Retrorockets in vogue?

louis · 2008-05-26 17:42:23

Retrorockets have been getting a lot of good publicity in the coverage of the Phoenix lander.

Surely now is the time to give serious consideration to retrorocket landing for a minimal human mission on Mars.

In principle, if we can refuel a Mars lander in earth orbit, couldn't we make a 100% retro landing?

SpaceNut · 2008-05-26 20:13:42

Not 100% due to heat shield needing to be in place until parachutes can be deployed.

Commodore · 2008-05-26 21:09:41

The ultimate goal is landing a horizontally configured Ares V class cargo.

GCNRevenger · 2008-05-27 06:58:01

You mean slowing to near-zero relative to Mars before reaching the atmosphere? The fuel bill would be enormous.

Commodore · 2008-05-27 07:14:59

You mean slowing to near-zero relative to Mars before reaching the atmosphere? The fuel bill would be enormous.

So would the payload.

SpaceNut · 2008-05-27 07:44:49

Current methods of landing employ slowing down from a speed more than 12,000 mph (19,200 kilometers per hour) to a 5 mph (8 kph) touchdown using a combination of friction, parachute and thrusters.

Here is the timeline used by Pheonix

Note that the heat shield comes off some 200 seconds before touch down and is traveling still at about 120 meters / sec. which is a far cry from atmospheric entry of 5.6 km /sec.

GCNRevenger · 2008-05-27 09:23:15

You mean slowing to near-zero relative to Mars before reaching the atmosphere? The fuel bill would be enormous.
So would the payload.

No. The payload would not be bigger, and in fact would be much smaller.

Think about it, if thrust-to-weight ratios or atmospheric drag aren't an issue, it will take you about as much rocket fuel to slow down and land as it takes to launch into orbit (or transit) from the ground. Even with the reduced Martian gravity, you'd therefore need a lander of the same scale as the Ares-V rocket.

But it doesn't just double the fuel bill, oh no, remember that the fuel bill increases exponentially with Delta-V, so if the same vehicle must launch from Earth and land on Mars, the rocket would have to be truly and hugely immense. It would easily dwarf the NOVA and SeaDragon superheavy rockets. The all-retro-rocket lander would be Ares-V size, imagine the size of rocket you would need to launch a fully-fueled Ares-V to Mars!

Or you want to employ orbital refueling? Then you would need to lug an Ares-V sized amount of fuel into orbit! Millions of pounds of rocket fuel, massive tankers, huge launch facilities able to build and fire dozens of heavy lifters annually...

So, in summary, retro rockets are not a particularly good idea for slowing down from orbital/transit velocities. Heat shields at least and perhaps parachutes too are the obvious way to go. The retro-burn for the Altair Lunar lander is the major reason why the Lunar surface payload is less than a sixth of the Ares-V LEO payload.

louis · 2008-05-27 16:06:37

You mean slowing to near-zero relative to Mars before reaching the atmosphere? The fuel bill would be enormous.

[b]Yes, that's what I was getting at. It would be enormous but would it be a price worth paying in terms of reduced complexity and increased safety? There must be some mass saving on heat shield and parachute paraphenalia.

I would want orbital refuelling and possibly not just orbital refuelling - maybe also midway (or maybe Mars orbit) refuelling and on Mars refuelling.

Also, I wouldn't want the fuel for orbital refuelling to come from earth. I'd want that to come from the Moon. If we are confident of establishing a human ISRU mission on Mars, we would also be able to put in place a lunar base with lunar fuel production.

Further I would favour LEO assembly of maybe two or three parts.

If you put all that together, then I think you may be back to manageable proportions.

Rune · 2008-05-27 17:11:16

You mean slowing to near-zero relative to Mars before reaching the atmosphere? The fuel bill would be enormous.
[b]Yes, that's what I was getting at. It would be enormous but would it be a price worth paying in terms of reduced complexity and increased safety? There must be some mass saving on heat shield and parachute paraphenalia.
I would want orbital refuelling and possibly not just orbital refuelling - maybe also midway (or maybe Mars orbit) refuelling and on Mars refuelling.
Also, I wouldn't want the fuel for orbital refuelling to come from earth. I'd want that to come from the Moon. If we are confident of establishing a human ISRU mission on Mars, we would also be able to put in place a lunar base with lunar fuel production.
Further I would favour LEO assembly of maybe two or three parts.
If you put all that together, then I think you may be back to manageable proportions.

Actually, if you are so confident about ISRU, you could launch aditional disposable flying-tanks from the moon to refuel after the boost phase...

