Fiction Science #5: SOEIVs - Feet First Into Science

 

INTRODUCTION

This series looks to take a deep dive into some of the events and places of the Halo franchise and determine how sciencey they are. I make a point to give as much leeway to the fiction, so long as it does not directly contradict with science, since this is, you know, fiction.  I always make a point to warn people if there might be spoilers ahead, even if they are incredibly minor, because there are new people getting into the Halo lore every day, and what might be basic and obvious to a lore veteran, may not for a new recruit.  If nothing else, this article might spur some interest in the extended Halo universe, so if at the end you want to learn more, check out the related media for a good place to start.
 

RELATED MEDIA (VERY MINOR SPOILERS)

 
 

THE QUESTION

HOW PLAUSIBLE IS THE SINGLE OCCUPANT EXOATMOSPHERIC INSERTION VEHICLE (SOEIV / ODST DROP POD) AND WHAT WOULD IT FEEL LIKE TO BE IN ONE?
 

ESTABLISHING CONDITIONS

So this is simultaneously going to be a complicated and simple question.  On one hand, we are just looking at a Single Occupant Exoatmospheric Insertion Vehicle (SOEIV), or ODST drop pod, and what would be required for that to work in real life.  On the other hand there are several confounding factors that make this much more complicated, such as distance from the surface, drop acceleration, and braking deceleration.  To make this a determinable question, we need to place some constraints on the situation, and then see if our numbers fit within those constraints.  For the purposes of this analysis, I looked at the following constraints:

  • Minimum distance from surface that would be considered in orbit

  • Maximum drop acceleration without experiencing redout

  • Maximum braking deceleration without experiencing loss of consciousness (G-LOC)

  • Maximum survivable impact speed without injury

I'm going to look at each one of these separately, figure out the worst (or best) case value I can reasonably set for it, then see if those values fit within the realm of reasonableness.
 

MINIMUM ORBITAL DISTANCE FROM SURFACE (160 KM)

Generally when we talk about drop pods, we are talking about a drop from a ship in orbit around the planet.  There is no real reason I can think of that you couldn't use drop pods from much closer to the ground, but UNSC warships are generally designed to be operated in space, not on the ground, and the third letter in SOEIV stands for Exoatmospheric, or outside the atmosphere. Of course orbital height is going to vary based on each planet, but for Earth, low earth orbit starts around 100 miles (160 km).  For the sake of this analysis, I will use 100 miles as my minimum drop distance.
 

MAXIMUM DROP ACCELERATION (30 m/s²)

So there are two scenarios I see when considering launching a drop pod from orbital distances.  The first, and most likely, is a drop from a ship in orbit around the planet.  This drop would require an propelled launch, as "dropping" something while in orbit wouldn't actually accomplish anything.  The pod would just sit there, since both the frigate and the drop pod are in free-fall while orbiting.  If this doesn't initially make sense, its because orbital mechanics can be a little confusing sometimes.

As an example, the International Space Station is in orbit around the Earth at about 250 miles (400 km) above the surface.  While it looks like there is no gravity on the ISS when you see astronauts floating around, the gravity at that distance is actually about 90% of the gravity you are feeling right now.  The reason the astronauts are floating is because they, along with the entire ISS, are in free-fall around the Earth.  They are literally falling towards the Earth, but are moving so fast around it, that they keep falling around the "edge" of the planet. This would be the same situation for a frigate in orbit, so when the drop pod is released, it would just stay there, unless some sort of boosting mechanism were used to launch the pod.

The second scenario is a drop from orbital distances, but with the ship "hovering" over the planet, with no lateral velocity at all.  This would accommodate a traditional "drop", though the ship itself would have to be using considerable energy to keep itself from falling onto the planet.  We know from the lore (Halo 2 comes to mind) that frigates at least can hover over the surface, so this is within the realm of plausible from a lore perspective, though it would likely be less than ideal from an energy usage perspective.  The drop while in orbit would use less fuel than the hovering scenario, and in combat, the ship would likely want to drop the pods and get out of the area anyway.

While researching this, I looked into the maximum negative vertical acceleration a person can withstand.  That would be the force felt if the drop pod launched down towards the surface rather than free-falling.  A quick search found that the maximum negative g-force would be around -2 or -3-Gs, or 30 to 40 m/s² (gravity is about 9.8 m/s², so a free-fall at 9.8 m/s² is actually 0-Gs, double that is 1 G, and so on).  To be clear on what the lore says in regards to drop pod design, Halo Waypoint provides the following:

Drop pods are typically deployed from high altitude or from an exoatmospheric location, being fired or soft-ejected toward the planet’s surface until the atmosphere is breached. At this point, the occupant is free-falling at terminal velocity until the pod’s automated braking-chute system is initiated, thrusting the pod backward prior to landfall. The top-mounted braking chute system itself is an extended cable attached to four petal-like flaps which automatically activate at low altitude, reducing the pod’s momentum. The combined efforts of reverse thrusters and airfoils slow the craft enough to ensure that impact is not (usually) fatal.
— HaloWaypoint.com

Based on this, it appears that drop pods can support either a simple "drop" or a launch towards the surface.  Watching the opening cutscene from Halo 3: ODST and the cutscene leading into the Halo 2 mission Delta Halo confirm that a propelled launch is used, with what is seemingly a continual acceleration until the pod reaches the bulk of the atmosphere and deploys the braking chute to begin deceleration.  Check out the drop shown in Halo 3: ODST to see what I mean.

