1. Fixed Main Engine versus Gimbal Mounted Alternative
Current Tasks:
Gimbal Mount Alternative
Separation Landing Alternative
Minimizing Thruster Cost
2/19/09
Kris Ezra
Attitude Group
Translunar Phase 1

2. Gimbal System Explanation
Advantages: Thrust vectoring implies possible decrease in attitude control prop
Disadvantages: Extra complexity and mass
Gimbal Mount Specifications:
Approximate mass of 6 kg
Angular maximum motion of 20º
3 Axis Gimbal
Relatively low complexity
Design, build, and test in house <$1,000
20º
1.06 m m
Kris Ezra
Attitude Group
Translunar Phase

3. Comparison of Alternatives
Determination Method
Prop. Mass (kg)
Moment Calculation (no rxn wheels Engine)
228
Moment Calculation (no rxn wheels Attitude) 80
Moment Calculation (rxn wheels)
Specific Thrust Manip. (Attitude Desaturation)
6.287
Specific Thrust Manip. (Engine Desaturation
17.925
Lowest Total Mass Penalty:
Attitude Thrusters: kg
Gimbaled Engine: kg
Recommendations:
Because of limited use of gimbaled main engine in other phases due to thrust magnitude, recommend fixed main engine
Gimbaled Thrust Additional Costs:
kg mass increase
< $1,000 Design, build, test cost
Additional power sufficient to run 3 actuators
Gimbal controllers (mass, power, cost yet unknown)
Kris Ezra
Attitude Group
Translunar Phase

4. Mass Calculation Methods
m is the time averaged mass
M is the induced moment
g is the gravitation constant of m/s^2
Isp is the specific impulse of H202
L is the distance from the thruster to the center of mass
Method 2:
M is the total propellant mass
D is mission duration in days
n is the average number of desaturation maneuvers per day
J is the maximum torque provided by reaction wheels (Nm)
T is an estimate of the specific thrust of the propellant for a given engine (N/kg)
L is the moment arm of the desaturation device (m)
Kris Ezra
Attitude Group
Translunar Phase

5. Specific Data Method 1: Method 2: Reaction Wheel Torque (Nm) 0.02
Mission Length (days)
365
Engine Thrust (N)
0.2
Desaturation Maneuvers (#/day) 6
Thrust Offset (m)
0.022
Max Reation Wheel Torque (Nm)
0.03
Isp (s)
161
H202 Specific Thrust (N/kg)
9.5
Attitude Moment Arm (m)
1.1
g (m/s^2)
Engine Moment Arm (m)
0.3858
Mission Length (s)
Total mass (kg) (Attitude DS)
Without rxn Wheel Torques:
Total mass (kg) (Engine DS)
mbar (kg/s) (no rxn wheels)
E-06
Total Mass (kg) (no rxn wheels)
With rxn Wheel Torques:
mbar (kg/s)
Total Mass (kg)
Kris Ezra
Attitude Group
Translunar Phase

6. Determination of Specific Thrust
FUEL STORAGE
H2O2 %=
90.0
Kappa=
1.33
R =
369.67
M,gasmix
Density=
1.391
kg/liter
Run time=
120
sec
V =
4.4
liter
1.17
US gallons
w =
6.2 kg
13.7
lbs
FUEL PUMP
Fuel Flow=
0.0513
kg/sec
3.1
kg/min
0.04
lit/sec
2.2
lit/min
0.11
lbs/sec
0.59
gpm P=
23.7
bar(e)
FLOW DISTRIBUTION TRAY
Total tray hole area= 8
mm2
Pr.Drop=
0.1
bar (0.1 to 0.3 bar is recommended)
Hole velocity=
4.43
m/s
Hole diam 4 mm
Number of holes 1
pcs
Obtained and modified from
Kris Ezra
Attitude Group
Translunar Phase

7. Determination of Specific Thrust
CATALYST PACK
Temp=
739 oC
1012 K
Catalyst activity= 75
kg H2O2/liter catalyst,minute D= 17 mm
Rem: The practical experiance so far is that maximum about 100 kg hydrogen peroxide
Length=
187
373
screens (when using stacked silver screens ,0.5 mm thick)
per liter of silver catalyst volume and minute, can be decomposed
Volume=
0.04
liter
Tot. screen area
0.082 m2
Weight of screen pack = 82
gram
Pressure drop over the catpack =
14.6
bar
Liquid feed load=
0.014
kg/mm2,min
(typical value=0,010)
lb/in2,min
ROCKET NOZZLE
D, inlet=
A,inlet =
0.0002
P,inlet= 10
bar (abs)
147
psia
c,inlet= 87
m/s
v,inlet=
m3/kg
D,throut=
7.7
A,throut=
4.66E-05
P,throut
5.4 79
c,throut=
654
T,throut=
v,throut= C
D,exit=
11.0
A,exit=
9E-05
P,exit 1
14.7
c,exit
1148
THRUST=
58.86
N = 6
kgf =
13.33
lbf
Kinetic power= 34
kW = 45 HP
Obtained and modified from
Kris Ezra
Attitude Group
Translunar Phase

8. References
Bengtsson, Erik. “Peroxide Propulsion” Accessed 18 Feb 2009, URL:
Vaughan, David A., “Gimbal Development for the NEXT Ion Propulsion System” AIAA Joint Propulsion Conference & Exhibit, 2005.
Lukasak, J. “Propellant Requirements: Attitude Control Thrusters with Low Thrust Orbit Trajectories,” 12 Feb
Erson, B. “Attitude Control Systems,” 19 Feb
Kris Ezra
Attitude