Not a bad argument at all, and I hope it is seriously considered, because the infrastucture neccesary to make it happen would prove incredibly useful for any other space proyect... But it may be a little too expensive for a first mission, on the other hand. And would require a fully functional moon base.

Rune. Everything has disadvantages.

louis · 2008-05-27 17:44:20

Glad you think this approach is not completely off the wall Rune. Another point I didn't mention, is that I would like us to build in some "hopping" ability for the lander. This could be crucial in enabling exploitation of water and other raw material resources. So refuelling would be part of a strategy designed to ensure we arrive with a good fuel surplus which could be used for that purpose. A mission design might then involve a number of initial "hops" to gather raw materials from around the planet.

Commodore · 2008-05-27 22:09:44

So, in summary, retro rockets are not a particularly good idea for slowing down from orbital/transit velocities. Heat shields at least and perhaps parachutes too are the obvious way to go. The retro-burn for the Altair Lunar lander is the major reason why the Lunar surface payload is less than a sixth of the Ares-V LEO payload.

Why would I not make use of aerobraking, heat shields, parachutes, or any other innovations to land such large payload? I'm simply saying that eventually your going to have to employ retrorockets for at least the last few miles to have a controlled landing.

GCNRevenger · 2008-05-27 22:55:11

So, in summary, retro rockets are not a particularly good idea for slowing down from orbital/transit velocities. Heat shields at least and perhaps parachutes too are the obvious way to go. The retro-burn for the Altair Lunar lander is the major reason why the Lunar surface payload is less than a sixth of the Ares-V LEO payload.
Why would I not make use of aerobraking, heat shields, parachutes, or any other innovations to land such large payload? I'm simply saying that eventually your going to have to employ retrorockets for at least the last few miles to have a controlled landing.

Hey don't look at me, its Louis' idea.

louis · 2008-05-28 01:40:03

Sorry GNC!

I think my issues would be:

1. Parachutes are a safety issue for me. They have been associated with mission failures before now.

2. Heat shields add mass and are a maintenance burden. How much effort goes into Space Shuttle maintenance as a result?

I suppose I look at this way the other way round: if we can organise things so that we rely entirely (or perhaps substantially - not ruling out aerobraking element completely I think) on retro rockets, why wouldn't we?

My aim would be to make launches easier, build in fuel stops, create a "hopping" ability for the lander and ensure true reusability.

noosfractal · 2008-05-28 02:28:23

why wouldn't we?

It is just too tempting to get the 6.4 km/sec delta vee for "free". The problem is that the mass ratio (inital/payload) is an exponential function of delta vee. All else being equal, your payload will be reduced by a factor of 1/10 (or your rocket has to be 10 times bigger initially). You'll always take the aerobrake risk if it's available. For chemical rockets anyway.

***EDIT to use a more reasonable specific impulse.

Rune · 2008-05-28 02:36:54

Sorry GNC!
My aim would be to make launches easier, build in fuel stops, create a "hopping" ability for the lander and ensure true reusability.

Have a look at the Delta Clipper design. If it is feasible for SSTO, it should work for Mars with proper ISRU. It may not be much cheaper than a non-reusable system, though, as the shuttle program has proven. And developing the ISRU capabilies is definitely not going to be cheap.

Rune. There is always a way... and probably somebody saw it first.

JoshNH4H · 2008-05-28 13:40:26

If we use a cheap, disposable heat sheild, maintenance won't be an issue. What about the possibility of going directly from aerobake to glider?

Rune · 2008-05-28 14:59:04

What about the possibility of going directly from aerobake to glider?

Instead of just a parachute? Show me a design aerodinamically sound that doesn't weight more than the needed fuel for a controlled landing (wich is quite little) and I'll think about it, but think of it as bringing a plane with you all the way from Earth. And it would have to fit behind a heatshield and have some kind of landing gear.

Rune. After all, a parachute technically is a glider. So they are already doing it.

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#1 2008-05-26 17:42:23

Re: Retrorockets in vogue?

#2 2008-05-26 20:13:42

Re: Retrorockets in vogue?

#3 2008-05-26 21:09:41

Re: Retrorockets in vogue?

#4 2008-05-27 06:58:01

Re: Retrorockets in vogue?

#5 2008-05-27 07:14:59

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#6 2008-05-27 07:44:49

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#7 2008-05-27 09:23:15

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#8 2008-05-27 16:06:37

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#9 2008-05-27 17:11:16

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#10 2008-05-27 17:44:20

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#11 2008-05-27 22:09:44

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#12 2008-05-27 22:55:11

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#13 2008-05-28 01:40:03

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#15 2008-05-28 02:36:54

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