How large would a mars sample return ascent stage need to be

[xenomorph555]

Just a quick question that has been bugging me, NASA and others keep talking about a Mars SRM but... wouldn't they need to bring a large multistage rocket?

Mars has atmosphere, is much larger then the moon and has more gravity. It's not as simple as a moon SRM.

[Starman4308]

Mars has atmosphere

The atmosphere on Mars is barely there: it is ~0.006 atm at surface level. This means that, in theory, you could get into Mars orbit with a rocket approximately the size of a pencil. Disregarding practical issues like fuel flow, you can scale down infinitely and retain the same delta-V, so long as your payload/fuel mass and specific impulse remain the same.

The reason we need large rockets on Earth is because the atmosphere is much thicker: going through it is about the same as launching through a 10-meter column of water. Scaling up has huge benefits: a large rocket will have a larger inertia/surface area ratio, and as such, punching through the atmosphere is easier when you have a big rocket. This is why the smallest LEO rocket that I know of was about 6 tonnes. On Mars, however, as mentioned, the atmosphere is much thinner, so you don't have to worry as much about economies of scale: a small rocket will still do it.

Edited November 8, 2014 by Starman4308

[Bill Phil]

Depends on payload mass.

Here's a question: how much sample do you want?

After that, I can't necessarily help you out.

[xenomorph555]

The atmosphere on Mars is barely there: it is ~0.006 atm at surface level. This means that, in theory, you could get into Mars orbit with a rocket approximately the size of a pencil. Disregarding practical issues like fuel flow, you can scale down infinitely and retain the same delta-V, so long as your payload/fuel mass and specific impulse remain the same.

The reason we need large rockets on Earth is because the atmosphere is much thicker: going through it is about the same as launching through a 10-meter column of water. Scaling up has huge benefits: a large rocket will have a larger inertia/surface area volume, and as such, punching through the atmosphere is easier when you have a big rocket. This is why the smallest LEO rocket that I know of was about 6 tonnes. On Mars, however, as mentioned, the atmosphere is much thinner, so you don't have to worry as much about economies of scale: a small rocket will still do it.

So say you had an orbital module to take the samples home, MOR, (the ascent stage would therefore be smaller) and you wanted to take about 100 grams of soil back, could the ascent be the size of luna 24 ascent stage, possibly with a small solid upper stage.

[Tommygun]

I've heard of some estimates that put the return stage at about the size of a phone booth, but I don't remember the sample size.

Edit: I take that back, the phone booth size rocket only takes it to orbit, it then transfers it to a Earth return vehicle.

http://www.parabolicarc.com/2011/08/09/a-non-toxic-fuel-from-the-mojave-desert/

Edited November 8, 2014 by Tommygun

[Camacha]

I've heard of some estimates that put the return stage at about the size of a phone booth

Obvious solution is obvious.

[vger]

What a difference an atmosphere makes, eh? You think your worst enemy as a rocket is gravity, when in fact, it's air.

[Camacha]

What a difference an atmosphere makes, eh? You think your worst enemy as a rocket is gravity, when in fact, it's air.

The problem with the Mars atmosphere is that it is too thin to be useful, but too thick to ignore. Everything just gets thrown off while barely being useful for flight or aerobraking.

[Bill Phil]

I've heard of some estimates that put the return stage at about the size of a phone booth, but I don't remember the sample size.

Edit: I take that back, the phone booth size rocket only takes it to orbit, it then transfers it to a Earth return vehicle.

http://www.parabolicarc.com/2011/08/09/a-non-toxic-fuel-from-the-mojave-desert/

http://www.parabolicarc.com/wp-content/uploads/2011/08/Mars_Sample_Return_SSTO.jpg

Wow, that rocket is kind of cute, in a toy-ish way. Judging from size I'd say only a few kilograms for sample mass.

I kind of want to build a rocket like that IRL, if it was possible for me to do so. *sigh*

[1greywind]

The reason we need large rockets on Earth is because the atmosphere is much thicker:

What? In real world we don't have KSP's "souposphere". Average air drag losses for Earth launch vehicle is ~0.5-3%. Gravitiy losses are ~15-20%. So we need big rockets because of of air, but due to exponent over mass ration in rocket equation.

[metaphor]

What? In real world we don't have KSP's "souposphere". Average air drag losses for Earth launch vehicle is ~0.5-3%. Gravitiy losses are ~15-20%. So we need big rockets because of of air, but due to exponent over mass ration in rocket equation.

That's true for big rockets, but for smaller rockets the air seems thicker. The rocket gets smaller, but the size of the incoming air molecules stays the same. Smaller rockets have higher surface area-to-mass ratios, and so have a higher drag. That's why as you go down in rocket size, a rocket's payload mass ratio gets smaller (the Saturn V was one of the most efficient rockets with respect to payload mass ratio). A rocket the size of a person would have too much drag to get into orbit.

