rockets is that the mass percentage of TPS scales as a less than linear function and TPS is one of if not the biggest dry mass addition for a reusable rocket.  Does that also then support the argument for short stubby rocket stages since they will have reduced surface area to hold a given amount of fuel and propellant and that is less to shield?  At least maybe that argument is valid for something entering ballistic, maybe it no longer applies if you need to generate lift as well.  I also understand that a rocket stage shaped like a beach ball would introduce other challenges for construction and transportation. 

As far as transportation, I had a University Professor over 20 years ago who liked to say that the SRB diameter for the Shuttle was set by the width of a horses ass..........Initial rail lines in Europe and N.A. where of course based on a standardized track width to worth with a standardized train size.  Many of those lines were built on high volume roads that had existed for centuries which needed to allow passage of horse drawn traffic in both directions.  Due to SRBs being built in Utah and the Shuttle launching from Florida, rail transit was highly desired so the segments had to fit onto rail cars.......as part of trains.......running on standardized tracks.........okay so maybe embellished a little, but a maybe a little bit of truth to it.  So the so what for me is that once your rocket stage is already to big to put on railroad cars or pass under highway bridges or load onto airplanes, why not just make it in the optimized shape/size relative to the mass of your launch vehicle?