Masters Special Problem 9800

8/18/2019 Masters Special Problem 9800

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Viability of Dual Propellant Mars Transportation

Vehicle

Marius D Popescu1 Georgia Institute of Technology, Atlanta, GA, 30318

Electric Propulsion methods have been at the forefront of vehicle concepts that

travel to Mars.

Nomenclature

g =gravitational constant, 9.81m/s2

µ =gravitational parameter

I. Introduction

A significant cost

II. Literature Review

It is known that varying specificimpulse and thrust to weight on a vehicle can

be advantageous. Historically this has almost

exclusively been done by staging, but is

advantageous even if only one stage is used.

In chemical propulsion, multiple fuels are

often used to vary thrust to weight and

impulse between stages, but it has also been

shown to be theoretically practical in a single

stage simply due to the tradeoff between

specific impulse and density impulse.Furthermore there is slight more benefit in

having a single engine than separate engines

burning in parallel [1] which led to a few

conceptual designs in dual fuel dual expander

rocket engines, which burned both

hydrocarbon and hydrogen fuel within

concentric combustion chambers and

expanded through a shared nozzle. In

addition to reducing propellant mass fraction,

using multiple propellants may increase

thrust efficiency, benefit the propellant tank

system design, and reduce cost impulse (that

is that one fuel may be cheaper to provide the

same ΔV). In general the same advantageous

could be applied to electric propulsion, and

be even more effective. An electric propulsion

device that can vary its impulse and thrust

over a large range means it could be in a more

optimal specific impulse regime for both

planetary escape and interplanetary travel

parts of a mission. However, even a small

range can increase performance; it also

benefits from better flexibility and

management of power levels throughout a

mission. For instance, as mass decreases

during a mission, the optimal Isp increases,

and changes in power level or large gravity or

drag losses may optimize Isp lower. Being able

to vary modes also may make it possible to

burn continuously which further reduces trip

times. It has been shown that varying specificimpulse in any propulsion device and electric

propulsion in particular can provide

significant reduction in propellant cost and

mass and/or trip time for interplanetary

missions as shown in the figure below. [2] It

is therefore of interest to develop an electric


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propulsion device that can vary its impulse

aka Variable Specific Impulse Electric

Propulsion (VSI EP). This is precisely the

reasoning behind the development of

VASIMR.

The above table shows that despite

additional gravitational losses and not being

able to utilize the Oberth effect, low thrust

NEP can reduce the amount of propellant

needed, ignoring time constraints. As shown

in Figure 1, adding variable impulse to the

device can further improve mass fraction.

Furthermore the same paper by Acta

Astronautica states:

“ It is quite interesting to notice how the variable-Isp

thruster permits a 10% mean reduction of the

propellant consumption. This value is slightly lower

(about 9%) if the unavoidable upper and lower

limits for the Isp are imposed at reasonable levels,

but it can double in special cases (like for long

missions with gravity assist and low total

propellant mass). It has to be recognized that up to80% of the achieved propellant mass savings could

be obtained using dualmode thrusters, considerably

simpler to develop and qualify.” [2]

There has been some research done

exactly as to what range of specific impulse is

necessary and optimal for a given mission or

vehicle. Unfortunately, the referenced paper

does not explicitly state the bounds, but one

can infer the range to be about 2800-3500s

for the mission to Mars, furthermore the

paper is not clear on what the maximum and

minimum specific impulse they found when

the bounds were not imposed.[2] A similarstudy conducted by the company that created

VASIMR, Ad Astra, found that on a mission to

Mars allowing the specific impulse to vary

between 4000s and 30000s vs a constant

specific impulse of 5000s given a power input

and initial mass allowed them to save about

15% of propellant. [6] It is worth noting that

the paper did not state whether or not the

constant specific impulse value was

optimized, and that this range of specificimpulse is not particularly close to the

currently realized range of 2000-10000. Both

papers nevertheless demonstrate significant

predicted propellant savings.

Considering the above results, there is

currently some amount of work being done

on variable specific impulse or multi-mode EP

devices. Currently, specific impulse is

principally varied by modulating electrical or

heating parameters in devices. Hall effectthrusters can vary their specific impulse by

increasing or decreasing the discharge

voltage and beam current. The range between

minimum and maximum specific impulse for

currently available hall thrusters is

approximately 500 to 1000s with the typical

maximum specific impulse around 3000s.

