How to land over one thousand tons of cargo with a single 9m diameter Starship on Mars.

Fun idea! One issue is the limiting factor for cargo per Starship to Mars is largely aerobraking on arrival.

The sims we have seen show Starship needing to pull a 5 g inverted aerobraking with the lift vector pointed at Mars just to stay in the atmosphere long enough to slow down.

Your stretched ship with 1000 tons of cargo has a much higher ballistic coefficient than a normal ship, making aerobraking much harder. To compensate, you would need to also increase the aerobraking cross section of the stretched Starship, which will increase dry mass and much of the gains evaporate.

You can do the same thought experiment with non stretched Starship. It has plenty of delta-v to leave LEO (or a high energy orbit) with a higher payload mass than it can aerobrake.

The ship is in this case on a slow transfer from Earth to Mars so it should be fine. It can also use multiple passes to slow down. The old ITS design had a surface area of about 600 m² and a mass of 850t (150t dry mass, 450t payload and 250t propellant) when it entered the atmosphere. The long Starship have a surface area of 1080m² and a mass of 1200t, it should actually have a easier time than ITS.

A “slow transfer” doesn’t mean you don’t have to aerobrake, you are still going from an interplanetary transfer to stationary on the surface.

I believe modeling showed ITS had a significantly higher lift to drag ratio so you'd need account for that in the comparison.

The L/D ratio on ultra long Starship is not going to be great, meaning higher g load on entry.

It also much more mass efficient to make a fat stubby ship strong horizontally. Long Starship is going to have some big horizontal loads to deal with while aerobraking, meaning comparatively much higher structural mass.

-2

The long cylinder would not have much bending stress during reentry because all the mass is distributed relatively evenly over the entire length of the rocket, also the 3bar pressure in the main tanks help a great deal with stability.

cshotton

Why would you think that? With aero surfaces at each end, the aerodynamic forces are higher at the ends than in the middle, if nothing else. Given the longer moment arm, the difference stresses/loads in the center would more or less be double that of a ship that was half the length, assuming even cargo distribution.

When using the flaps to adjust angle of attack while decelerating at 5 g, the forces needed move the heavy ship quickly enough are quite high.

sebaska

Even with slow pass you must drop about 0.8km/s in one go, or you're going back into a heliocentric trajectory.

That's the key difference between aerobraking and aerocapture. The latter has a minimum ∆v which must happen in one go.

KnifeKnut

Would Starship be able to aerocapture without an engine burn instead of trying to do it all in a single dive into the the martian atmosphere?

Yep and that's one of the options often explored. Do one aerobraking pass to slow to under escape velocity, let the heat shield and structure cool off, then do a second pass.

It mostly helps with reducing heat soak into the structure. Peak heating (the hottest bit) and peak forces happen very early on so are not reduced by multiple passes.

Even with a fairly leisurely 7 km/s entry velocity, half the overall kinetic energy needs to be be shed when aerobraking from 7 km/s down to 5 km/s. (the other half is shed going from 5 km/s down to zero.)

dgkimpton

I can't help but think a rocket that tall is going to wobble like a wet noodle. There comes a point where to keep the rocket straight the walls would have to be so thick it just isn't worth it over a larger diameter rocket. I don't know what that point is, but this certainly looks iffy.

toastedcrumpets

You're talking about the fineness ratio and falcon 9 maxes this out. Just compare against that!

The real issue is wind shear.

dgkimpton

That's the term I was hunting for indeed.

Yes, perfect weather is adviced when launching the long ship. Luckily only 1 out of 18 launches need good weather.

WorkO0

Isn't the structural integrity mostly held up by pressure of the internal tanks?

dgkimpton

Yes and no. End-to-end loading yes, but flexing isn't quite so easy to deal with (at least from my understanding) once it gets past a certain ratio of length to diameter, presumably due to all the leverage that is brought to bear. Sadly I'm not a rocket engineer and don't know all the details but I remember it being talked about with F9 and how they couldn't extend it further without running into those issues.

NateHotshot

❄️ Chilling

At what point would it be better to add another stage in between?

It's not worth it in my opinion, only 1 of 18 launches is done with the long ship. It's better to refill a striped down Starship in a highly elliptical orbit and add 8km/s above Earth escape velocity.

cshotton

If it is only 1 in 18, it makes no sense to incur all of the added development cost and risk. Just do a few more launches of the standard design and dispense with all of the R&D and risk of this "long" version.

MoonTrooper258

I actually thought I was on r/shittyspacexideas until I was reading the comments.

-2

I just like to take things to the extreme. The ship dry mass is about 200t, I think it could get into orbit weighing 400t, so we could make it ~twice as long. 2000t cargo to Mars may be possible!

You’re ignoring the extensive stiffening and reinforcements that would be needed.

