Orion Mars Mission Propulsion Module Specific Impulse

(Exhaust Vel) 2,500 s

(24,500 m/s) Thrust 3,500,000 N Dry Mass 91,000 kg Pusher Plate

Diameter 10 m Height 21 m

This is from Manned Planetary Exploration Capability Using Nuclear Pulse Propulsion by Paul R. Shipps. Basically it shows how an Orion-powered Mars mission is so superior to a chemically powered mission that it just isn't funny.

The family of Mars missions uses the basic 10-meter pusher plate propulsion module, since that can be lofted by a Saturn V. If you limit mission designs to non-multistage missions, it still has outrageous amounts of delta-V. The study found it could handle mission with delta-V ranging from 12,000 to 34,800 m/s and payloads from 45,000 to 200,000 kilograms.

The Orion can do the same miniscule mission as a chemically powered rocket if the Orion has a total Initial Mass In Low Earth Orbit (IMLEO) of only 290,000 kg. But that's where the chemical rocket maxes out while the Orion is just getting started. You can load it with metric tons of extra propellant and do the mission in 200 days flat instead of three years. Or you can increase the mission to 400 days, but add lots more scientist to the crew along with tons of scientific instruments. You can even add more fuel and return to an elliptical orbit around Terra using a brute-force rocket thrust braking instead of barbecuing the ship with aerobraking. Orion has power to spare and then some.

The standard Mars missions designed use multiple Saturn V launches to loft the components into orbit. But if you want to cut costs and have the political will, you can boost the Orion spacecraft into orbit with one Saturn V launch — if you don't mind it switching to Orion nuclear pulse drive while still in the atmosphere, starting at an altitude of 50 nautical miles (93 km). This is not totally risk-free, but the risk is manageable. But just try explaining that to your hysterical constituents.

click for larger image

Nucler Pulse Propulsion Module

Internal details of the engine can be found here. It has a specific impulse of 2,500 s (exhaust velocity of 24,500 m/s), a dry mass of 91,000 kg, and an effective thrust of 3,500,000 N. "Effective" because the thrust is not continuous, the nukes go off at about 1 second intervals.

The interesting thing is all the various Mars missions can be performed by the basic Orion propulsion module as is. All you have to do is change the number of nuclear pulse units it carres. The raw might of nuclear fission makes this engine very flexible.

The propulsion module does have limited internal space for internal magazines, but the bulk of the nuclear pulse units are a carried in external magazines, which are ejected when empty.

Propulsion module (hot pink in diagram) does not include payload spine (green) nor the (empty) external propellant magazines with magazine support structure (gold). In other words the 91,000 kg dry mass is just the hot pink part. Especially since the number (and mass) of magazines varies with the mission.

The study authors wanted to avoid a lot of tedious calculation so they used a simplification. To do calculations for all the missions, the correct way is to total up the the mass of the needed empty magazines, the RCS propellant, and whatnot to be added to the "dry mass" for mass ratio calculation. This takes forever. The study authors found out that you get much the same answer if you simply downgrade the propulsion module's specific impulse by a fixed percentage. For this module (with some internal magazine storage capacity), a 4% downgrade of specific impulse would account for magazine weight, magazine support structure, and the Reaction Control System (RCS) fuel. But with lots less math.

For the above reason, instead of calculating delta-V as if the propulsion module had a specific impulse of 2,500 seconds, they instead used 2,500 / 1.04 = 2,405 seconds (and an exhaust velocity of 23,593 m/s).

The mass of the empty magazines, magazine support structure, and RCS fuel is more or less considered to be part of the payload (via the 4% downgrade trick). Naturally the mass of the nuclear pulse units proper is considered to be propellant mass (part of the "wet mass").

Mars Mission Velocity Requirements

Because NASA reports contain eternal optimism the report writers analyzed a Mars mission departing Terra in 1982, a mere 17 years from when the report was written.

This was to be a simplistic, inelegant, brute-force mission. All the maneuvers were done by rocket thrust, no fancy aerobraking was used to reduce delta-V requirements.

They only figured the delta-V for two maneuvers in a given mission: Terra-to-Mars (ΔV out ) and Mars-to-Terra (ΔV back ). You can get away with this simplification if your spacecraft does not use multistaging. The only reason the study authors used two delta-V measures instead of one is because the spacecraft mass changes so drastically. Lots of payload is consumed or left at Mars, particularly the two Aeronutronic landers.

