Why are even the biggest rockets so light?

Pds314 · May 15, 2020

Rockets feel like they're huge. They look huge. But even a very rudimentary comparison to aircraft or oceangoing vessels show that the empty mass of the largest rockets ever constructed is still less than 200 tonnes.

Compare, the largest aircraft, around 300 tonnes empty.

Or the largest aircraft carriers, over 100000 tonnes empty.

Land vehicles like large bucket wheel excavators can be over 14000 tonnes empty.

And even tanks are competitive with the largest rockets in empty weight, with the Panzer VIII Maus being nearly 200 tonnes empty.

Even steel-and-wood sailing ships reached something like 8000 tonnes empty.

So the question is, why is this the case? Clearly rocket designers are more likely to shave weight, even at the cost of increased expense, but why has nobody built, f.e. a rocket an appreciable fraction the size of the Seawise giant, but lighter when empty? Is it lack of demand for the ultra-heavy lift payloads such rockets would have to fly? Is it some kind of structural or hydraulic problem of making rockets hundreds of meters tall full of fuel? Surely it can't be sheer loading from atmospheric forces, as rockets really are only designed to take much stress, albeit several G of it, from one primary direction.

Anyway, why don't we have rockets with empty stage masses of thousands or tens of thousands of tonnes and fuel loads of hundreds of thousands or millions of tonnes?

Edited May 16, 2020 by Pds314

magnemoe · May 16, 2020

For an ship dry weight don't matter much, only cargo capacity, safety and cost. its an benefit to make ships very large if the cargo capacity demand is there as fuel use, crew cost and maintenance grow slower than cargo capacity as you scale up.
Downsides is that you now only have one giant ship who can not fit through canals and is limited to the ports it can use. Some very large oil tankers was build back then the Suez canal was blocked. They became uneconomical once it re-opened
For stuff on roads, above 50-70 ton and you are special transports. Say you have an brigade of tanks on flatbeds you get restrictions on how many you could have on an bridge. If you simply transport iron bars its cheaper to just use more trucks than getting into the special category.

For rockets it has not been much demand for very heavy payloads after the moon landings and satellites and other space stuff has an long development cycle and nobody designs payloads to heavy to lift.

Starship and probably new Glen plans to kick down that wall , starship + superheavy will probably have an higher dry mass than most planes.
Planes has the same issue, not much demand for very heavy airlift. Yes its exist but is limited, https://en.wikipedia.org/wiki/Antonov_An-225_Mriya can lift 250 tons but only one is build, next down the list An-124 is 150 ton similar to Starship estimate.

mikegarrison · May 16, 2020

Rockets are structurally light because we want them to be light. We spend a lot of money and effort to make them light. It almost sounds like you think we should want them to be heavier.

May 16, 2020

3 hours ago, Pds314 said:

Is it some kind of structural or hydraulic problem of making rockets hundreds of meters tall full of fuel?

Pretty much that. Try that in KSP and see how far this gets you. Also, there's a matter of launch facilities. You need a VAB that's taller than the rocket, or you need to be able to erect your rocket from horizontal to vertical. Neither of these is a trivial problem, and both get worse with increasing height.

Another matter is that large engines are hard. RD-170 had a 6-chamber version proposed, and that would be, I think, the most powerful rocket engine ever. With too large chambers, you get combustion instability. You could just cram in more chambers and/or more engines, but that's a plumbing nightmare. Generally, unless you're going for economics of scale (a new approach, one that requires extremely reliable engines), the less engines, the better.

Big Dumb Booster is a concept, and rockets of that sort have been proposed, such as Sea Dragon. This never got very far, the closest we got are Russian rockets, which are rather simpler than the US ones, but they do get the job done. Generally, there is indeed no market for something that big.

kerbiloid · May 16, 2020

Because they don't have wings.

And eat more fuel per kg of payload (as the plane's oxidizer is outside).