 
 

While trained fighter pilots can withstand continual G-forces around 9x Earth's gravity in the downward direction without blacking out, the limits for forces in the reverse direction are far lower.  While people will blackout in a positive G situation when the blood drains from their head, they will experience what is called redout in the negative direction as blood pools in their head.  As you might expect, this can be very dangerous for the occupant, and can easily cause serious injury.  

Because you don't want injured, or even woozy ODSTs exiting a drop pod into battle, I'm going to put an upper limit on drop pod acceleration at 30 m/s².  This equates to a force on the occupant of about 20 m/s², since you are still in a gravity well and about 10 m/s² is negated by Earth itself.
 

MAXIMUM DROP DECELERATION (80 m/s²)

If the first half of this journey sounds intense, the landing is almost definitely worse.  There is nearly no discernible atmosphere until you get to about 32 km (20 miles) above the surface of Earth.  Until then the pod is continually accelerating towards the surface.  After that, the pod is desperately trying to slow down so as to keep from pulverizing the drop pod and the ODST inside.

As a quick aside, imagine a drop pod launched from orbit that experiences a total braking system failure.  As the pod hits the atmosphere, it will quickly slow down to terminal velocity.  Without the braking system, however, the pod won't slow to survivable impact speeds, and would hit the ground at around 600 to 800 kph (370 to 500 mph).  How awful would that be?  Pretty damn awful.  To help you imagine what it might look like, watch this video of a fighter jet impacting a concrete wall during a test at about 800 kph (500 mph).  It would be something like that.

So continuing the discussion from earlier, fighter pilots can withstand up to 9 Gs of positive vertical acceleration, which comes to around 88 m/s².  This is in a seated position, but the forces imparted on the human body would be similar while standing, or at least in a very upright seated position.  I'll round down a bit to 80 m/s², though I don't think we will need anywhere close to 9 Gs to slow down the pod before it impacts the Earth.
 

MAXIMUM SURVIVABLE IMPACT VELOCITY (80 kph)

We now know the absolute maximum deceleration the human body can withstand, but we also need to know the maximum speed the drop pod can hit the ground and maintain the ODST intact and in fighting condition.  From the video above, I know it is less than 500 mph (800 kph).  This number is hard to pin down, since there isn't good data on the survivability of a crash feet-first.  Drivers and pilots of cars and planes are generally seated, so the crash effects the human body far differently than it would an ODST in a drop pod.  The issue is made more confounding by the fact that I have no idea how well the harness and impact absorption systems in a drop pod help the occupant survive the impact.  I am going to have to somewhat guess at a reasonable speed, but based on the copious amounts of car crash data out there, I'd say an upper limit to the survivable (without injury) is about 50 mph (80 kph).  Here is what a 50 mph car crash looks like for reference.

 
 

SETTING UP THE EXPERIMENT

Now that I think we have established the required parameters for analyzing dropping an SOEIV from orbit, lets run through how we are going to test out how feasible the entire ODST concept really is.  To do that, I am going to make a quick timeline of basic events that we can use to break up the drop from launch to landing.
 

1 - INITIAL CONDITIONS

For this test, I am going to assume the SOEIV is on the Forward Unto Dawn, in orbit 100 miles (160 km) above the surface.  We will look at the scenario again for different distances, though they shouldn't have a huge effect other than transit time.  The ship and pod are not moving vertically (i.e. maintaining the same orbital height).  Normal orbital velocity for that altitude is around 17,500 mph (28,000 kph), but for the purposes of this analysis, I am not going to take the orbital velocity into consideration.  Were the Forward Unto Dawn in orbit when launching the pods, however, the SOEIV trajectory would be a ballistic arc rather than just a straight line down.  This would increase the amount of air resistance experienced, though we don't really know what measures drop pods use to negate this lateral motion during transit, so for all I know the drop pod normally slows lateral motion down to zero by the time it contacts the atmosphere.  Long story short, there are too many unknowns to account for this, and frankly, I don't think it will make much of a difference in the end.
 

 The force of gravity on Earth as a function of distance from the surface.

The force of gravity on Earth as a function of distance from the surface.