If this isn’t a shitpost dear god man.

dotancohen

He's also ignoring thrust. High Isp is great when you're already in orbit, but how does he plan on getting that first stage off the planet? SRBs?

Yeah, I’m not really taking this too seriously because I would lose my mind knit picking.

The long ship will not have any payload when it is launched and it only need about 1200t of propellant to reach orbit. The full stack mass of the long ship will actually be lighter than a ordinary Starship launch.

dotancohen

Can you roughly point out where the center of mass will be in that configuration? And the center of pressure? Are you sure that the first stage engines can gimbal enough for that without needing fins at the bottom? Are you familiar with the term "fineness"?

Just eyeballing it, probably at the gridfins, so 70m upp. ~3/5 off the rocket cylinder is above the center of mass. The launch profile would be modified to give a gentle maxQ and the rocket should only launch during excellent weather conditions. SpaceX plans to make Starship 150m tall so I don't think a 33% taller rocket is a deal breaker.

dotancohen

You can't eyeball center of mass on a rocket. The densities of the propellants are different, and nowhere near the density of the engines. Of course the center of pressure is near the gridfins. Where near the gridfins? How much AoA can the raptors gimbal to account for in that configuration?

gentle maxQ

So, incredibly horrible gravity losses?

the rocket should only launch during excellent weather conditions.

So will have a low launch cadence and be missing schedules with many expensive scrubs?

I'm not knocking your idea. I'm trying to see how practical it is.

-3

I could have worded it better, essentially the tank needs to be 4mm to handle the hydrostatic pressure when the rocket is accelerating at 3g near hot staging. So I modell the tank as 5mm to account for the stringers. The same logic applies to the cargo area(2.5mm skin and stringers equivalent of 1.5mm skin for a total of 4mm).

An extra mm of steel isn’t going to support a 1000 metric ton payload at max-q.

Your mass calculation omit the stringers and other structural parts as far as I can see.

Jak_Extreme

I think at that height, it would be a no brainer to increase diameter. Would help with structural integrity and you could make it not so tall or at least give it more power

CremePuffBandit

⛰️ Lithobraking

Delta V is important, but only if it can actually get off the ground. I know the current booster has a pretty high TWR at launch, but this has got to be pushing it.

ResidentPositive4122

After a certain point it would be better to just increase the number of stages. I still have an intuition that we'll see that coming later. A stage 2 that is fully reusable and just pushes "stage3" as much as it can and then lands downrange. Basically a shorter Starship, no payload section, full tanks. S3 can be anything, doesn't even need to return, can be space station modules or whatever.

-1

Orbital refilling is the stage 3 in the Starship architecture, so there is no need to make the design more complicated for little gain.

One question remains: why not land directly but enter a low Mars orbit?

sebaska

They stated it: To cut down ∆v of the mission ship by 1.5km/s.

Such a heavy vehicle would require several hundred tonnes of landing fuel.

To cut down ∆v of the mission ship by 1.5km/s.

How do you get to that number?

sebaska

Ask the OP. But this sounds about right for the landing burn if you're landing 1200t rather than 200t vehicle. You need to do much more propulsive braking.

brekus

Because it won't have enough fuel to land.

HomeAl0ne

It takes much more delta-v to enter Martian orbit (especially a low one) than to use aerobraking to shed most of your velocity and land directly.

That's not what I meant nor what is discussed in the post.

Aerobraking is used in both cases.

But OP wants to enter LMO instead of landing. Why?

Because it's easier for the tanker launched from Mars to get into a low Mars orbit than a high one. By refilling in Mars orbit the ship only need to provide 4km/s for the TMI burn.

refilling in Mars orbit the ship only need to provide 4km/s for the TMI burn.

You can't make a Trans Mars Injection Burn in LMO. Do you mean a burn back to earth?

playwrightinaflower

Because it's easier for the tanker launched from Mars to get into a low Mars orbit than a high one

Why spend fuel to land the fuel on Mars and spend even more fuel to launch the fuel you previously landed from Mars again ?

VdersFishNChips

You're going to need (probably quite beefy) landing legs too.

HomeAl0ne

And a much longer elevator down from the top. I can just imagine the scene as the first 5 astronauts enter the cage and are slowly lowered down. While they are staring out over the Martian landscape there’s this tinny elevator Muzak playing in their helmets. They shuffle in place nervously, one looking at another and then down at his boots awkwardly…

The ship is caught by a tower on Mars.

cshotton

🙄

So not only do you have to engineer a special rocket for a 5% use case, but you have to design, engineer, ship, and construct a special landing tower? And also orbit a GPS satellite equivalent with ground beacons for precise guidance? You might want to model the economics of this next so you can understand why "tecnically feasible" doesn't equate to "economically viable."