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| From: Keith Lofstrom <keithl@gate.kl-ic.com> To: Narayanan Komerath <komerath2020@gmail.com> Cc: Bcc: keithl@gate Subject: Heat shield versus rocket Re: Space technology/business mailing lists Reply-To: keithl@keithl.com |
= PhobosMars == |
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| NK: The following is a question I would ask on a propulsion-related mailing list. You might be the right person to answer it, assuming you can step back far enough to imagine two "unrelated" problems as one. I'm pondering missions to Phobos, and comparing rocket delta V to heat shield delta V. I hope you can help me judge the relative merits. ---------- JAXA's Mars Moon eXplorer (MMX) proposes to arrive at Phobos with three rocket burns: |
JAXA's Mars Moon eXplorer (MMX) proposes to arrive at Phobos with three rocket burns: |
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| 2) Apo-apsis plane change and boost, delta V of 40 m/s (results in a higher peri-apsis at 9376 km Phobos orbit) |
2) Apo-apsis plane change and boost, delta V of 40 m/s (results in a higher peri-apsis at 9376 km Phobos orbit) |
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| Apo-apsis is very high, 70*Mars radius, so that plane change delta V is cheap. Time is money, so lower is more delta V but a somewhat cheaper mission |
Apo-apsis is very high, 70*Mars radius, so that plane change delta V is cheap. Time is money, so lower is more delta V but a somewhat cheaper mission. |
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| That's the "apple", slightly modified from published numbers, to enable "apple/orange comparison". |
That's the "apple", slightly modified from published numbers, to enable "apple/orange comparison". |
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| Note that each heat shield slowdown is in and out of upper Mars atmosphere, higher and smaller and much gentler than heat shield entry to Mars surface. WAG: 2 gees, not 12 |
Note that each heat shield slowdown is in and out of upper Mars atmosphere, higher and smaller and much gentler than heat shield entry to Mars surface. WAG: 2 gees, not 12 |
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| Note also that system failure may cause the spacecraft to burn up and impact Mars surface; a "planetary protection" problem. |
Note also that system failure may cause the spacecraft to burn up and impact Mars surface; a "planetary protection" problem. |
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| So, this approach reduces chemical rocket delta V by 920 m/s, (but does not eliminate it), and adds a heat shield (gently used twice) delta V of 1330 m/s. |
So, this approach reduces chemical rocket delta V by 920 m/s, (but does not eliminate it), and adds a heat shield (gently used twice) delta V of 1330 m/s. |
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| But heat shields are not free - they add mass and different failure modes. Also, a dependence on the highly variable Mars atmosphere, requiring two pre-entry estimates of Mars atmospheric density. |
But heat shields are not free - they add mass and different failure modes. Also, a dependence on the highly variable Mars atmosphere, requiring two pre-entry estimates of Mars atmospheric density. |
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| A heat shield designed for two uses, loaded to less than 5% of Earth entry heating, will be cheap, but not free or massless. Hundreds of hours between uses, so there is time to repair or resurface it if necessary. |
A heat shield designed for two uses, loaded to less than 5% of Earth entry heating, will be cheap, but not free or massless. Hundreds of hours between uses, so there is time to repair or resurface it if necessary. |
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| You are an aeropropulsion expert, but hypersonic aircraft must also be heat resistant, so you can probably estimate the mass and cost of a heat shield dissipating perhaps 300 MJ/m2 of slowdown heat, over two multi-minute passes. |
You are an aeropropulsion expert, but hypersonic aircraft must also be heat resistant, so you can probably estimate the mass and cost of a heat shield dissipating perhaps 300 MJ/m2 of slowdown heat, over two multi-minute passes. |
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| And how that weight and cost (and reliability) "orange" scales to many extra minutes of non-cryo-fueled rocket engine "apple". 920 m/s delta V is a LOT of heavy years-storable propellant. |
And how that weight and cost (and reliability) "orange" scales to many extra minutes of non-cryo-fueled rocket engine "apple". 920 m/s delta V is a LOT of heavy years-storable propellant. |
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| Ponder that for a while. Perhaps you know of a paper or textbook that compares these apples and oranges. |
Ponder that for a while. Perhaps you know of a paper or textbook that compares these apples and oranges. |
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| When I have this figured out, my next study will be "Phobos indirect to Mars". Given the enormous planetary protection consequences, my idea of a manned "plant the flag" mission to Mars is to send a large manned mission to a robotically prepared semipermanent base on Phobos, and spend 98% of mission elapsed time there. |
When I have this figured out, my next study will be "Phobos indirect to Mars". Given the enormous planetary protection consequences, my idea of a manned "plant the flag" mission to Mars is to send a large manned mission to a robotically prepared semipermanent base on Phobos, and spend 98% of mission elapsed time there. |
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| Radiation shielded, 1 gee centrifuge, closed cycle algae food and air farm. Controlling hundreds of surface robots from that base; that's 90% of what the expedition does. Also examining tonnes of rocks sent up on ascent vehicles. The real work gets done on Phobos. |
Radiation shielded, 1 gee centrifuge, closed cycle algae food and air farm. Controlling hundreds of surface robots from that base; that's 90% of what the expedition does. Also examining tonnes of rocks sent up on ascent vehicles. The real work gets done on Phobos. |
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| Nobody has ever walked on the Moon. They've walked on boots that walked on the Moon, and they've looked at the Moon through glass. "Walking on Mars" with predictive- adaptive tele-presence, perhaps wearing "sensor socks" so astronauts can actually feel the sharp gritty surface with their "bare" feet in real time, may be "more real" than actually being there. |