The mission assumes the spacecraft returns to a Terran elliptical orbit (Terran approach velocity of 11,000 m/s), have a reserve of 300 m/s RCS outbound and 460 m/s inbound, plus a 3% performance reserve. The crew is transferred from the spacecraft to Terra by a separate pickup vehicle based in orbit or on Terra (not carried by the Orion).

For each mission, a 40-day Mars orbit capture period is included in the durations. So the scientists landed on Mars can do as much science as they possibly can in one and one-third months.

Mission Delta-Vs Mission

Duration ΔV out ΔV back ΔV total 450-day 9,100 m/s 12,500 m/s 21,600 m/s 350-day 14,000 m/s 14,600 m/s 28,600 m/s 240-day 18,000 m/s 16,800 m/s 34,800 m/s

Remember these mission are brute-force. NASA trajectory analysis can reduce the trip times by about 50 days or so by using swing-by maneuvers and other fancy mission optimizations. Others reduce the delta-V. For instance, NASA has a Venus swing-by maneuver which can do a 450 to 500 day Mars mission for a low-low total delta-V of 12,000 m/s

Mission Delta-Vs Mission

Duration ΔV out ΔV back ΔV total 450 to

500-day 7,600 m/s 4,400 m/s 12,000 m/s

Again, the point is all these missions can be performed with the exact same Orion propulsion unit by simply modifying the amount of nuclear pulse units carried. Other propulsion systems would need staging or totally different designs of propellant tanks sizes. With Orion you just stack another layer of standard bomb magazines in the rack.

Mars Mission Payload and Duration Options

Figure 2

103 LB = 450 kg

106 LB = 450,000 kg

In figure 2, the ordinate is the Orbit Departure Weight (IMLEO) and the abscissa is Total Payload. Abscissa is in units of one-thousand pounds (103 LB or 450 kg). Ordinate is in units of one-million pounds (106 LB or 450,000 kg) on the left, and in units of uprated Saturn V payloads on the right (127,000 kg).

The total payload is assumed to be split 50-50 into the so-called "round-trip" payload and the "destination" payload. The former is payload carried both to and from Mars, the latter is assumed to be all consumed or abandoned on Mars. 50-50 sounds arbitrary, but as it turns out lots of carefully planned mission studies have something very close to that split.

The dotted line at the bottom contains the anemic chemically-powered miniscule mission previously referred to, helpfully labeled with "Minimal Manned Landing Mission". Rubbing salt in the wound, the report authors point out that this chemical rocket can only carry a small number of crew (requiring each person to have multiple functions, and increasing each person's work-load) and the rocket will need a high degree of expensive subsystem development and optimization because Every Gram Counts. Neither of which apply to a rocket driven by exploding nuclear bombs.

Just in case you might have forgotten what you read in the last ten seconds, the report authors reiterate that one single standard Orion propulsion unit can perform any of the mission on the chart, no expensive development and optimization required. The report authors also wrote that at the top of figure 2, just because.

The two points marked "Reference Designs" are based on specific payload breakdowns of about 145,000 kg. The 450-day reference design has an IMLEO of 522,000 kg (about x4 Saturn V payloads), the 250-day reference design has an IMLEO of 839,000 kg (about x7 Saturn V payloads).

Remember the missions assume that the spacecraft does not carry any pickup vehicle. Once it returns to Terran elliptical orbit the crew is rescued by a separate vehicle stationed in orbit or on Terra.

If you examine the chart you will be interested to find that reducing the mission duration does NOT create an outrageous increase in IMLEO. You want a half-year Mars mission? No problem!

Table 2

units are in pounds, 1 pound = 0.45 kg

Table 2 contains the weight statements for the two reference designs.

The "radiation shelter" is the over-sized storm cellar, found in all Orion control rooms. All long-duration spacecraft need storm cellar to protect the crew from solar proton storms. All Orion need extra-strength storm cellars because being propelled by the equivalent of a small nuclear war is not healthy for children and other living things.

The storm cellar mass is enough to reduce the radiation exposure from the Orion drive to only 0.5 Sievert per mission. This cellar will keep the dose from solar proton storms at 1 Sievert per mission. The two reference missions have the same storm cellar mass. The 250 day mission has a shorter solar exposure than the 450 day mission, but a higher nuclear pulse exposure because more bombs are needed to shorten the trip. So it equals out.

The propulsion periods when the crew has to retire to the storm cellar are usually short, from a few to about 20 minutes. The nuclear pulse units radiaton flux do not cause significant neutron activation so the crew can access any part of the spacecraft a short time after propulsion shutdown.