2 - CONTACT WITH BULK ATMOSPHERE

While the actual atmosphere gradually gets denser as you get closer to the surface, for modelling purposes I am going to assume the atmosphere starts at about 32 km from the surface.  99% of the atmosphere exists below 32 km, so for simplicity's sake we will pretend that other 1% has no effect.  In reality it would have a small effect, but it would be negligible.  That means the during the transit from launch (160 km) until contact with the atmosphere (32 km), there is effectively nothing slowing down the drop pod.  I'll run the analysis using the maximum drop acceleration of 30 m/s², then again using an average gravitational acceleration of 9.5 m/s².  For reference, the force of gravity at the surface is about 9.8 m/s², and is a little over 9 m/s² at 160 km up.  I'm going to use an average number because it is a lot easier to do the math and will result in a nearly identical result.  As you can see in the chart on the right, the plot is nearly linear for the altitude we are using until 32 km.
  

3 - CONTACT WITH THE GROUND

For the last 32 km, the drop pod is slowing down via the air-brake and just the air resistance the pod experiences.  To survive the drop, the pod has to slow down to less than 80 kph and the deceleration from 32 km to 0 km has to be less than 80 m/s².  I will first look at air resistance itself and how much that will slow down the drop pod without additional assistance, then look at what would be needed to slow down the craft to less than 80 kph.  From cutscenes, we know the drop pod uses an air brake for a portion of its descent, then releases the brake and free-falls to the ground.  It isn't clear whether the pod uses retro rockets after that point to slow the descent further, but Halo Waypoint mentions their existence, so we can see how that would effect the landing speed.
 

THE RESULTS

Now that we have made our assumptions and determined the setup of the experiment, lets get to the fun part: doing the math and figuring out whether or not the ODST inside the drop pod is going to live to fight in the battle, or get splattered all over the inside of the pod.  Note, for the following calculations I ignore consideration number 3 and assume a 0 kph impact speed.  This is more conservative than needed, but 80 kph is effectively 0 anyway when you are looking at entry velocities in the thousands of kilometers per hour.
  

1-2 - LAUNCH UNTIL CONTACT WITH ATMOSPHERE

Since we made the simplifying assumption that the SOEIV is not going to experience appreciable effects from the atmosphere until 32 km in altitude, the first 128 km of travel will be a straight acceleration.  First off we are going to use the maximum survivable downward acceleration of 30 m/s².  This equation is made a lot easier by the fact that the initial velocity is zero, and the acceleration is constant.  I'm not going to show my work here for the sake of not confusing anyone unfamiliar with calculus, but the basic equations are as follows:

  • Acceleration = 30 m/s²

  • Velocity = acceleration x time

  • Distance = 1/2 x acceleration x time squared

We of course know the acceleration (30 m/s²), and we made assumptions for the distance traveled (128 km).  We can use the equations to figure out the final velocity, but you need to know the travel time in order to get that number.  Using the third equation lets us figure out what the transit time is, then using that we can figure out how fast the SOEIV is going when it hits the atmosphere.  Plugging in 128,000 meters for the distance and 30 m/s² for the acceleration, the transit time comes out to 92.4 seconds.  That's just over a minute and a half to get to the atmosphere.  From watching the Halo 3: ODST opening cutscene, it depicts a transit time of closer to 1 minute for the whole ordeal, but I'll chalk that up to artistic licensing and/or a much lower initial drop height.

Now we know the transit time, so we can figure out just how fast the pod is traveling when it hits the atmosphere.  accelerating at 30 m/s² for 92.4 seconds gets you a final velocity of 2,771 m/s, or 9,977 kph or 6,200 mph.  That sucker is moving.

Before we move on, lets look at how this would be different from different altitudes and different accelerations.  There isn't any particular max altitude that would be "too high", though at some point the transit time would be too long and the pod would become an easier target for enemy ships in the area.  I'll set a maximum altitude at 1,000 km, which sounds very far, but from the perspective of orbital distances is not far at all.  We already said our minimum acceleration is the force of gravity, though in reality you could set any transit acceleration you wanted.  It would just be impractical at some point.  Note, I use 9.5 m/s² as the average gravitational acceleration from 160 km, and 8.5 m/s² from 1000 km.  See the above chart as to why.

I'm not going to go into all the math again for each possibility, but it will all be the same equations, just with somewhat different numbers.  Here are all the results:

  1. INITIAL HEIGHT: 160 km
    MAX ACCELERATION: 30 m/s²
    TRANSIT TIME TO 32 KM: 92 seconds
    SPEED AT 32 KM: 9,977 kph

  2. INITIAL HEIGHT: 160 km
    MAX ACCELERATION: 9.5 m/s²
    TRANSIT TIME TO 32 KM: 164 seconds
    SPEED AT 32 KM: 5,614 kph

  3. INITIAL HEIGHT: 1,000 km
    MAX ACCELERATION: 30 m/s²
    TRANSIT TIME TO 32 KM: 254 seconds
    SPEED AT 32 KM: 27,436 kph

  4. INITIAL HEIGHT: 1.000 km
    MAX ACCELERATION: 8.5 m/s²
    TRANSIT TIME TO 32 KM: 477 seconds
    SPEED AT 32 KM: 14,604 kph

You can see that we are quickly getting to very long transit times at 1,000 km.  Interestingly, while 27,000 kph sounds really fast, and it is, that is nearly the same reentry speed of the Space Shuttle.  Entry speeds of the Apollo capsules were even faster, so this is definitely in the ballpark of feasible.
 

