To my understanding a lot of it had to do with poor radio communication, being on the defensive, the Germans attacking first and disease, but i'd love to hear and in-depth answer to why and how the Soviets suffered such a loss and how exactly they won.
Paging /u/georgy_k_zhukov basicaly.
Start with the fact that the Soviets didn't lose 860 000 people: the death count was at best half that(as per the original source of the same author that published the 863k figure, Krivocheev). Sick and wounded can often still fight and thus shouldn't be counted as casualties in the classic sense.
The Russian version of the wiki page that mentions the 863k goes into much deeper detail:
According to '93 declassified files(also written about by Krivocheev), only 254 thousand were deaths, captures or missing (listed as 'unrecoverable').
Krivosheyev, G. F., ed. (1993). Declassified: Losses of the Soviet Armed Forces in wars, military operations and conflicts(in Russian). ISBN 5-203-01400-0
A staggering 653 thousand are listed as 'sick or injured'. This does not mean they were not capable of fighting!
The German losses also differ depending on who you ask. Russian sources list over 300,000 'casualties'(deaths and captured) for the entire Kursk Offensive, versus only 109k 'deaths' from official German Sources and 433k wounded and otherwise afflicted.
'Rise and Fall of the German Air Force: 1933-1945(in Russian)': ISBN 5-699-18349-3
I think the bottom line is that the Soviets did not lose four times as many soldiers as the Germans did, but the losses were much closer to 1:1 than you'd think.
The last source I have is (again) my own copy of 'Book for Future Admirals' http://militera.lib.ru/h/0/pdf/mityaev_av01.pdf that, despite being heavily propagandic in places, lists less than 300,000 deaths or missing for the soviet side and over 280k dead, captured, or missing German soldiers during the time of the Kursk offensive.
How did the Soviets win? That's uniform across many sources:
A combination of heavy tank divisions(the soviet war machine reached critical peak in armoured production at the time), artillery shelling and the return of the airforce, combined with the intense hatred for the invaders seem to have done the trick. The soviet forces aimed to split and destroy with fast and mobile tank divisions. The German forces weren't used to defending(most of the war to this point was offensive) and certainly not against a well planned critical advance that was aided by some critical information from the British Intelligence at the time: the cipher to top level Nazi communications, giving away all their plans.
The Soviets also had many more tanks, artillery(almost 2x the german amount) and aircraft on the scene, as well as more people. To add to that, the Soviet command pulled its best units from all over the front(where possible) to aid in the offensive, leading to a critical mass of units that were battle tested and more numerous than the enemy. Couple that with knowing almost everything the enemy does and you have a recipe for success.
Another edit to appease /u/sowser that I didn't write the Book of Future Admirals
Poor radio communications were not as much of an issue as may first be thought given that the Soviets were operating from established, well prepared positions which diminished the drawbacks of field telephone communications. Poor radio communications certainly reduced the effectiveness of counterattacks by elements of the 1st, 2nd and 5th Guards Tank Armies but Soviet commanders enjoyed good to excellent communications with most of their forces throughout the operation. Soviet intelligence on German forces and their dispositions was excellent and by 1943 the Germans were facing an increasingly competent Red Army at every level.
Soviet casualties were high, though permanent losses as has been shown previously, have historically been overstated for the Soviet side and understated for the German side. At Kursk the Soviets created an echeloned defense in depth with the intention of dissipating the anticipated German summer offensive in order to exhaust the Germans, deplete their reserves and facilitate the subsequently planned Soviet Belgorod-Kharkov Offensive Operation (part of Polkovodets Rumyantsev) and Operation Kutuzov.
How did the Soviets “win”? The Soviets ground down and dissipated the offensive strike power of the German army in the East, denying them the ability to conduct offensive operations on a strategic scale for the rest of the war. It is important to remember that for the Soviets, the Kursk Strategic Defensive Operation was a sequential operation facilitating the planned Soviet summer and autumn offensives of 1943. However “severe” the casualties inflicted by the Germans on the Red Army were, they were not sufficient to divert significant reserves and striking power away from the planned Kutuzov and Polkovets-Rumyantsev offensive operations, the results of which would prove decisive for the war in the East. Not only did the Germans fail to cut off and reduce Soviet forces in the Kursk salient, the Germans also failed to fully draw in Soviet Armored forces and destroy their capability to mount offensive action.