The majority of the destination payload is the two Aeronutronic Mars Landers (tail-sitter version). These were designed for a different spacecraft but as it turns out they fit on the 10-meter nuclear pulse rocket with only minor modification (payload spine has to be flattened).

The Exploration Vehicle Configuration

Figure 3

click for larger image

The payload stack consists of the payload spine supporting the flight station at the top, next lower is the personnel accomodations, then the Mars payload including the two landers. The bottom of the payload spine provides crew access to the propulsion module. The lower part of the spine passes through the center of the magazine stack, and encloses a repair-bay/spares-storage room (3 meters diameter by 7.6 meters tall).

The payload spine is flattened in two places to accomodate the landers. If the required pulse unit magazine stack is too tall to fit under the Mars payload, the payload spine might have to be lengthened a bit. This is the only modification the Orion spacecraft is likely to need.

The personnel accomodations is "upside down" because the entire spacecraft is a tumbling pigeon. The center of gravity (CG) of tumbling pigeon rotation moves aft as nuclear pulse units and landers are expended.

Options in Personnel Complement

Figure 4

In figure 4, the ordinate is the Terran Orbit Departure Weight (IMLEO) just like figure 2, but the the abscissa is the number of crew members.

The dotted line shows how rapidly the IMLEO rises with the number of crew for an 850 second multi-stage NERVA-style nuclear thermal rocket. The solid lines show how modest the IMLEO increase is for extra crew with an Orion boom-boom rocket. Again the report writers harp on the fact that Orion is not subject to Every Gram Counts. With other anemic propulsion systems designers have to have the maximum payload determined at the start of the design process. The max payload is carved in stone. Once you have produced the spacecraft, adding more payload makes it impossible for the spacecraft to do the mission. With the mighty Orion on the other hand, adding more payload just means you just have to add a few more bomb magazines.

Figure 4 illustrates a useful concept called "loading factor".

With the 400-day mission, adding an additional person increases the round-trip payload by about 4,500 kg, once you add in the extra food, water, and air. This additional mass needs additional propellant (pulse units) to propel it. The extra payload plus extra propellant increases the IMLEO by about 11,300 kg. So the loading factor is 2.5 to 1. Which means for every unit of extra payload mass you add, the IMLEO mass increases by 2.5 units.

In other words, for each additional 100 kg of inert weight added (telescopes, cornflakes, meteoroid protection, heavier structure) you need only add 150 kg of propellant to carry it through the journey! (100+150 = 250, which is a loading factor 2.5 to 1) No vehicle change is required, just add more propellant.

For the 200-day mission the loading factor is more like 4.3 to 1.

Options in Terra Return Conditions

The reference design missions assume the spacecraft returns to Terra and uses a modest amount of thrust to enter an economical but wildly ellptical Terran orbit (approach velocity about 11,000 m/s). The missions do not waste payload mass by lugging along a little Terra reentry vehicle, they assume the crew will be rescued by a local vehicle stationed in Terra orbit or on a surface base. The Orion spacecraft will remain in elliptical orbit, available for restocking and reuse.

The report authors looked into two other options.

Orion spacecraft can be braked into a nice circular LEO orbit, if you are willing to carry additional propellant





If the priority is to save propellant and reduce IMLEO: you reduce propellant stock, carry a reentry vehicle, and the crew bails out in said vehicle as the Orion goes streaking past Terra on a one-way trip into the dark of the Solar system. The Orion passes by Terra at about 15,000 m/s relative. Another study estimated that the mass for a reentry vehicle for 8 crew and 15,000 m/s is about 6,990 kg.

Figure 5

In figure 5, the ordinate is the Terran Orbit Departure Weight (IMLEO) just like figure 2.

The pair of bars at 50,000 ft/sec (15,000 m/s) is the propellant-saving "abandon ship" option.

The pair of bars at 35,000 ft/sec (11,000 m/s) is the standard reference missions.

The pair of bars at Circular Orbit is the propellant-wasting circular LEO option.

As you can see the "abandon ship" option has a lower IMLEO, though the mass of the reentry vehicle reduces the savings somewhat. And you cannot reuse the Orion. The circular orbit option does have a higher IMLEO, but not by an overwhelming amount.

Single Launch Mission Capacity

The reference missions assume multiple Saturn V launches to loft the components into orbit, where they are assembled. There is a way to use just one Saturn V launch. Unfortunately it involves using the Orion drive. In Terra's atmosphere.

