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#1 2016-02-13 10:24:23

kbd512
Member
Registered: 2015-01-02
Posts: 1,187

Advanced Mars EDL Technologies for Manned and Unmanned Missions

As the title of this thread suggests, we're here to discuss technologies that improve our landed tonnage per unit mass of propellant expended.  While propulsive landings permit any tonnage to be landed for which sufficient propellant can be expended, even when combined with other EDL technologies like expandable or inflatable aerodynamic decelerators, propulsive landings require a rough 50/50 split between EDL hardware and payload for payload masses over 1t.  With propellants that provide substantially better specific impulse, such as LOX/LH2, it is possible to improve on that a bit.  However, any EDL solution that makes substantial use of retro-propulsion will ensure that a substantial percentage of the tonnage sent to Mars will be EDL hardware.

Unfortunately for us, Mars is a particularly challenging EDL environment.  Let's review NASA's MSL mission to illustrate just how challenging it is:

Reentry occurs at roughly 125 km in altitude above the surface of Mars at a velocity of more than 5.8km/s.  Peak heating is up to 2100C over certain areas of the heat shield, largely due to flow turbulence.  Peak deceleration is 12.9G.  The heat shield is subjected to almost 48t of compressive and shear loads, generating considerable deflection under load.  The supersonic parachute generates a peak drag force of more than 29t.  All that pushing and pulling on the reentry vehicle and payload generates pretty extreme performance requirements.

The reentry vehicle mass is 2455 kg and 899 kg of payload is landed.  That's 73% EDL hardware and 27% payload.  The problem is even more severe when we consider the fact that 530t worth of aerospace hardware was required to deliver that payload.  Landed payload mass works out to just .17% of the mass required to deliver it.  That's pretty poor by any standard.

While there are other things we certainly can and should do to improve the mass fraction of the payload with respect to the propulsion system mass required to deliver it, here we're going to focus on what we can do to substantially reduce the mass of the EDL hardware.  Whatever the ultimate solution is, resolution of this issue is on the critical path for human exploration of Mars.

There are three basic aerodynamic deceleration technologies we can work with:

1. Ballons

Pros:

Assists with deceleration

Can be quite light and can contain light gases like hydrogen and helium

Fabrication is straightforward and well understood

Cons:

Requires advanced materials to survive heating associated with reentry

Likely difficult to make precision landings with

Requires a rapid inflation deployment mechanism

2. Parachutes

Pros:

Assists with deceleration

Even lighter than a ballon

Fabrication is straightforward and well understood

Cons:

Requires advanced materials to survive hypersonic deployment

Likely difficult to make precision landings with

Requires a forced inflation deployment mechanism for any substantial payload

3. Wings

Pros:

Creates aerodynamic lift

Control surfaces make precision landings possible

Potentially provides the most complete solution, from reentry to just before touchdown

Cons:

Fabrication requires advanced materials for sizes relevant to landing any substantial payload

Occupies a much larger physical volume than a ballon or parachute for sizes relevant to landing any substantial payload

Requires extensive aerodynamic testing to assess performance

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