Nobody has ever walked on the Moon. They've walked on boots that walked on the Moon, and they've looked at the Moon through glass. "Walking on Mars" with predictive-adaptive tele-presence, perhaps wearing "sensor socks" so astronauts can actually feel the sharp gritty surface with their "bare" feet in real time, may be "more real" than actually being there. |
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| There will also be a three-day, two-astronaut "Mars lander from Phobos" sub-mission, for a brief "show the flag, say the words, check the pulse, and collect the souvenirs" visit. This will be to a site already scouted, sampled, and prepared by robots. Some of those robots operated by the same two astronauts, pre- and post- landing. |
There will also be a three-day, two-astronaut "Mars lander from Phobos" sub-mission, for a brief "show the flag, say the words, check the pulse, and collect the souvenirs" visit. This will be to a site already scouted, sampled, and prepared by robots. Some of those robots operated by the same two astronauts, pre- and post- landing. |
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| Then ride an ascent vehicle back to the base on Phobos, and back to experiencing the entire planet using tele-presence. |
Then ride an ascent vehicle back to the base on Phobos, and back to experiencing the entire planet using tele-presence. |
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| If similar lander/ascenders move both rocks and astronauts, there will be plenty of system tests before risking humans (and Mars), as well as a spare ascender if the ascender the astronauts rode down on fails. The lander might need an entry heat shield, but the ascender does not; an open frame with a nosecone, and mesh seats for space suits, would be adequate. Keith -- Keith Lofstrom keithl@keithl.com |
If similar lander/ascenders move both rocks and astronauts, there will be plenty of system tests before risking humans (and Mars), as well as a spare ascender if the ascender the astronauts rode down on fails. The lander might need an entry heat shield, but the ascender does not; an open frame with a nosecone, and mesh seats for space suits, would be adequate. |
= PhobosMars ==
JAXA's Mars Moon eXplorer (MMX) proposes to arrive at Phobos with three rocket burns:
1) Mars capture with a near-Mars delta V of 630 m/s 2) Apo-apsis plane change and boost, delta V of 40 m/s (results in a higher peri-apsis at 9376 km Phobos orbit) 3) Peri-apsis insertion into Phobos orbit, delta V 850 m/s
Apo-apsis is very high, 70*Mars radius, so that plane change delta V is cheap. Time is money, so lower is more delta V but a somewhat cheaper mission.
Total chemical rocket delta V, 1520 m/s
That's the "apple", slightly modified from published numbers, to enable "apple/orange comparison".
My imagined alternative: 1) Mars capture with a HEAT SHIELD delta V of 630 m/s 2) Apo-apsis plane change (no boost), delta V of 30 m/s (max) 3) Second Mars HEAT SHIELD slowdown, delta V of 700 m/s
- (results in an apo-apsis at 9376 km Phobos orbit)
4) Apo-apsis insertion into Phobos orbit, delta V 570 m/s
Total chemical rocket delta V, 600 m/s Total heat shield delta V, 1330 m/s
Note that each heat shield slowdown is in and out of upper Mars atmosphere, higher and smaller and much gentler than heat shield entry to Mars surface. WAG: 2 gees, not 12
Note also that system failure may cause the spacecraft to burn up and impact Mars surface; a "planetary protection" problem.
So, this approach reduces chemical rocket delta V by 920 m/s, (but does not eliminate it), and adds a heat shield (gently used twice) delta V of 1330 m/s.
But heat shields are not free - they add mass and different failure modes. Also, a dependence on the highly variable Mars atmosphere, requiring two pre-entry estimates of Mars atmospheric density.
That's the "orange", which I hope to compare.
A heat shield designed for two uses, loaded to less than 5% of Earth entry heating, will be cheap, but not free or massless. Hundreds of hours between uses, so there is time to repair or resurface it if necessary.
You are an aeropropulsion expert, but hypersonic aircraft must also be heat resistant, so you can probably estimate the mass and cost of a heat shield dissipating perhaps 300 MJ/m2 of slowdown heat, over two multi-minute passes.
And how that weight and cost (and reliability) "orange" scales to many extra minutes of non-cryo-fueled rocket engine "apple". 920 m/s delta V is a LOT of heavy years-storable propellant.
Ponder that for a while. Perhaps you know of a paper or textbook that compares these apples and oranges.
When I have this figured out, my next study will be "Phobos indirect to Mars". Given the enormous planetary protection consequences, my idea of a manned "plant the flag" mission to Mars is to send a large manned mission to a robotically prepared semipermanent base on Phobos, and spend 98% of mission elapsed time there.
Radiation shielded, 1 gee centrifuge, closed cycle algae food and air farm. Controlling hundreds of surface robots from that base; that's 90% of what the expedition does. Also examining tonnes of rocks sent up on ascent vehicles. The real work gets done on Phobos.
Nobody has ever walked on the Moon. They've walked on boots that walked on the Moon, and they've looked at the Moon through glass. "Walking on Mars" with predictive-adaptive tele-presence, perhaps wearing "sensor socks" so astronauts can actually feel the sharp gritty surface with their "bare" feet in real time, may be "more real" than actually being there.
There will also be a three-day, two-astronaut "Mars lander from Phobos" sub-mission, for a brief "show the flag, say the words, check the pulse, and collect the souvenirs" visit. This will be to a site already scouted, sampled, and prepared by robots. Some of those robots operated by the same two astronauts, pre- and post- landing.
Then ride an ascent vehicle back to the base on Phobos, and back to experiencing the entire planet using tele-presence.
If similar lander/ascenders move both rocks and astronauts, there will be plenty of system tests before risking humans (and Mars), as well as a spare ascender if the ascender the astronauts rode down on fails. The lander might need an entry heat shield, but the ascender does not; an open frame with a nosecone, and mesh seats for space suits, would be adequate.