 SOEIVs dropping into combat on  Installation 05 , November 2nd, 2552.

SOEIVs dropping into combat on Installation 05, November 2nd, 2552.

2-3 - DECELERATION THROUGH THE ATMOSPHERE

Once the capsule hits the atmosphere, it will start to slow down rapidly.  The three major concerns during this phase are having enough distance to slow down the pod, doing so slowly enough to keep the ODST from blacking out, and slowing down without melting the pod.   Right off the bat I am going to ignore the third question because we already have materials that can withstand reentry heat generated at the velocities we are talking about, so having these materials in five-hundred years doesn't worry me.  Instead I will just focus on the first two.  Referencing the Wikipedia page on atmospheric entry, there are four major considerations regarding designing a craft to enter the atmosphere, and only the third, peak deceleration, will be the focus of this analysis.  The other three are related to loading onto the actual shell of the craft, both physical and thermal.  Since I already said we aren't going to consider that in this article, that just leaves peak deceleration as our focus.

In researching this, I looked at the data from all the spacecraft we have deorbited over the years, from the Apollo capsules to the Space Shuttle to the Soyuz capsule.  All three (obviously) reenter the atmosphere successfully and keep their occupants alive.  In addition, their reentry speeds are in excess of the reentry speeds we calculated above.  This should be a no-brainer.  However, there is one big difference.  All spacecraft we have that reenter the atmosphere do so on a ballistic trajectory.  That is, they are moving very quickly parallel to the Earth's surface, in addition to down toward it.  This allows for a much longer distance traveled until they reach the surface, resulting in lower G-forces.  The drop pod scenario we are considering is straight down, no lateral motion at all.

Straight down is actually a worst-case scenario since the distance is minimal.  If the drop pod and occupant can survive that descent, they can definitely survive a descent where they are moving horizontally as well as vertically.  I will use 27,436 kph as our entry speed since that was the fastest velocity we calculated, and 32 km as the distance since that is the minimum distance we are looking at.  Anything slower or longer would be constrained by these numbers.

I will also say that reentry forces are not actually going to be even throughout the descent.  The atmosphere gets thicker as you get lower, and the speed is dropping as well.  Since we know the pods have thrusters for use in effectively any direction, and for the purposes of this exercise have unlimited fuel, as long as the average deceleration required is less than 80 m/s², you can just assume the pod uses those thrusters to balance out the G-forces during the descent.  I mean, we have slipspace engines and entire artificial planets, I think this is a pretty minor allowance.

For these calculations we will use the same set of equations as we did in the last section.  For review they are as follows:

  • Acceleration = 80 m/s²

  • Velocity = acceleration x time

  • Distance = 1/2 x acceleration x time squared

In this case acceleration is 80 m/s² rather than 30 m/s², and distance is now 32 km.  Also, the assumed initial velocity is 27,436 kph, or 7,340 m/s.  Since the knowns are slightly different, I will approach the equations a little differently.  We know initial velocity and max acceleration, so plugging those into the second equations gives us a minimum transit time of 91.75 seconds to slow down the pod from over 27,000 kph to 0 kph.  Using that time and the same acceleration value in equation three, the minimum required stopping distance is... 337 km.  Uh oh.  I think we killed our ODST.  Doing some more math, at that initial velocity and distance, the drop pod is still travelling at about 5,000 m/s, 18,000 kph, or 11,000 mph when it impacts the ground. For reference, here is what a projectile travelling at half that speed looks like on impact.  Note, there is NO EXPLOSIVE in this firing.  Everything you see is from kinetic energy alone.

 
 

So at the initial speed of 7,340 m/s, what would the deceleration need to be to stop the drop pod, regardless of occupant safety?  Plugging in some more numbers gives an average deceleration of about 850 m/s².  That's ten times what we assumed to be our maximum.  Converting to G-forces, that's about 86 Gs.  That isn't intense.  That is a death sentence.  For reference, the highest recorded G-force survived was 46.2 Gs during rocket sled testing in the 1950s.  That was also for 0.6 seconds.  This is nearly double the force, and the ODST would experience that for 8.6 seconds.  Even if he somehow survived this ordeal, there is no way the ODST would be in fighting shape.

Alright.  Now that we have literally blown up our ODST and turned him and his pod into a quite impressive projectile weapon, lets look at some other scenarios.  We know the worst case scenario is not good.  Lets look at a better case.

Maybe using the slowest speed we calculated, 5,614 kph, the same distance of 32 km, and a maximum deceleration of 80 m/s² will give us better results.  Using the second equation again, we get a minimum transit time of 19.5 seconds to slow down the pod from 5,600 kph to 0 kph.  That sounds better.  Taking that number and plugging it into the third equation along with 80 m/s², the minimum required stopping distance is... 15.2 km.  We survived!  And with quite a bit of distance to spare.  Phew!  For a minute there I thought were going to have to question the entire SOEIV concept, which would have been super disappointing, because they look so freaking cool.

