Spacecraft Propulsion Tech Committee
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Proposal for an Ascent Stage Propulsion/Control System

15 March 1997

This is a proposal for the Artemis Reference Mission Ascent Stage Boost Phase Propulsion/Control Thruster system. It can essentially be separated into two parts: one part that deals with performance and controls issues, and one that deals with the functionality of the propulsion system.

Please note that this proposal will only address pitch and yaw attitude control. In exoatmospheric boost flight, roll control is often a secondary concern. As such, the issues pertaining to roll control will not be addressed.

Performance and Control

System Description

The propulsion system I am proposing uses three main thrusters concentrated near the center of the Ascent Stage to provide the majority of the vehicle's thrust. It also uses four aft-facing attitude thrusters placed on the outside of the Ascent Stage's main structural ring to provide the vehicle's pitch and yaw control. The main thrusters are Kaiser Marquardt R-40A thrusters. These thrusters are used on board the Space Shuttle for attitude control. They were chosen because they were just about the only flight-proven thrusters in the proper thrust range that I could find in the Propulsion Technical Committee's engine spec list. A rear view (slightly above and to one side) of the Ascent Stage is shown below.

Figure 1

In addition to the aft-facing thrusters, there are also four forward-facing attitude thrusters that may be used for pitch and yaw control during the boost phase. They have been angled 45 degrees out of plane because these thrusters will also be used to slow the Ascent Stage as it approaches the LTV for rendezvous; the 45 degree angle will help direct their exhaust away from the LTV and thereby help avoid contaminating it. A front view of the Ascent Stage is shown below.

Figure 2

Please note that the vehicle sketches shown in the above figures are not meant to represent the exact thruster placements and orientations that the Ascent Stage will use. The attitude thrusters will likely be placed as far outboard as is practical in order to increase their effectiveness. The main thrusters will probably be positioned and angled so that their thrust lines bound the vehicle's center of gravity uncertainties. It should also be obvious that a large portion of the Ascent Stage's other structures have not been shown correctly (or at all) in these drawings.

Engine Characteristics

Relevant characteristics of the main thrusters have been obtained from the Propulsion Technical Committee's engine spec list, and are summarized below:

Engine              Kaiser Marquardt R-40A
Thrust              3,870 N  (range from 3,114 to 5,338 N)
Mass                10.25 kg
Isp                 281 sec  (range from 281 to 306 sec)
Propellants         MMH/N2O4
O/F                 1.6
Propellant Pressure 16.4 atm
Area Ratio          20  (range from 20 to 150)
Burn Time           500 sec max for a single burn,
                     over 15,000 sec total lifetime

Nominal Functioning

During a nominal ascent (i.e., one in which all thrusters are functioning properly), the three main thrusters will fire continuously for the duration of the boost from the lunar surface until a Low Lunar Orbit (LLO) is achieved. The four rear-facing attitude thrusters will also be fired nearly continuously during the boost. However, the attitude thrusters will be "off-pulsed" for pitch and yaw control. That is, each attitude thruster will be turned off whenever needed to keep the vehicle at its desired pitch and yaw attitudes. The use of the four attitude thrusters should result in a modest reduction of structural loading on the vehicle due to a distribution in the thrust forces. It should also slightly increase the vehicle's performance as well due to a reduction in gravity losses.

The four forward-facing attitude thrusters will not be used during a nominal ascent, but will likely be used during the LTV rendezvous.

Failure Tolerance

This system has been designed to able to cope with the loss of any one of the seven thrusters (three main, four attitude) that it would nominally use during an ascent from the lunar surface. Loss of attitude thrusters nominally used during the rendezvous with the LTV is a separate issue and will not be addressed.

The loss of one attitude thruster will likely necessitate the shutdown of all of the other aft-facing attitude thrusters, in order to prevent unbalanced moments from causing the vehicle to tumble. In such an event, pitch and yaw control will be transferred to the four forward-facing attitude thrusters shown in Figure 2. These thrusters will use an "on-pulsed" scheme to maintain control (i.e., the thrusters are turned on as needed to maintain the desired pitch and yaw attitudes). This is done to minimize the performance loss that will occur when the thrust from the attitude thrusters opposes the thrust from the main engines. The resulting ascent to orbit will take longer, but should still be doable. Even though these thrusters are angled 45 degrees out of plane, the distances of their thrust lines from the center of gravity should only be slightly less than those of the aft-facing thrusters. As a result, they should have nearly as much control effectiveness as the aft-facing thrusters. The shutdown of the aft-facing thrusters will unfortunately eliminate the structural benefits of distributing the forward thrust, and for this reason this case could become the limiting case to which the structure is designed.

The loss of one main thruster may not affect the control scheme used at all. If the main thrusters are positioned and angled as suggested above, it is quite possible that the four aft-facing attitude thrusters will retain enough control authority to maintain control (although the resulting pulsing commands will be significantly different). If it turns out that the aft-facing thrusters do not have enough control authority, the forward facing thrusters could be "on-pulsed" to work in conjunction with the aft-facing thrusters. The performance of the vehicle in this case will be noticeably affected, due to increased gravitational losses. This may be the limiting performance case. Takeoff thrust should still be more than sufficient, though. (The reason three main thrusters were chosen was to ensure adequate liftoff thrust if one fails; if only two thrusters were used and one failed, the vehicle's takeoff thrust to lunar weight ratio would be dangerously close to one.)


