Mars Hopper Project: Baseline studies to validate RELAP5 using project results calculated for a radioisotope powered, impulse driven, long-range, long-lived mobile platform for exploration of Mars

Presented by: R. SchultzWork done by:Steven D. HoweRobert C. O’Brien, William Taitano, Doug Crawford, Nathan Jerred, Spencer Cooley, John Crepeau, Steve Hansen, Andrew Klein, James Werner

July 27, 2011

Outline

Summary: Mars Hopper projectSummary: approach for performing validation

study.

Planetary exploration is getting tougher Every mission has returned

knowledge different than what was expected

But planetary exploration is getting increasingly expensive

Orbital platforms are good but need surface exploration- more expensive

MERs did great but covered only 15 km total after 5 years

Surface landings necessitate flat, safe landing site but science may be in nooks and crannies

We need numbers on the ground

Need more science per $

Interest in canyon walls, mountainsides, deep canyon bottoms

Olympus Mons

Valles Marineris

ConceptInitiated June, 2009 in the CSNR Summer

Fellows programThe Mars Hopper concept utilizes energy from

radioisotopic decay in a manner different from any existing RTGs, i.e. as a thermal capacitor. ◦ Radioisotope sources have very high specific energy, j/kg,

while having rather low specific power, w/kg. Pu-238 has a specific energy of 1.6x106 MJ/kg which is

160,000 times the specific energy of chemical explosives. Factoring in the 25% conversion to electricity, the system

may have 4x105 MJ/kg of electrical energy compared to the 0.72 MJ/kg for Li-ion batteries.

By accumulating the heat from radioisotopic decay for long periods, the power of the source can be dramatically increased for short periods.

ConceptThe basis for the concept is to utilize the

decay heat from radioactive isotopes to heat a block of material to high temperatures. ◦ While the heating is taking place, some of

the thermal power is diverted to run a cryocooler.

◦ The cryocooler takes in the Martian atmosphere and liquefies it at 2.8 MPa.

◦ Once the tank full, the power convertor is turned off and the core is allowed to increase in temperature.

◦ After a peak temperature of 1200 K is reached, the liquid CO2 is injected into the core, heated, expanded through a nozzle, and allowed to produce thrust.

◦ Part of the CO2 propellant is “burned” for ascent. After a ballistic coast, the remaining propellant is used for a soft landing.

◦ Once landed, the process repeats.

Universal Encapsulation - Common technology for reactor fuels and radioisotope sources

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The distribution and encapsulation of radioisotope materials and nuclear fuels in an inert carrier matrix will address several issues and requirements for space power applications:◦ Potential to address non-proliferation

security requirements.◦ The ability to survive re-entry into Earth’s

atmosphere and impact under accident conditions.

◦ Assembly & handling safety◦ Reduction in material self interaction such

as -n reactions.◦ Self-shielding properties.

The SPS acquired with a INL LDRD grant enables fabrication of tungsten parts at nearly full theoretical density

Core subsystem -- Thermal issuesSeparate

heating from cooling geometries

Allow radiative losses only

Utilize radiative loss as source for power conversion

Why beryllium

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Material Tmelt (K) Thermal Conductivity(W/m-K)

Heat Capacity(J/kg-K)

Carbon 3823 165 710

Tungsten 3695 173 130

Beryllium 1551 200 1820

[

finished cylindrical Be elements

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STAR-CCM model of the steady state temperature reached by a radioisotope encased

• Three major issues exist in thermal management

• The thermal isolation of the low power thermal source is critical in order for the core to reach the required temperature in a practicable time period.

• The heat transfer requirement impacts on the length of the core and its mass.

• The thermal cycling

qualification of the design will impose lifetime limits for the entire system.

Thermal Isolation and management is crucial

Evaluation of insulator thickness and temperature profiles

A STAR-CCM+ model of the core was built and run in steady state and time dependent modes

overall core temperature increases non-linearly as the insulator thickness is increased. T

The average surface titanium radiation temperature decreases due to the increased surface area.