Hall thrusters are currently the most

commonly studied potential bimodal electric

propulsion device and most already have

some specific impulse flexibility over a small

range. Although Ion thrusters would in theory

derive the most benefit from dual propellants,

they suffer from inefficiencies at lower

specific impulse operation and scalability

making them generally impractical for

manned interplanetary missions. There are


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attempts modifying ion engines to be partially

bimodal, the GIE NEXT attempts to improve

upon throttle-ability and specific impulse, but

over a large range of power and suffers

technical issues. Although no other flown

models have been specifically designed forvariable specific impulse, there are some

models being tested and designed, including

the T-220HT-HET currently being developed

and tested in Georgia Tech’s HPEPL. In

addition to more conventional Hall thrusters,

there are other short and medium term

technologies being pursued. The same paper

from Acta-Astronautica mentions some of

these: Hybrid electrostatic systems like

HET/GIE (ex. QinetiQ) which combines thetwo propulsion systems either in parallel or

as an integrated system (much the same

principle as the chemical rockets), and double

stage hall-effect thrusters (ex. SPT-MAG and

LABEN-ALTA DSHET), which separates the

ionization and acceleration regions of hall

effect propulsion. Shorter term Nested Hall

Thrusters (NHT) have a big potential in

covering a broad range of specific impulses

and thrust levels. Of course, there are also

longer term and more novel technologies

such as the VASIMR, PIT and HIIPER concepts

to approach the goal of VSI EP.

Currently the most prominent design

considering variable specific impulse for

electrically powered mars transit vehicles is

VASIMR. VASIMR stands for VAriable Specific

Impulse Magnetoplasma Rocket and is being

developed by the Ad Astra Company founded

and led by former astronaut Chang Diaz.

Simply stated, VASIMR essentially develops

thrust by heating a gas and converting its

perpendicular motion into parallel motion,

fundamentally similar to chemical rockets. It

consists of three stages which comprise three

main subsystems: the plasma injection stage

utilizing a helicon antenna, the heating stage

utilizing an Ion Cyclotron Resonance Heating

(ICRH) antenna, and the expanding stage

utilizing a magnetic nozzle. The design is

electrode less, meaning that the high

temperature plasma does not erode the

device and can handle high power densities.This is achieved by magnetic confinement

which ties all three stages together and using

RF power to produce and heat the plasma.

Thrust and specific impulse is varied

primarily by selectively partitioning the RF

power to the helicon or ICRH systems, along

with adjusting the propellant mass flow.

VASIMR experiences a few

technological challenges at the moment.

Contributing many of issues are the large andstrong magnetic fields it requires. This

presents 4 main issues: charged particles

remaining attached to field lines causing

greater beam divergence, losses, and charging

of the spacecraft, the powerful

superconducting magnets required are both

complex and heavy, the shielding that would

be required for both communication and

health considerations, and induced torques or

movement of charges due to electromagneticinteractions. In addition to this, VASIMR

suffers from significant thermal management

considerations, requiring rather large and

potentially heavy radiators. The latter issue is

only more profound when the power systems

are taken into account, which represents the

most important consideration with any

electric propulsion device. VASIMR gets

significant benefits from increasing power

levels, and since its proposed roles are

primarily interplanetary and/or large

payload coupled with fast transit, it is

generally advocated that the VASIMR

operates in MWs of power. Due to this Dr.

Chang Diaz has suggested nuclear power,

which additionally needs significant thermal

management, and is likely heavier than solar


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power. VASIMR has also yet to demonstrate

long term firing over the ambitioned specific

impulse range. As of today the VX-200 can

only be fired for less than a 60 seconds with

1.2 seconds being the average firing length

and needs to be cooled down over anextensive period of time, and the specific

impulse has only been optimized and

controlled between 780s – 4900s, far less

than the ambition range of 3000 – 30000s

[6][7].

So far, the demonstrated VX-200 has

been able to achieve maximum 51mN/kW at

1660s and 35 mN/kW at the maximum thrust

level, and thrust efficiencies varied from

around 10% to 72% at the highest thrustlevel with around 30% thrust efficiency

around the maximum thrust to input power,

for short periods of time. [7] On the other

hand Hall effect thrusters have demonstrated

levels of 90mN/kW at similar efficiencies or

better over a similar range. NASA’s 457Mv2

Hall thruster has demonstrated 76.4 mN/kW

at low power and 46.1mN/kW at max power,

with anode efficiencies between about 55 to

70%. [8] Furthermore the specific power ofVASIMR is projected to only come down to

about 1.6kg/kW for a 1MW case, 3kg/kW for

250kW case. [9] Whereas the hall effect

thrusters have demonstrated 1.3 kg/kW and

below at 6kW which improves with scaling.

[10] Nested hall effect thrusters are expected

to improve those values even further to

perhaps 0.5kg/kW for MW levels and be able

to vary impulse between 1000-5000s.[11] It

should be also considered that Hall effect

thrusters have been flown and tested

extensively, and can be fired for 1000’s of

hours. They have also been shown to be fuel

flexible with the 400M model operated on

both krypton and xenon. Benefits for dual

propellants can benefit any electric

propulsion device including VASIMR, which

can virtually run on any propellant and has

considered propellants such as deuterium

and krypton. However, until VASIMR has

demonstrated to be more competitive with

current and shorter term technologies, the

focus of the research will be more focused ondemonstrated propulsion devices such as hall

effect thrusters and electrostatic ion engines.

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Masters Special Problem 9800