NASA Titan Submarine mission proposal

[Enorats]

http://mentalfloss.com/article/78049/plan-send-submarine-titan-saturns-largest-moon

I read this article about a proposal to send a sub to Titan, a plan which is still in its infancy, and couldn't help but think "wtf?" at a couple points. Since its driving me nuts I figured I'd ask here to see if I'm right.

At one point in the article the author quotes the NASA scientist saying that the reentry velocity at Titan would be similar to that at Earth, and thus a craft like the X-37b could survive.

This seems wrong to me. Titan is much smaller than the Earth, so I'd assume an orbiter would be moving slower. Assuming they circularize the orbit prior to reentry, reentry velocities should be significantly lower. If they aerobrake (which I assume they would) then coming from amy interplanetary transfer the velocities would be vastly higher, and would likely require significantly greater shielding. Am I wrong here?

Anyone who plays RSS done this before? The closest stock comparision I can think of is Laythe, which is far closer in size to Kerbin than Titan is to Earth. Laythe's atmosphere is quite a bit less dense than Titan's as well. Titan has a greater atmospheric pressure than Earth, another factor I'd think they want to take into consideration..

[FancyMouse]

You misread. It said "entry velocity" equals Earth's "orbital velocity".

[Enorats]

True, but it amounts to the same thing. The speed you enter the atmosphere at Titan (entry velocity) should not be the same as the orbital velocity of the Earth.

If you're aerobraking directly from a transfer then you're going to be going faster than Earth orbital velocity.. otherwise you wouldn't have left Earth in the first place. If you're entering from a stable orbit you'd be going far slower. The only way I can see them being the same is if they're entering from a highly eccentric orbit. I suppose they could do that, but it'd be harder on the craft than necessary so I'm not sure why they would plan it like that.

[FancyMouse]

Several things.

1. You definitely do want to aerobrake/aerocapture. That's much much more mass efficient than burning. Galileo probe "only" has heat shield as heavy as the probe itself, and it manages an entry of 47km/s+ into Jupiter. I think it's safe to assume heat shield wiped at least 40km/s of its speed - how much fuel do you need if you want to burn?

2. So from the above, entry velocity is usually even higher than escape velocity - but it's only higher than Titan's escape velocity. It doesn't by itself tells how it compares with Earth's (unless you bring in their mass/radius/etc. and calculate). Velocity needs transform between reference of frames, so your "otherwise you wouldn't have left Earth in the first place" is an invalid point. Since you mentioned Laythe - did you try to encounter Laythe when it's at different points of its orbit, and see what's the encounter velocity? They can be drastically different even you start from a similar speed/energy from outside.

[Enorats]

I guess I get what you're saying regarding the relative velocities, but I'm trying to say that a transfer to Saturn is going to have a pretty giant velocity difference when you get there. It's been awhile since I did a transfer out to Jool but I seem to recall something like 7500 m/s orbital velocity there coming in from a transfer from Kerbin at a window. Though, I suppose the relative velocity to the moon in question would vary wildly from that number depending on how you're approaching it. If it's coming at you the velocity is going to be much higher than if it's moving in line with you and you come up behind it. Maybe it would be possible for an entry to Titan to be similar to one at Earth if you hit it at the right point. 

[Red Iron Crown]

Moved to Science & Spaceflight.

[peadar1987]

Hmm... What's Titan's orbital velocity at Saturn? And what velocity would a probe be coming in at? Without knowing the numbers, it seems at least plausible that if you took away the orbital velocity of Titan from the incoming velocity of a probe, it might coincidentally be roughly equal to the velocity of earth reentry?

Edited April 5, 2016 by peadar1987

[LeuZ]

When going to Saturn with a Hohmann Transfer you are leaving earth with about 15 km/s I think, but when arriving at Saturn you are much slower than Earth orbital velocity.

[peadar1987]

58 minutes ago, LeuZ said:

When going to Saturn with a Hohmann Transfer you are leaving earth with about 15 km/s I think, but when arriving at Saturn you are much slower than Earth orbital velocity.

Yes, but as you are moving slowly at the apoapsis of an elliptical orbit, Saturn will be catching you quickly, meaning there will still be quite a high relative velocity. (Saturn orbits at 9.6km/s).

After bashing the vis-viva equation about for a little bit, I get the orbital velocity of a probe travelling from Earth to Saturn as 4.2km/s relative to the sun at apoapsis, which means Saturn will be overtaking the probe at 5.4km/s. The probe will then accelerate further as it falls into Saturn's gravity well.

This chart shows that it takes 1.3km/s of delta-V to get from Titan orbit to Saturn escape, so if you lined up your orbits right, you could probably intercept Titan at less than 7km/s, which is indeed just less than earth's reentry velocity.

Edited April 5, 2016 by peadar1987

[fredinno]

18 hours ago, Enorats said:

True, but it amounts to the same thing. The speed you enter the atmosphere at Titan (entry velocity) should not be the same as the orbital velocity of the Earth.

If you're aerobraking directly from a transfer then you're going to be going faster than Earth orbital velocity.. otherwise you wouldn't have left Earth in the first place. If you're entering from a stable orbit you'd be going far slower. The only way I can see them being the same is if they're entering from a highly eccentric orbit. I suppose they could do that, but it'd be harder on the craft than necessary so I'm not sure why they would plan it like that.

This shows LEO reentry velocity is about 1km/s lower than Titan reentry from Saturn orbit. Skip reentry could probably shave entry velocity off to 9.4 m/s

Edited April 5, 2016 by fredinno

[-ctn-]

Huygens entered Titan's atmosphere at around 6 km/s. When I was trying to plan my short film, I calculated a "direct transfer," by creating a Hohmann transfer to Neptune and timing it so that the probe slams into Titan's atmosphere halfway through the transfer. The re-entry speed was somewhere around 12 km/s, which a PICA-X heatshield can handle.

[PB666]

On April 5, 2016 at 11:29 AM, peadar1987 said:

Yes, but as you are moving slowly at the apoapsis of an elliptical orbit, Saturn will be catching you quickly, meaning there will still be quite a high relative velocity. (Saturn orbits at 9.6km/s).

After bashing the vis-viva equation about for a little bit, I get the orbital velocity of a probe travelling from Earth to Saturn as 4.2km/s relative to the sun at apoapsis, which means Saturn will be overtaking the probe at 5.4km/s. The probe will then accelerate further as it falls into Saturn's gravity well.

This chart shows that it takes 1.3km/s of delta-V to get from Titan orbit to Saturn escape, so if you lined up your orbits right, you could probably intercept Titan at less than 7km/s, which is indeed just less than earth's reentry velocity.

But it could catch titan specifically orbit around saturn on the sun side, how fast dies titan orbit saturn?

On April 5, 2016 at 1:52 PM, fredinno said:

This shows LEO reentry velocity is about 1km/s lower than Titan reentry from Saturn orbit. Skip reentry could probably shave entry velocity off to 9.4 m/s

Poor pluto, doesn't even get pock chopped anymore. 

[fredinno]

6 hours ago, PB666 said:

But it could catch titan specifically orbit around saturn on the sun side, how fast dies titan orbit saturn?

Poor pluto, doesn't even get pock chopped anymore. 

Here's one with EVEN more destinations. And also Pluto.

http://yumonstudios.deviantart.com/art/THE-Ultimate-Delta-V-Map-of-the-Solar-System-559376250