Red Dragon Discussion Thread (1)

[go4mars]

If they do they should patent it and then get it out in the open for us all to gawk at and discuss openly! Maybe save some tax dollars by preventing others from spending 10 times as much landing a fraction of the payload on Mars.

They don't do patents.  They don't want China or other places to steal the ideas. 

[DaveH62]

If they do they should patent it and then get it out in the open for us all to gawk at and discuss openly! Maybe save some tax dollars by preventing others from spending 10 times as much landing a fraction of the payload on Mars.

They don't do patents.  They don't want China or other places to steal the ideas. 

Is there any DOD process to protect patentable trade secrets that other countries would not respect? Must be a high risk aspect of companies like SpaceX to not be able to use the patent system.

[Jim]

Is there any DOD process to protect patentable trade secrets that other countries would not respect? Must be a high risk aspect of companies like SpaceX to not be able to use the patent system.

That is where trade secrets come from. 

[MickQ]

Does this mean that the solution to supersonic retro-propulsion is a correctly positioned air spoiler or two

Mick.

[ugordan]

Does this mean that the solution to supersonic retro-propulsion is a correctly positioned air spoiler or two

It may also mean they haven't really analyzed the Mars EDL case with Dragon and just assume it would work since it works on Earth...

[simonbp]

Does this mean that the solution to supersonic retro-propulsion is a correctly positioned air spoiler or two

Possibly; or simply the willingness to accept the cosine losses of angling thrusters at high angle of attack...

[Robotbeat]

Does this mean that the solution to supersonic retro-propulsion is a correctly positioned air spoiler or two

Mick.

Supersonic retro isn't a problem unless you do it with a single engine right in the middle of your heatshield. And even then, it can still work (just not as efficiently as otherwise).

[Kaputnik]

Supersonic retro isn't a problem unless you do it with a single engine right in the middle of your heatshield. And even then, it can still work (just not as efficiently as otherwise).

Source for that?

Anyway, supersonic retro isn't a complete cure. The propellant mass becomes quite significant.

How fast would a ten tonne Dragon be going as it plummeted through the atmosphere? Pretty fast.
A 120m2 parachute only slowed MPF down to 270m/s. By comparison Dragon presents a 10m2 drag area- just 8.4% of the area yet it would mass ten to twenty times as much. So it's going to be hitting the ground pretty fast- we are talking on the order of kilometres per second, not metres.
To knock a km/s off the Dragon's speed, you'd need to burn about a third of its mass in propellant.
If we assume a ten tonne vehicle at entry, that leaves you with about 2.5t to cover all of your Mars landing modifications and payload. Not very efficient.

Now, if SpaceX, perhaps with help from NASA, were to use their PICA and Draco technologies to build a lander optimised for Mars, things could get much more interesting. They could make it have a much wider diameter, whatever the maximum allowed by FH is, and probably end up landing a larger payload with less risk.

[Nathan]

Haven't spacex said they intend to use parachutes to slow the craft? I think that not having to reconfigure the craft for landing (ie dropping a heat shield, back shell etc) buys additional time for deceleration as well. Robertzubrin didn't seem to think there was an issue with heat shield parachute combination followed by powered descent.

[Kaputnik]

Haven't spacex said they intend to use parachutes to slow the craft? I think that not having to reconfigure the craft for landing (ie dropping a heat shield, back shell etc) buys additional time for deceleration as well. Robertzubrin didn't seem to think there was an issue with heat shield parachute combination followed by powered descent.

It's not the principle, it's the numbers.
A conventional Dragon is going far too fast to open a parachute, it will hit the ground whilst still supersonic, or maybe even hypersonic. So SpaceX would need to develop and qualify a completely new design and type of drag device. IMHO they do best when refining the existing state of the art rather than brekaing totally new ground, so this would be a big challenge.

For comparison, NASA have only ever developed/qualified large supersonic parachutes in the context of the Viking program. This was a hugely expensive project requiring tests on sounding rockets, and they have shied away from the difficuly and expense of ever doing these sorts of tests again.

[baldusi]

Haven't spacex said they intend to use parachutes to slow the craft? I think that not having to reconfigure the craft for landing (ie dropping a heat shield, back shell etc) buys additional time for deceleration as well. Robertzubrin didn't seem to think there was an issue with heat shield parachute combination followed by powered descent.