Edited May 16, 2020 by kerbiloid

cubinator · May 16, 2020

How far a rocket can go is greatly determined by its full/dry weight ratio. This is the part inside the natural log in the rocket delta-V equation. Let's say you have a rocket that is 50% fuel and 50% rocket, which would be pretty high for an airplane but low for a rocket. Here's the delta-V equation:

$\Delta v=v_{\text{e}}\ln {\frac {m_{0}}{m_{f}}}=I_{\text{sp}}g_{0}\ln {\frac {m_{0}}{m_{f}}}$

Let's say the rocket is 100 tons when full, and it has an Isp of 300. Putting those numbers into the delta-V equation (g₀ is Earth's gravity btw), the result is that the rocket can accelerate to 2039.9 meters per second in open space. Now, if the rocket is 75% fuel and 25% rocket, the result changes: 4079.9 m/s. And if the rocket were only 40% fuel and 60% rocket (a value typical for an airliner) it could only go 1503.4 m/s. In the 50/50 rocket, a quarter of your velocity budget comes from that last 10% of fuel.

So we can see that it's really best to optimize a rocket to have a very high full/dry weight ratio, no matter how heavy the rocket is overall. That's why engineers like to make rockets that have mass fractions of 80% or 90% fuel, because it means they can push the remaining 20% or 10% of payload even further, sometimes making the difference between one planet and the next.

Bill Phil · May 16, 2020

This is because tanks to hold liquids are a well understood technology and the structural strength of the materials used is much larger than needed. So supporting the fully loaded rocket against the engine thrust doesn't require as much structural mass as it may initially seem and the tanks themselves are optimized to have low weights but hold huge amounts of propellant.

As to why there aren't larger rockets... they're not needed, for one. And even if there was some desire for them they would require infrastructure that just doesn't exist. Except maybe a Sea Dragon, though that's a big maybe. And there are limits to the rocket scaling. Though where those limits are depends on the exact vehicle design.

KerikBalm · May 18, 2020

Saturn V loaded mass is nearly 3,000 tons. That's not light...

wumpus · May 18, 2020

On 5/16/2020 at 10:14 AM, cubinator said:

How far a rocket can go is greatly determined by its full/dry weight ratio. This is the part inside the natural log in the rocket delta-V equation. Let's say you have a rocket that is 50% fuel and 50% rocket, which would be pretty high for an airplane but low for a rocket. Here's the delta-V equation:

$\Delta v=v_{\text{e}}\ln {\frac {m_{0}}{m_{f}}}=I_{\text{sp}}g_{0}\ln {\frac {m_{0}}{m_{f}}}$

Let's say the rocket is 100 tons when full, and it has an Isp of 300. Putting those numbers into the delta-V equation (g₀ is Earth's gravity btw), the result is that the rocket can accelerate to 2039.9 meters per second in open space. Now, if the rocket is 75% fuel and 25% rocket, the result changes: 4079.9 m/s. And if the rocket were only 40% fuel and 60% rocket (a value typical for an airliner) it could only go 1503.4 m/s. In the 50/50 rocket, a quarter of your velocity budget comes from that last 10% of fuel.

So we can see that it's really best to optimize a rocket to have a very high full/dry weight ratio, no matter how heavy the rocket is overall. That's why engineers like to make rockets that have mass fractions of 80% or 90% fuel, because it means they can push the remaining 20% or 10% of payload even further, sometimes making the difference between one planet and the next.

Oddly enough, this equation still leaves the question of "why do rockets have such a low dry mass" unanswered for the first stages ignited.

The last stage has a dry weight of itself+the payload. The next stage has a dry mass of itself + the last stages wet weight + the playload (and so on). So the earlier you light the stages, the less important dry weight becomes (assuming you can still meet the overall delta-v requirements). Finally for extra boosters (whether a 2.5 stage configuration or those SRBs that ring some uncrewed rockets), TWR is nearly all that matters, and both Isp and dry mass mostly change delta-v linearly.

- On the other hand, I think the delta rocket family (or a similar ULA rocket) switched from steel SRBs to carbon fiber. Presumably that tiny bit of delta-v was worth it (or they were gold-plating it, a real possibility).