Ok, so we know an entry velocity of 27,000 kph is not manageable, and an entry velocity of 5,600 kph has a lot of room to spare.  So, given the conditions we have set, what is the maximum entry velocity a drop pod can be travelling and still survive the descent and keep the ODST alive and conscious?  We know our distance is still 32 km, and the maximum deceleration is still 80 m/s².  Using these numbers, we get a transit time of 28.3 seconds.  Using that transit time and plugging it into the velocity equation, the maximum entry velocity would be 2264 m/s, or 8,150 kph or 5,064 mph.  That's still pretty fast, and that would be one hell of a ride.  For reference, that is nearly the same speed as the railgun in the above video.  I don't think intense even begins to describe it.

Reviewing the results we calculated, we have the following results:

  1. INITIAL HEIGHT: 1,000 km
    MAX ACCELERATION: 30 m/s²
    INITIAL OCCUPANT STATUS: Alive

    FINAL HEIGHT: 0 km
    FINAL VELOCITY: 18,000 kph
    FINAL OCCUPANT STATUS: Dead

  2. INITIAL HEIGHT: 160 km
    MAX ACCELERATION: 9.5 m/s
    INITIAL OCCUPANT STATUS: Alive

    FINAL HEIGHT: 0 km
    FINAL VELOCITY: 0 kph
    FINAL OCCUPANT STATUS: Alive

REANALYZING THE SCENARIOS

In hindsight, it would have been easier to start with the last set of calculations we did and work backwards.  But where is the fun in that?  I think it can be easier to visualize the scenario the way we approached it anyway, rather than working backwards in time.  Plus we got to turn an ODST into a projectile weapon.

So knowing that 8,150 kph is the upper limit for a straight-down reentry, we can do a few things.  We could use the 30 m/s² maximum acceleration to calculate the maximum altitude a launch could be performed at and have no additional acceleration limits.  Using those numbers, and working backwards, the resulting drop altitude would be 117.5 km, or 73 miles.  Pretty high up, and a reasonable launch distance, though that would still be too low to be considered "in orbit".  But no worries, it is still a reasonable scenario, which is good.

Another way to look at this would be to take our original altitudes and figure out what the max acceleration could be, given that there is still a 8,384 kph limit on reentry velocity.  Using 160 km first, the max average downward acceleration would be 20 m/s².  Seems like a reasonable drop scenario also.  Doing the same math for a 1,000 km drop, the maximum average drop acceleration would be 2.65 m/s².  That is probably not very plausible.  For one thing, it would take over 14 minutes to get to the atmosphere, and another thirty seconds to get to the ground.  Second, assuming again you start at 0 orbital velocity, the acceleration due to gravity would be more than 2.65 m/s².  The pod would have to actually fire in reverse a little just to keep the pod from falling too fast.  1,000 km seems like too far a drop distance.
 

FINAL RESULTS

The third way to look at this is to figure out what a reasonable maximum drop altitude is.  Again, we know 8,150 kph is as fast as we can get to, and I am going to make the assumption that the acceleration for the first half of the drop has to at least be the normal force of gravity so you aren't fighting the Earth on the way down.  Of course the average force of Earth's gravity varies based on altitude, so without making the calculation more complicated than I want to, we will just look at some snapshots of different altitudes and determine the required acceleration that way.  Here are several different drops from differing heights:

  1. INITIAL HEIGHT: 117.5 km (73 miles)
    AVERAGE GRAVITY: 9.7 m/s²
    MAX ACCELERATION: 30 m/s² (-2.07 Gs)
    MAX DECELERATION: 80 m/s² (9.2 Gs)
    MINIMUM DROP TIME: 104 seconds

  2. INITIAL HEIGHT: 160 km (100 miles)
    AVERAGE GRAVITY: 9.5 m/s²
    MAX ACCELERATION: 20 m/s² (-1.17 Gs)
    MAX DECELERATION: 80 m/s² (9.2 Gs)
    MINIMUM DROP TIME: 142 seconds

  3. INITIAL HEIGHT: 250 km (155 miles)
    AVERAGE GRAVITY: 9.4 m/s²
    MAX ACCELERATION: 11.75 m/s² (-0.24 Gs)
    MAX DECELERATION: 80 m/s² (9.2 Gs)
    MINIMUM DROP TIME: 221 seconds

  4. INITIAL HEIGHT: 300 km (186 miles)
    AVERAGE GRAVITY: 9.3 m/s²
    MAX ACCELERATION: 9.6 m/s² (-0.03 Gs)
    MAX DECELERATION: 80 m/s² (9.2 Gs)
    MINIMUM DROP TIME: 264 seconds

  5. INITIAL HEIGHT: 315 km (196 miles)
    AVERAGE GRAVITY: 9.1 m/s²
    MAX ACCELERATION: 9.1 m/s² (0 Gs)
    MAX DECELERATION: 80 m/s² (9.2 Gs)
    MINIMUM DROP TIME: 277 seconds

So looking at that data, It appears that drops from 315 km (196 miles) and lower are both survivable and manageable.  This is actually quite a small range of orbital heights, but is well within the established lore and a good degree of believability.  While we did not take every factor into account, the bounds that we set means a more detailed analysis would likely find this range is a little larger.  Even at 315 km, however, the drop time is getting uncomfortably long, and if you can't get a capital ship any closer to the surface due to the enemy, there's a good chance you aren't getting any drop pods through either.