It's not the principle, it's the numbers.
A conventional Dragon is going far too fast to open a parachute, it will hit the ground whilst still supersonic, or maybe even hypersonic. So SpaceX would need to develop and qualify a completely new design and type of drag device. IMHO they do best when refining the existing state of the art rather than brekaing totally new ground, so this would be a big challenge.

For comparison, NASA have only ever developed/qualified large supersonic parachutes in the context of the Viking program. This was a hugely expensive project requiring tests on sounding rockets, and they have shied away from the difficuly and expense of ever doing these sorts of tests again.

What would be the problem of simply adding a crasher stage?

[Kaputnik]

What would be the problem of simply adding a crasher stage?

Do you mean a stage that would fire to decelerate from orbit, prior to entry? That would need a pretty collosal delta-v, 3 or 4 km/s I reckon. I.e. for a state-of-the-art hypergolic stage, you're quadrupling the mass that needs to be put in Mars orbit.

On the other hand you can let the atmosphere perform the majority of that deceleration for you, and fire your 'crasher' shortly before reaching the surface. But that means it has to be able to fire into a supersonic airstream and that is, again, breaking new ground.

[baldusi]

What would be the problem of simply adding a crasher stage?

Do you mean a stage that would fire to decelerate from orbit, prior to entry? That would need a pretty collosal delta-v, 3 or 4 km/s I reckon. I.e. for a state-of-the-art hypergolic stage, you're quadrupling the mass that needs to be put in Mars orbit.

On the other hand you can let the atmosphere perform the majority of that deceleration for you, and fire your 'crasher' shortly before reaching the surface. But that means it has to be able to fire into a supersonic airstream and that is, again, breaking new ground.

I meant the second type. With a mean pressure of less than 30 pascals (I'm assuming 30km above Mars), I can't believe that would be a problem. But even if it was, you'd only need to develop a rocket that gimballed up to 90 degrees, and stat the thrust orthogonally to the hypersonic flow. If the entry step was acute enough, you could actually start it pointing downwards and thus mitigate some of the cosine losses as straight against the gravity gradient.

[Kaputnik]

What would be the problem of simply adding a crasher stage?

Do you mean a stage that would fire to decelerate from orbit, prior to entry? That would need a pretty collosal delta-v, 3 or 4 km/s I reckon. I.e. for a state-of-the-art hypergolic stage, you're quadrupling the mass that needs to be put in Mars orbit.

On the other hand you can let the atmosphere perform the majority of that deceleration for you, and fire your 'crasher' shortly before reaching the surface. But that means it has to be able to fire into a supersonic airstream and that is, again, breaking new ground.

I meant the second type. With a mean pressure of less than 30 pascals (I'm assuming 30km above Mars), I can't believe that would be a problem. But even if it was, you'd only need to develop a rocket that gimballed up to 90 degrees, and stat the thrust orthogonally to the hypersonic flow. If the entry step was acute enough, you could actually start it pointing downwards and thus mitigate some of the cosine losses as straight against the gravity gradient.

In some ways, the first way would be playing to SpaceX's strengths more- they hope to have a low-cost launcher, so just throw mass at the problem.

For the second, there has been some study of supersonic retropropulsion on Mars, and it *is* considered to be a non-trivial problem. I think your idea of firing obliquely into the stream is considered one of the components of making such a system work.

However, as I said earlier in the thread, the Dragon would need to have on the order of 1km/s or more delta-v to perform this deceleration. Using Dracos, and ignoring the potentially large cosine losses (as well as any losses caused by plume interaction) you need more than a third of the entry mass to be propellant. That is a pretty hefty portion.

I would suggest it would be safer and more efficient to just apply SpaceX's existing technologies (heatshield, draco) to a more conventional design, using as wide a heatshield as is possible within the FH PLF.

One other thought occurs- to my knowledge (Soviet probes may be an exception) all previous Mars entry vehicle used a 70 degree sphere-cone shape. I have read, somewhere, that this is to do with the CO2 atmosphere behaving somewhat differently to Earth's. So how is a semi-spherical shield like Dragon going to behave?

[simonbp]

If we assume a ten tonne vehicle at entry, that leaves you with about 2.5t to cover all of your Mars landing modifications and payload. Not very efficient.

Actually, it's pretty good. Phoenix had an entry mass of 602 kg and a payload mass of <80 kg.

Also, they might brake to Mars orbit before descent. This would allow a shallow, high L/D lifting trajectory (like Viking), allowing it to burn off much more energy aerodynamically before it has to ignite the rockets.