The Orion is used as the top stage, starting at an altitude of 93 kilometers. The savings are substantial, the risks are manageable. But the thought of detonating *Two* *Hundred* *Nuclear* *Bombs* per launch will cause any nukeophobic person to scream in your face at the top of their lungs. Especially if you are a politician and they are one of your constituents.

The reference mission has one Saturn V launch to loft the Orion propulsion module, one launch for operational payload (personnel accomodations unit, remaining vehicle structure, some supplies), one launch for the Mars excursion modules, and a couple of launches carrying nuclear pulse units and miscellaneous small payloads.

And as is typical for any space system, the direct operating costs are dominated by the cost of boosting the stuff into orbit. "Halfway to Anywhere", remember? Reducing the number of Saturn V launches will cut the costs dramatically. Not to mention avoiding the nightmare of orbital assembly.

Figure 6

Figure 6 shows a fully assembled Orion with a gross weight of 635,000 kg (1.4×106 LB) being boosted by a Saturn V with an uprated S-1C stage (since the standard S-1C cannot structurally handle that much payload, plus it needs more thrust and delta-V).

The Orion ignites at an altitude of about 98 kilometers (53 nautical miles) and starts nuking away. This is high enough to protect the eyesight of idiots who cannot be bothered with warnings of not being too close to the launch site and staring directly at freaking nuclear explosions. The Orion arrives at LEO with its mass reduced to 476,000 kg due to burning 159,000 kg of nuclear pulse units.

The Orion then performs some shakedown maneuvers to get all the bugs out. After that the IMLEO mass is about 454,000 kg (1×106 LB). Looking it up in figure 2 we can see that is enough for quite a few mission options. It can do a total payload 250×103 LB (113,000 kg) in a 400 to 450-day mission returning to elliptical Terra orbit. Or even 430×103 LB (195,000 kg) if you are willing to settle for a 450 to 500-day minimum ΔV mission.

You will, however, need one additional launch to boost the crew into orbit. Trying to man-rate a nuclear Orion boost into orbit would be a nightmare. Just man-rating the Orion for deep-space operations is hard enough.

Since the initial Orion gross weight is 635,000 kg and the effective thrust is 3,500,000 N you can see the initial thrust-to-weight ratio during orbital boost is 0.55. This is a pretty low ratio compared to chemical rockets. However the report assures us that detailed trajectory computations (that they do not elaborate on) reveal that for a 2,500 sec I sp rocket this thrust-to-weight ratio actually maximizes the amount of weight delivered to LEO.

SYSTEM ADVANTAGES AND SYSTEM PROBLEMS

There are other advantages to the Orion, besides the flexibility of a single design that the report authors keep mentioning every five minutes. And of course there are disadvantages as well.

Single Vehicle Operational Advantages

Pretty much all the the other Mars mission spacecraft rely upon mult-staging, whether chemical or nuclear-thermal. But not Orion.

One major advantage is a single-stage vehicles can do several test flights and shakedown cruises. You can't do that with multi-stage craft, not if they have to jettison parts of themselves as part of the test. Which means the the brave crew of a mult-stage craft have to set forth on a mission to distant Mars IN AN UNTESTED VEHICLE.

Nothing works perfectly the first time. Shakedown cruises allow debugging the systems, and allow the crew to become familiar with the peculiarities. It also allows any incipient or "break-in" failures to be fixed before departure. Instead of becoming a life-or-death emergency 54.6 million kilometers from the closest help.

Shakedown cruises also allow actual operating performance to be verified, the spacecraft's center of gravity can be trimmed, and unexpectedly high-loss or high-consumption expendables can be supplemented.

Plus any unexpectedly discovered overwhelming problems will result in merely cancelling the mission, instead of a spacecraft lost with all hands in the black depths of space.

Test flights and shakedown cruises are standard procedure in the aircraft, marine, automotive, and other transportation fields. Setting forth on a long voyage without such test is unthinkable, except in the ad-hoc one-shot mult-stage rocket biz. Orion will allow a return to rational testing.

Economic Advantages

The flexibility of a single design raises its head again, reminding us that it is a vast cost saving to just make one design and reuse it. Instead of making and debugging a freaking new design for every single new mission. This also costs savings in shakedown cruises, since the design bugs will have mostly been already discovered only the specific ship idiosyncrasies will have to be found.