More than three dozen of these R-40A thrusters fly on each Space Shuttle mission. A number have also been used on satellites. These thrusters should be easy to obtain. Also, it would seem reasonable that by now NASA and the manufacturer have ample performance characterization data on the engines. Some of this data might even be available (at least to U.S. citizens).

Attitude Thrusters

At this point, I have not yet come upon any small thrusters in the Engine Spec List that stand out as the best choice for the attitude thrusters we'll need. Initially, I was leaning toward the Kaiser Marquardt R-1E, which are the thrusters the Space Shuttle uses for Vernier (extremely precise) attitude control. However, these thrusters are rather heavy for their thrust capability; they have a mass of 3.7 kg and produce only 110 N of thrust nominally. It may be wise to hold off selecting a baseline set of attitude thrusters until we have a better idea of what their performance requirements will be. (Their performance requirements will be based on a number of factors, including their locations, the main thruster locations, the stage's center of gravity, and the stage's moments of inertia.)


One possible concern with this engine is its maximum time for a single burn. The Ascent Stage will need to reach LLO in about eight minutes, even in an engine-out scenario. This should be investigated further in trajectory simulations.


A functional schematic of the proposed propulsion system is shown below in Figure 3. This system is a blowdown system, i.e., one which uses only the initial pressure inside its tanks to expel propellants. There are four propellant tanks in this system, two for MMH and two for N2O4. The propellants themselves will occupy half of the volume of these tanks when full, and will be contained inside positive expulsion bladders. The positive expulsion bladders will ensure that the propellants are always properly seated for use and should allow propellant feed to continue normally until the tanks are very nearly empty. The remainder of the tanks will be filled with a helium pressurant, initially at 24 atm, through the He Fill/Drain valve. Both fuel and oxidizer will be conducted into one common feed line (for each), to help ensure equal usage from both (fuel or oxidizer) tanks.

Figure 3

Please note that any "valve" depicted in the schematic could be one of a number of different types of valves, and could even represent multiple valves needed at the same location. I did not think it was necessary at this stage to pin down every valve in the system; I just put valves where I was pretty sure something would be needed.

Tank Jettison

This system should allow for reasonably easy disconnection (and subsequent discarding) of the Ascent propellant tanks after docking with the LTV has been accomplished. Getting rid of the tanks in LLO will serve two purposes. First, it will reduce the dry mass that the LTV must return to LEO. Second, and more important, it will help to significantly reduce the uncertainty associated with the Ascent Stage's return mass. If the tanks are not jettisoned in LLO (or if Ascent propellants are not expended in some other fashion), the LTV will need to carry extra propellant to cover cases in which the Ascent Stage is the most efficient in the use of its propellant. This amounts to a performance penalty on the whole translunar system because part of it performs above expectations, and in my opinion is unacceptable. In any event, new tanks will probably be brought from Earth for future missions.


A blowdown system is a very simple system. The only moving parts are the electrically-operated valves on the thrusters themselves. (The other valves depicted in Figure 3 may need to be opened or closed only once before or after the flight.) A pressure regulated system (the only other option for pressure-fed thrusters) would require the addition of a high pressure pressurant (He) tank and a regulator.

Sizing Data

Current calculations indicate that about 1500 lb of propellant will be needed on the Ascent Stage. Using an oxidizer to fuel ratio of 1.6 (the nominal ratio of the R-40A), this corresponds to about 925 lb (10.22 ft3) of N2O4 and 580 lb (10.56 ft3) of MMH. Assuming that half of each propellant tank volume is initially filled by He pressurant, two tanks of each of the above volumes will be needed. The MMH tanks will have a radius of 1.361 ft and the N2O4 tanks will have a radius of 1.346 ft. Using an initial tank pressure of 24 atm (which will yield a final pressure of 12 atm), spherical aluminum tanks (with a tensile yield strength of 47 ksi and a safety factor of 1.5) of the required volumes would weigh about 31.1 and 30.1 lb each for MMH and N2O4, respectively.

Performance Impacts

Using a blowdown system will have an effect on the system's performance. Initially, the thrusters will produce more thrust at a higher specific impulse because they are operating at a higher pressure than normal. This is beneficial because it will provide extra thrust and control authority when the vehicle is the most massive. As fuel is expended, the system will gradually lose pressure, and as a result, some performance. Near the end of the flight (when the tanks are nearly empty), the thrust will be at its lowest. This will be somewhat desirable, since it will lower the minimum impulse bit the attitude thrusters can produce during the rendezvous with the LTV. A lower minimum impulse bit will allow the Ascent Stage to have more precise maneuvering control during the rendezvous.


One possible concern with this schematic is that the N2O4 may not react well with the rubber materials commonly used to make bladders. If so, another method of ensuring oxidizer expulsion from the tanks and preventing unauthorized mixing of fuel and oxidizer will have to be developed.

Another concern is that MMH and N2O4 will have noticeably different head loss (of pressure) in their feedlines. If that is true, different starting pressures will need to be used for the MMH and N2O4 tanks.

Concluding Remarks

This proposal should not be viewed as the final word on the Ascent Stage propulsion system. Rather, it should be taken as a starting point, one which can evolve and be used for comparison with other concepts. I think it would be appropriate for some trade studies to be done on this conceptual design. A few that come to mind include:

Spacecraft Propulsion Tech Committee

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