As power conversion units, CO2 tank (with CO2 in it) and other instrumentation are included, a greater heat sink and increased surface area for heat loss to the atmosphere will be produced.

An approximate usable thermal energy was calculated based on the specific heat of the beryllium and core PuO2/W matrix.

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Thickness [cm] Core Average Temperature [K]

Average Radiation Temperature [K] (Titanium)

Stored “Usable” Thermal Energy [MJ]

Thermal System Mass [kg]

1.00 1262.3 730.0 10.2 13.41.25 1320.0 723.0 11.2 13.6

1.50 1340.0 715.2 11.6 13.7

Co2 liquefaction The Hopper concept requires that a low mass, low power carbon dioxide

(CO2) liquefaction system The liquefaction system will collect CO2 gas from the Martian atmosphere

over a period of 7-8 days. Due to the high pressure ratio needed and low power available to compress

the necessary CO2, a mechanical compressor was unable to complete the task.

The most successful approach was to freeze the CO2 to a heat exchanger using a cryocooler to remove the heat.

The frozen CO2 would then be heated and pressurized in a closed volume (an intermediate pressure vessel) to make liquid CO2.

The design that meets the requirements:◦ weighs 6.5 kg (less than the required 28 kg); ◦ uses 220 W (less than the required 250 W); ◦ liquifies 0.6 kg in 10 hours (extrapolating this amount and considering the use of two

cryocooler systems results in a total of 22 kg being liquified in seven and a half Martian days)

◦ provides a low maintenance system with minimal moving parts

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Point design

 OverallTotal energy stored (J) 1.48e7Isotope thermal power (W) 1000Core max temperature (K) 1200 Core SpecificationsMass Pu-O2 (kg) 2.5Mass tungsten matrix (kg) 4.55Length tungsten source (m) 0.30Radius tungsten source (m) 0.0129Beryllium mass(kg) 6.068Outer beryllium radius (m) 0.0728Thickness of insulation (m) 0.015Inner pressure vessel rad. (m) 0.1868Pressure vessel wall (m) 0.001Core length (m) 0.30Rad curvature of plenums (m) 0.1268CO2 tank radius (m) 0.183Nozzle length (m) 0.3Total ship length (m) 1.50 

Envisioned Architecture• Small scale Hoppers with a 10 kg payload would

weigh around 52 kg dry for a 5-10 km hop

• Each could accommodate 2-3 instruments with low power demand (e.g. NAA, n detector, XRD, etc.)

• Build operational platform that provides power, propulsion, data acquisition, and data transmission

• Provide 12-15 for Mars• World-wide university competition for instrument

packages and data collection

• Hop samples to centralized location for the Mars Sample Return ascent vehicle

Summary The CSNR is designing a pulse power mobile platform that can cover large

areas of Mars within a few years using local in-situ resources

The platform can “hop” every 5-7 days and cover 5-10 km per hop

If several such platforms could be simultaneously deployed from a single launch vehicle, a surface network of science stations would be possible that provided long term assessment of meteorological conditions.

The concept can be demonstrated on Earth using an electrically heated core and existing power conversion technologies for modest cost

Other applications of the pulse power capability of the “thermal capacitor” concept may include satellite station keeping and burst communications

The Hopper can enable samples from all over Mars to meet the Mars Sample Return descent vehicle.

The Mars Hopper can revolutionize planetary exploration

Validation studies…• Three calculational efforts underway:

• Hand calculations--baseline• CFD: Rich Martineau

• Benchmarked code developed at U Idaho• RELAP5 calculations

• Calculation performed for blowdown of CO2 tank and choking in the beryllium flow passages stemming from friction and heating.

• Boundary conditions: initial pressure in tank is 2.8 MPa, 270 K and Martian atmosphere is at 630 Pa

• Beryllium is initially heated to 1200 K