- this is one of those things KSP teaches you wrong. SRBs are not all that cheap nor reliable, and thus risking the payload (or worse, the crew) on an SRB not exploding is avoided when possible.

magnemoe · May 18, 2020

5 hours ago, wumpus said:

Oddly enough, this equation still leaves the question of "why do rockets have such a low dry mass" unanswered for the first stages ignited.

The last stage has a dry weight of itself+the payload. The next stage has a dry mass of itself + the last stages wet weight + the playload (and so on). So the earlier you light the stages, the less important dry weight becomes (assuming you can still meet the overall delta-v requirements). Finally for extra boosters (whether a 2.5 stage configuration or those SRBs that ring some uncrewed rockets), TWR is nearly all that matters, and both Isp and dry mass mostly change delta-v linearly.

- On the other hand, I think the delta rocket family (or a similar ULA rocket) switched from steel SRBs to carbon fiber. Presumably that tiny bit of delta-v was worth it (or they were gold-plating it, a real possibility).

- this is one of those things KSP teaches you wrong. SRBs are not all that cheap nor reliable, and thus risking the payload (or worse, the crew) on an SRB not exploding is avoided when possible.

First stage dry mass still matter, its just 5-10 times less critical. Note that for an disposable rocket, increasing the first stage size will reduce the TWR but increase dV a bit is beneficial as fuel and larger tanks are cheaper than engines.
For reuse you want much higher TWR as you want to get second stage up to speed fast so you can land first stage, this is even more critical if you do boostback.

Composite SRB, well that sounds a bit weird, SRB is about trust not ISP or TWR, yes it could be gold platting as in wanting to get composite experience.
Think most KSP players agree that 2.5 stage rockets with SRB, is the most efficient design, in KSP the second stage is also the payload in most cases this is also the most common design for modern rockets before falcon 9.

magnemoe · May 18, 2020

On 5/16/2020 at 8:56 PM, Bill Phil said:

This is because tanks to hold liquids are a well understood technology and the structural strength of the materials used is much larger than needed. So supporting the fully loaded rocket against the engine thrust doesn't require as much structural mass as it may initially seem and the tanks themselves are optimized to have low weights but hold huge amounts of propellant.

As to why there aren't larger rockets... they're not needed, for one. And even if there was some desire for them they would require infrastructure that just doesn't exist. Except maybe a Sea Dragon, though that's a big maybe. And there are limits to the rocket scaling. Though where those limits are depends on the exact vehicle design.

This, also the fueled rocket is pressurized who increases structural strength.
As for larger rockets its no need, yet. Both SLS, long march 9/10, Starship and to some degree New Glen is politically / ideologically motivated. For SLS, China its return to the moon and later mars, for SpaceX its colonizing mars, for Blue Origin its colonizing space.

It was no need for gasoline before cars, the first oil wells was about kerosene for lamps and oil for lubricant on wheels and machines. You burned off the gasoline as it was dangerous, Like gas was burned off 30 years ago.
Some year after producing oil they found they could run steam engines on oil, this was critical on warships as it was denser, burned cleaner and you don't need crew showing coal. All nice if you had plenty of oil and geared to fight all over the world.

Bill Phil · May 18, 2020

10 hours ago, KerikBalm said:

Saturn V loaded mass is nearly 3,000 tons. That's not light...

And the Saturn V dry mass was much, much smaller. The S-IC represented the bulk of the rocket when fully loaded with propellant. Nearly 2300 tonnes full, but only around 130 empty. Compared to the total mass of the vehicle when fully loaded, the empty mass was very light. S-IVB empty was less than 15 tonnes, and the S-II was around 36 tonnes empty.

kerbiloid · May 19, 2020

It's funny when you count the tanks in tanks (i.e. big metal barrels in armored combat vehicles) and put aside an empty rocket and corresponding number of tanks.

Soyuz ~33 t

Spoiler