[Camacha]

Also don't forget that rockets are shaped almost exclusively to deal with that first little bit of the journey. If you would not have an atmosphere to contend with, these craft would look very, very different. Just look at the only non-atmospheric manned spacecraft we ever had - the Apollo LEM. It's a different game altogether. No sleek, long, arrow like fire sticks any more.

Edited November 8, 2014 by Camacha

[Starman4308]

What? In real world we don't have KSP's "souposphere". Average air drag losses for Earth launch vehicle is ~0.5-3%. Gravitiy losses are ~15-20%. So we need big rockets because of of air, but due to exponent over mass ration in rocket equation.

You are misunderstanding what is going on. The delta-V requirement for typical rockets specifies a mass fraction, but not total mass. Taking the Ariane V as a guideline, minus atmosphere, we could launch a 1-gram payload on a 49-gram rocket.

The reason no model rocketeers have launched 1-gram payloads to LEO is very much atmosphere. As you scale down, atmosphere becomes an increasingly daunting problem: smaller rockets have a smaller inertia-to-area ratio, and have a harder time punching through atmosphere. As you scale up dimensions, surface area exposed to atmosphere, and thus drag, goes up as the square of dimension, while mass/thrust/inertia all go up as the cube, meaning atmosphere becomes increasingly irrelevant as you go big.

In stock KSP, if you had the parts (ignoring differences in TWR, Isp, etc) you actually could get away with your 1-gram payloads, because the terribleness of the stock aero model means there are no advantages to aerodynamic rockets. If you move to FAR/NEAR, you would see limits on micro-rocketry, however, because the smaller your rocket, the more delta-V you have to spend fighting aero drag, until you reached the point at which your rocket could not carry a single gram of payload to LKO.

EDIT: The reason you see the average air drag losses there is that rocket engineers know this, and very deliberately choose to design rockets large enough to minimize aero drag. If they spend excessive amounts of delta-V fighting aero drag, their launch costs balloon; it would be more economically efficient just to go big from the start, and carry several payloads per rocket if small stuff needs to get to orbit.

Edited November 8, 2014 by Starman4308

[Kryten]

You are misunderstanding what is going on. The delta-V requirement for typical rockets specifies a mass fraction, but not total mass. Taking the Ariane V as a guideline, minus atmosphere, we could launch a 1-gram payload on a 49-gram rocket.

That example would only if we could make working Vulcains and Aestus' weighing less than a gram. Good luck getting any liquid motor working at that scale, nevermind a high-performance one.

[Starman4308]

That example would only if we could make working Vulcains and Aestus' weighing less than a gram. Good luck getting any liquid motor working at that scale, nevermind a high-performance one.

Well, there's that too, but I figured I'd ignore differences in rocket engine performance to illustrate the point about atmosphere.

[numerobis]

Just a quick question that has been bugging me, NASA and others keep talking about a Mars SRM but... wouldn't they need to bring a large multistage rocket?

You don't need a huge rocket, because the atmosphere is very thin, and the planet is 1/3 the size of Earth. Some proposals dock in orbit with a return stage, others go all the way home; either way, the transfer home is cheaper than the transfer to there.

The more reasonable proposals send an empty rocket, and fill up on Mars, which dramatically reduces the mass that needs to be sent.

[metaphor]

You don't need a huge rocket, because the atmosphere is very thin, and the planet is 1/3 the size of Earth. Some proposals dock in orbit with a return stage, others go all the way home; either way, the transfer home is cheaper than the transfer to there.

The more reasonable proposals send an empty rocket, and fill up on Mars, which dramatically reduces the mass that needs to be sent.

Yeah, the rocket would only need to have about 4 km/s of delta-v to get into Mars orbit, which is very possible to do single-stage. The MSR proposals I've seen have a total payload to the surface of about the same size as Curiosity or a little bigger. The ascent rocket would be only a few hundred kg, with a ~1 kg sample onboard (that's without ISRU since it's not really needed with such a small rocket).

[NERVAfan]

What? In real world we don't have KSP's "souposphere". Average air drag losses for Earth launch vehicle is ~0.5-3%. Gravitiy losses are ~15-20%. So we need big rockets because of of air, but due to exponent over mass ration in rocket equation.

But the average drag losses are so low because Earth launch vehicles are large. I believe Starman4308 is correct that a, say, 20kg rocket wouldn't be able to reach orbit on Earth even if it had the exact same mass ratio and Isp as a Falcon 9 or Proton.

(Although you also need the rocket parts to survive the pressure and stuff, so I doubt a pencil-sized LV would work even on an airless body - unless it was a really low-gravity one.)

[magnemoe]

Now if you did an mars sample return it would make lost of sense to use a group of rover, let them transfer the samples to the return rocket.