The end result is another good one, in that it seems the concept of the SOEIV, or at least the basic concept of dropping troops from orbit, is not a technical impossibility.  The only thing I notice in cutscenes that don't really match this data is the drop time, which is generally shown to be around a minute or less.  Even from 118 km up, the drop time is over a minute and a half.  This isn't really a huge deal, though, since it isn't THAT far off, and like I said earlier, I am just chalking it up to artistic licensing.  As for things like the actual braking system, I didn't get into the details of how that whole system must work, but since we don't have a ton of detail on the specifics, it would be hard to analyze.  What I can say is that the pod itself would slow down to somewhere around 800 kph or 500 mph without any brakes just from air resistance.  Adding in the air brake, you might get to a couple hundred mph, and the remaining brake would have to be done via retro rockets.  This is a workable basic idea and has been used in various configurations on Earth and Mars.  I see nothing that tells me this wouldn't work, so I am going to default to plausible.  So the next time you see a drop pod hurtling through space towards the ground, and your buddy asks "Is that even possible?", you can safely respond "Probably".
 

BUT WHAT WOULD IT FEEL LIKE?

The one question I haven't answered directly is the actual experience of dropping onto the battlefield.  Using the numbers above, we can see the experience would vary a bit, though the ending would pretty much be the same, and also the most intense.  From very low altitudes, the experience would actually be the worst, as your whole body would get shoved up into the harness for the first two-thirds of the trip, and then shoved down quite forcefully for the last thirty seconds.  From around 300 km out, the ODST would be in free-fall for nearly 4 minutes, then another intense 30 seconds of landing.  The one portion of this experience we didn't analyze specifically was the actual impact, which is obviously faster than 0 kph.  Based on the assumptions I made towards the beginning, the impact must be less than about 80 kph or 50 mph, but even that would be like getting into a bad car accident.  Oh, and then you get to do the hard part of actually fight in battle.  For those who want to try and imagine the whole experience from beginning to end, I'll give it a try.

 

"THE COMMUTE"

An alarm blares.  "Approaching the combat zone! Prepare to drop, ODSTs!" you hear your squad leader shout, as the entire deck suddenly becomes a flurry of activity.  Everyone grabs their gear and heads to their pods.  You grab an MA5D and your helmet, and climb into your assigned drop pod.  After securing your firearm to the interior of the pod and fasten your helmet, you latch yourself into the harness system of the SOEIV and the door closes.  The harness is tight.  Not uncomfortably so, but you don't have a lot of freedom of motion.  That's okay though, because there isn't anywhere to go.  There is barely enough room to move your arms around.  Just enough to reach the controls on either side of you.  Hopefully that harness holds, because if it gives way, you are going to come face-to-face with every surface of that pod.

The SOEIV deployment system on the ship moves the pods into their launch bays, and lowers them into the vacuum of space.  You can't see much, except the belly of the ship above you, the horizon of the planet below you, and about a dozen of your ODST brethren hanging in their own pods alongside you.  It feels like an eternity as you hang there, but in reality it is only a handful of seconds.  A small "THUMP" shakes the pod as the decoupling charge fires, freeing you from the grasp of the place you called home only minutes ago. For a brief moment the pod floats along with the much larger vessel above you, detached, but still within arms reach.  If you could move your damn arms.  Then the automated countdown begins, "beep, beep, beep, BEEP" and as that final chime sounds, your pod's insertion thrusters fire, launching the SOEIV away from the ship and towards its destination, Earth.  You, meanwhile, are forced up into your harness, carrying what is effectively twice your weight on your shoulders.  The feeling doesn't go away, though.  Its continuous, and as the seconds tick by, you feel the blood rushing to your head, causing pressure to build in your eyes and sinuses.  The feeling is so intense it gets nearly impossible to breath through your nose, though you stopped doing that the moment the alarm sounded back aboard the ship.  You don't start to blackout though.  You are experiencing the problem of too much blood in your head.  You are feeling the effects of redout.  As the blood pressure builds in your face, your lower eyelids actually start filling with blood and get pushed into your field of vision.  It is just enough force to be incredibly unpleasant without actually damaging your blood vessels.