Another advantage is that nuclear pulse units are nicely dense, highly storable, and mostly trouble-free. Other propulsion systems use liquid hydrogen which is pretty much the exact opposite. Liquid hydrogen is annoyingly non-dense, requiring monstrously huge tanks and thus lots of booster vehicles and launch facilities. Liquid hydrogen is not storable at all, suffering from boil-off and thus requiring power-hungry cryogenic cooling equipment. Boil-off also forces closely-spaced successive launches because the longer the hydrogen tanks loiter in orbit waiting for the rest the more hydrogen will be lost. Finally a spacecraft loaded with nice dense nuclear pulse units will have a high ballistic coefficient which will protect it from atmospheric drag deorbiting it. The poor spacecraft loaded with liquid hydrogen will have to depart quickly or suffer a fiery crash.

But the most significant economic advantage is designing mission subsystems while being free of the tyranny of Every Gram Counts. Instead of spending tons of money and time trying to make featherweight (yet reliable) versions of all systems, you can just slap them together out of boilerplate like old Soviet spacecraft. When an additional 100 kg can be carried by simply loading an extra 148 kg of propellant, many subsystem problems become easier to solve.

The main economic disadvantage of Orion is that the pulse units are shockingly expensive. Which is not surprising considering that they are loaded with highly-enriched weapons-grade uranium-235. The official price of HEU is classified, on the black market weapons-grade uranium has a spot price of $10,000 a gram. The back of my envelope says the propellant mass will be roughly 1.4% HEU. Liquid hydrogen on the other hand is about $0.70 US per kilogram.

Enroute Maintenance Capacity

The Orion is far more maintainable enroute than other spacecraft. Especially other nuclear ones.

Orion has very low residual radioactivity, even after a large delta-V maneuver. The nuclear pulse units use beryllium oxide as a channel filler plus tungsten as propellant in order to sop up the neutrons heading for the spacecraft. The idea is it is better to use the neutron energy to accelerate the propellant instead of wasting them and allowing them to turn the butt-end of the ship radioactive. It is safe for the crew to exit the storm cellar surrounding the flight station immediately upon propulsion shutdown. And only a short delay is needed for the neutron activation levels to die down to a safe level, allown crew access to the entire spacecraft. Even the pusher plate.

Nuclear thermal rockets, on the other hand, are neutron activation machines. Once the engine has been used it will be dangerously radioactive for decades to come.

The propellant is packaged in convenient discrete, dense containers instead of being large volumes of liquid hydrogen boiling away in cryogenically cooled propellant tanks. Trying to do maintenance inside a tank of -253°C LH 2 is a good way to die. Or trying to do maintenance nearby an LH 2 tank. Orions have no cryogentic components (except for maybe the RCS) so all the ship components are easily accessable at temperatures normal for the space environment. This also means the structural members can be composed of ordinary steels, aluminum alloys and titanium instead of exotic hard-to-fix stuff.

To take advantage of this easy access the Orion is designed with a large well-equipped repair bay and spare parts storage area. The ship can be worked on during coasting periods.

Developmental Problems

If this spacecraft is so great, why ain't NASA using them? Well, there are a few … problems.

The report mentions that there are some uncertainties about the development of the nuclear pulse units, which unfortunately they cannot talk about because it is classified. They are after all basically nuclear weapons.

All such programs have three classes of developmental problems: technical, programmatic (research and development), and poltical. Ordinary rocket projects usually have big problems with the first two classes, but the political problems are minor or nonexistent. With Orion, the bulk of the problems are political.

Orion has the technical problems well in hand, with lots of research and experimentation on ablation, explosive debris — pusher-plate interactions, and impulsive loading on structures.

Orion's programmatic problems are mostly due to the fact that there is no immediate "requirement" for a spacecraft with such a huge thrust and delta-V capacity. So the budgets are limited. The report is of the opinion that if Orion spacecraft are made available, rocket scientists will be falling over each other to take advantage of the oodles of delta-V and thrust they provide.

But Orion's political problems are where the poop hits the fan.

The report says the problem "rather obviously, stems from the fact that nuclear pulse propulsion uses in small scale the same energy source used for nuclear weapons". Translation: the voters are going to scream "OMG!!! YOU ARE TRYING TO MAKE A ROCKETSHIP THAT USES FREAKING ATOM BOMBS!!! ARE YOU CRAZY??!?"

A related political problem is that the Partial Test Ban Treaty forbids civilian nuclear detonations anywhere but underground. Which is a problem for a spaceship. The report optimistically mentions that the treaty provides procedures for its own amendment. Good luck with that.