[Nathan]

Haven't spacex said they intend to use parachutes to slow the craft? I think that not having to reconfigure the craft for landing (ie dropping a heat shield, back shell etc) buys additional time for deceleration as well. Robertzubrin didn't seem to think there was an issue with heat shield parachute combination followed by powered descent.

It's not the principle, it's the numbers.
A conventional Dragon is going far too fast to open a parachute, it will hit the ground whilst still supersonic, or maybe even hypersonic. So SpaceX would need to develop and qualify a completely new design and type of drag device. IMHO they do best when refining the existing state of the art rather than brekaing totally new ground, so this would be a big challenge.

For comparison, NASA have only ever developed/qualified large supersonic parachutes in the context of the Viking program. This was a hugely expensive project requiring tests on sounding rockets, and they have shied away from the difficuly and expense of ever doing these sorts of tests again.

Where are those numbers?
If I recall the articles on this correctly there was talk of a second heat shield for braking into orbit followed by descent using the regular dragon shield. This lowers the entry velocity immediately. Then there's a parachute followed by powered descent. Nowhere has supersonic retropropulsion been mentioned. I suspect a new parachute may be required though but again no mention of that anywhere.
I think we need to gather the links to the available info and work from there.

[Nathan]

I may be confusing zubrin's article on this with the Ames info

[Kaputnik]

If we assume a ten tonne vehicle at entry, that leaves you with about 2.5t to cover all of your Mars landing modifications and payload. Not very efficient.

Actually, it's pretty good. Phoenix had an entry mass of 602 kg and a payload mass of <80 kg.

Also, they might brake to Mars orbit before descent. This would allow a shallow, high L/D lifting trajectory (like Viking), allowing it to burn off much more energy aerodynamically before it has to ignite the rockets.

I was assuming all along that the descent would begin in orbit, not direct entry. I have also been assuming that Dragon attempts to gain as much lift during entry as possible. These would allow you to enter with a ballistic coefficient of around 200kg/m2; that figure makes a ten tonne Dragon 'only' five times too heavy for this to work.

[Kaputnik]

Haven't spacex said they intend to use parachutes to slow the craft? I think that not having to reconfigure the craft for landing (ie dropping a heat shield, back shell etc) buys additional time for deceleration as well. Robertzubrin didn't seem to think there was an issue with heat shield parachute combination followed by powered descent.

It's not the principle, it's the numbers.
A conventional Dragon is going far too fast to open a parachute, it will hit the ground whilst still supersonic, or maybe even hypersonic. So SpaceX would need to develop and qualify a completely new design and type of drag device. IMHO they do best when refining the existing state of the art rather than brekaing totally new ground, so this would be a big challenge.

For comparison, NASA have only ever developed/qualified large supersonic parachutes in the context of the Viking program. This was a hugely expensive project requiring tests on sounding rockets, and they have shied away from the difficuly and expense of ever doing these sorts of tests again.

Where are those numbers?
If I recall the articles on this correctly there was talk of a second heat shield for braking into orbit followed by descent using the regular dragon shield. This lowers the entry velocity immediately. Then there's a parachute followed by powered descent. Nowhere has supersonic retropropulsion been mentioned. I suspect a new parachute may be required though but again no mention of that anywhere.
I think we need to gather the links to the available info and work from there.

A good overview of the problem is here:
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100017668_2010017622.pdf
There's also a more in-depth paper which I don't have the link for at the moment but it explains the importance of ballistic co-efficient in detail.

For starters, using a heatshield for braking into orbit has just thrown away all of the cheap/easy/simple ideas. This is untried technology. No craft has ever achieved aerocapture before, only direct entry or 'aerobraking', which is very different.
SpaceX are good at doing the things other people did already, but cheaper. Aerocapture is not playing to their strengths.

As I've said numerous times now, even if you are 'merely' descending from Mars orbital velocity, assuming a 10t Dragon with its 10m2 heatshield, the craft is five times too 'dense' to slow down sufficiently to be able to pop out a parachute. It simply smacks into the ground whilst still highly supersonic.

So you need some sort of supersonic deceleration technique. In fact, make that hypersonic, because it needs to be deployed well before you reach the ground, when the craft will be going even faster.


So why not just make a new heatshield, about 5m diameter to fit inside the Falcon PLF, and make a proper Mars lander of around two or three tonnes. No new technology required.

[ChefPat]

SpaceX are good at doing the things other people did already, but cheaper. Aerocapture is not playing to their strengths.
[/quote]
Propellant cross feed.