And it just stays like that.  For over a minute your body experiences a feeling not unlike being hung upside down, but worse.  Worse because the acceleration forces you are experiencing are twice that of normal Earth gravity.  Even worse still because you aren't upside down at all.  You can see the planet rushing up below you.  Your eyes tell you that you are right-side up.  But every other sense is telling you otherwise.  Particularly your stomach, which at this moment is telling you that eating ten minutes before dropping was definitely one of the worst decisions you ever made.  And this is not the time to try to relieve that feeling.  For one, the feeling of being horribly nauseous is nothing compared to the feeling of vomit running UP your face, into your nostrils and eyes, and pooling at the top of your helmet.  Its also doubly unpleasant when you start the deceleration phase and feel that same vomit make a second pass back down your hair, your face, your body, and eventually pooling mostly around your feet.  Then, once you get ground-side, you actually have to fight in those conditions.  That is, of course, if you can still see out of your visor.  And it isn't like a shower is in your near future.  A catastrophic braking system failure would almost be a courtesy at that point.  Yeah, lets just not do that.

So your pod, along with the pods of your squad-mates, continue to accelerate toward the Earth.  As you approach the sixty-second mark, the sound inside the pod grows from the constant rumbling of the booster to what seems to resemble a roaring inferno coupled with endless wind.  The booster hasn't cut out yet, but you have begun to breach the bulk of the planet's atmosphere.  The feeling of hanging upside down continues, but it's beginning to abate as the air growing denser around your pod starts to fight against the force of the booster rocket over your head.  The phrase "stuck between a rock and a hard place" crosses your mind.

Then, in an instant, your whole body is thrust down to the floor of the pod as the insertion booster cuts out.  Now you carry your weight in your feet and crotch.  Quickly your weight grows.  This feeling continues to worsen until it literally feels like you weigh eight times the normal 250 pounds you are in full gear.  That's 2,000 pounds, but who's counting?  This feeling is way more intense than the drop, but at least you know in the back of your mind it will end a lot sooner.  Plus there isn't any time to think about it as your ODST training kicks in, flexing your leg and arm muscles to keep at least some of your blood in your head and torso to maintain consciousness.  Your vision starts to gray, then tunnel, but you keep from passing out.  The feeling starts to lessen a bit as your pod slows.  That is, until you hear the familiar thump of the air-brake deploying, thrusting you back down into your seat even harder than before.

As the ground rapidly approaches and your pod slows to near terminal velocity, you feel the weight of your body quickly transition from a full ton down to a much more manageable few hundred pounds.  The air is still screaming around you, but at least that reentry fireball has dissipated.  Almost there now.  You know what is coming next, and while it is thoroughly unpleasant, there is literally nothing you can do now to stop it.  All you can do is mentally prepare yourself.  Oh, and pray the pod's braking system functions as designed.  Not that you will have time to think about it if it fails.

At the prescribed altitude, the air-brake detaches from your pod, flying clear of the landing zone.  Marines often ask you why the thing that is literally there to keep you from going splat on the ground seemingly flies off the pod over a mile above the surface.  But you know better.  Drop School taught you the ins and outs of the SOEIV system, including the purpose for every phase of the drop.  They taught you that the air-brake detaches and flies clear of the drop zone so that it doesn't become a projectile, crashing down onto unsuspecting Marines, your squad-mates, or even you around the drop-zone.  And you know all too well that isn't just a hypothetical.  You have personally participated in two drops where an air brake failed to detach properly.

The first was five years ago, when one of the the squibs on a squad-mate's pod's Air-Brake Separation System (ABSS) failed to detonate, leaving the metal umbrella and it's support cable hanging by one of the four latches as the pod descended.  Wind, and the shaking caused by the unbalanced pod helped fully detach the brake, but less than half a mile from land-fall.  The brake cleared your squad-mate's pod, but landed about a quarter mile away, right beside a company of Marines, injuring three, but luckily killing no one.

The second incident happened only two months ago, and the participant wasn't so lucky.  You would think a total failure of ABSS would be better than the partial failure you witnessed what feels like a lifetime ago.  But it is so much worse.  As your pod completed its drop, everything was nominal, and you were the second of your six-man squad to impact the ground.  Your door detachment system charges fired and freed you to the sight of a raging ground battle in front of you, and your squad's remaining four descending pods above you.  Before you even had a chance to climb out of your pod, the next squad-member impacted the ground about fifty meters to your eleven o'clock, seemingly in the usual fashion, save for the air-brake that was still completely attached to the pod.  Well, attached isn't the right word.  More like permanently bound.  When the pod hit the ground, the chute's cable went slack, and the large, four-finned braking mechanism crashed down right on top of your squad-mates pod, becoming affixed within the metal top of the SOEIV.

The term chute always makes people think of light, canvas parachutes, but the SOEIV's "chute" is nothing of the sort.  It is designed to slow your pod from about a mile a second to a paltry few hundred miles per hour in about a dozen seconds.  That thing can handle both the force of the planet's atmosphere pushing with all its might to keep the intruding drop pod out, and the intense heat generated when so many air molecules impact the bottom surface of the fins, literally rubbing against the underside so hard it creates a wall of fire at the point of contact.  So yeah, the air-brake is absurdly strong.  And it ain't light either.

The impact crumpled the top of your squad-mate's pod down about three feet, and you immediately feared what you were taught could happen if your pod experienced an ABSS failure.  The engineers termed it an "Air-Brake Coupling Incident".  'Coupling' because the air-brake would become physically attached to the top of the drop pod as the metal of the pod crumpled around the fins, making it nearly impossible to detach the two without some heavy machinery.  Not that you would do that on the battlefield.  And not that it mattered, because as the door to your squad-mate's pod flew clear, you knew that no one was climbing out.  The impact had physically crushed the top of the SOEIV, and as you had been taught would happen, pressed that entire section of the pod down onto your squad-mate.  People think drop pods are strong, and they are, but they are designed for impact on the bottom of the pod, not the top.  A simple mechanical failure had taken the life of one of your squad-mates that day, and combat had yet to even begin for you.  Your squad would lose two more that day, but the accident in the pod is the death you will always remember.

All that rushes through your head as you descend through the clouds.  As you begin to make out individual people on the ground, you hear the familiar "POP!" of a successful ABSS activation.  And if you didn't trust the sound of ABSS, you sure do trust its effects.  For a moment you feel nearly weightless, as the brake that a moment ago was rapidly slowing your pod is thrown clear of your descent path.  In reality you aren't in weightlessness, but the transition from nearly 5 Gs to closer to 2 is the most freeing feeling you can experience as an ODST.  Except of course for exiting your pod alive and in one piece.

As your pod begins the last phase of the descent, you prepare yourself for the final, most intense part of the whole ordeal, land-fall.  The engineers had a term for this too, "Rapid Pod Deceleration Phase" is what they said, though your squad just calls it "the crash".  You had a buddy that used to call it "boom-time", but he was kind of an idiot.  Actually, you couldn't figure out how he ever got accepted to become an ODST, though you supposed he always managed to get through the training scenarios somehow, so he must not be that stupid.  He was gone now, anyway, killed in combat on Meridian in 2549, so in the end it doesn't really matter what he called it.

The ground is now coming up on you incredibly quickly, though your descent is continually being slowed by the retro-rockets firing underneath you.  As the land transitions from below you to in front of you, you keep an eye on your altimeter and speedometer.  1,000 feet and 343 mph. 500 feet, 117 mph. 200 feet, 87 mph. 100 feet, 58 mph.  50 feet, 42 mph.  At the last possible moment the rockets cutout, and for the last few feet you are in free-fall.  For the briefest of moments you are truly weightless, freed from the burden of all that weight you were supporting for the past half minute.  In the time it takes your brain to process the feeling and think to itself "I am weightless", however, the feeling ends, and does so abruptly.  You feel the drop pod impact the Earth, a feeling that is nearly impossible to recreate in real life, aside from maybe a serious car accident, but those are incredibly rare except when the automated traffic management system experiences an issue.  "Total Commute Control Failure" the engineers call it.  Goddamn engineers and their soulless terminology for shit that kills people.  How about "We F-ed Up... Again"?  That would be more accurate.  And it would free up a bunch of acronyms too.

During the split second your pod impacts the ground, about a dozen systems in the SOEIV activate to keep your equipment functional and you battle-ready.  It may not look like it, but there is actually an impressive shock absorbing system built into the pod that makes the landing, if not pleasant, at least not terrible.  You may have hit the ground at nearly 40 mph, but it felt like a gentle 20 mph crash to you inside the pod.  For a brief moment you are a little disoriented, shaken by the sudden change in velocity, but your ODST training kicks in yet again and you grab your MA5D and equipment like you do every drop and wait those precious seconds as the automated "Pod Door Detachment System",  or PODDS (go to hell, engineers) activates and pops your pod's door away and clear.  You can hear the sounds of fighting as you step out of the pod, which you were repeatedly told never to touch immediately after a drop, unless you feel like adding a severe burn to the list of crap you have to deal with today.  As you scan the horizon, however, you can't see any actual fighting.  Maybe all the fighting is over that hill off to your 2 o'clock? Your ears are still ringing from the whole ordeal when you hear through your helmet's comms "Hey, dumbass!  Turn around!"  You whip your head around to see your ODST commander, along with the four other members of your squad laughing at you as you try to regain all your wits.  You perform the traditional ODST one-finger stretch towards the squad, which only garners more boisterous cackling.  Plasma fire streaks over your squad's head, though they don't seem overly phased by it.  "Let's go to work, ODSTs!" you hear your squad leader shout, turning to face the enemy and return the favor.  You run to catch up to the rest of the squad.  Just another average start to the workday for an ODST.
 

P.S. You can't talk about ODSTs and drop pods for several pages without paying tribute to the fallen helljumpers.  In memory of those fallen in defense of Earth and her colonies:

 
 
 
 

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