Why high casualties? The Soviet defense was conducted mainly by rifle armies (infantry with some tank/AFV support) in prepared defensive positions, not primarily by armored formations as is often portrayed. Kursk is generally visualized as a massive tank battle which leads many analyses to become bogged down in distracting technical comparisons of the AFVs involved. While certainly characterized at times by massive armored clashes, the battle itself, and much of the destruction of AFVs which took place during it, occurred as German armor tried to push through prepared anti-tank defenses with integrated artillery support and minefields. It is particularly telling that only three of the Red Army’s five tank armies were involved in the operation, and in a piecemeal fashion at that. While elements of these three formations were involved in decisive local counterattacks at Prokhorovka (1st and 5th Guards Tank Army in the South), Ponyri and Okhovatka (2nd Guards Tank Army in the North), these forces were never fully committed and were not attrited sufficiently to prevent their involvement in subsequent offensive operations.
Most of the tank killing which occurred at Kursk was undertaken in a comparatively tedious and mundane fashion by Soviet infantry, artillery and anti-tank guns. A heroic struggle against hordes of T-34 tanks charging over the steppe makes for a much better narrative than the infuriatingly slow advance experienced by the Germans as they were forced to grind through the Soviet defenses strong point by strong point, unable to identify the next until it took them under fire. A particularly interesting sentiment contained in German analysis of the Citadel operation is the widespread belief that the operation failed largely as a result of a lack of sufficient infantry forces rather than the performance of German armor which was considered successful.¹ For a cost in manpower the Soviets were able to inflict horrific, unrecoupable losses upon the German armored forces while minimizing the damage to their own armored forces to a level where they could still play a key role in the following counter offensives. A constant refrain in any German tank commanders memoir is their loathing of Soviet anti-tank guns and by Kursk the Soviets had large numbers of sufficiently capable anti-tank guns deployed in the salient. Most importantly these weapons were organized and employed systematically as part of a well developed and effective anti-tank defense. These defenses took a telling toll on the German Armored spearheads, allowing them to be blunted and ultimately stopped by Red Army armored counterattacks. The striking power of German armored forces at Kursk was attritted and dispersed significantly by these stubborn defenses as well as the increasing inability of the German Army's overstretched infantry to secure the flanks of the armored spearheads as they penetrated Soviet lines, forcing the armored forces to divert fighting power from the front to shore up their vulnerable flanks. The defeat of the cream of Germany’s armored forces by endless hordes of suicidally employed T-34 tanks was ultimately a much easier narrative to swallow than the alternative.
Finally the fruits of increased aircraft production, fielding of new modern types and structural reorganization of the Soviet Air Forces finally came to bear on the battlefield. The Soviets were able to mount a severe challenge to the Luftwaffe and were able to deny air superiority to the Germans over the battlefield, reducing the striking power of German offensive spearheads as well as disrupting their offensive operations. Soviet pilots were increasingly competent, and although on average not nearly as skilled as their german opponents they were increasingly aggressive and tactically astute, and fighting proficiently with newly developed tactics. Furthermore successful and systematically employed Soviet counterintelligence measures and maskirovka efforts often prevented the accurate plotting and targeting of Soviet defensive positions beyond the line of contact for aerial and artillery fires. German armored thrusts suffered without the reliable flying artillery of the Luftwaffe which found itself operating in a highly contested aerial environment against targets which were often fleeting and difficult to locate. German airpower was unable to play the same decisive role it had contributed to the 1941 and 1942 summer offensives.
Glantz as usual provides a great look into the battle:
Colonel David Glantz Soviet Defensive Tactics at Kursk, CSI Report No. 11 July 1943 http://usacac.army.mil/cac2/cgsc/carl/download/csipubs/glantz2.pdf
Steven H. Newton “Kursk the German View” http://www.e-reading.club/bookreader.php/138784/Newton_-Kursk-_The_German_View.pdf
Highly recommend panzer warfare on the Eastern Front 1943-45 by Robert Forcz. He goes through all the modern sources on Kursk to explain in great detail the many faults the panzerdivisionen faced in 1943. Critical failures in supplying parts to repair panthers, tigers, or even pz ivs. The battery problems in the ferdinands, and the utter failure of Manstein to request proper mine clearing support. Probably, the biggest issues was in his C2 command and utter failure to check his subordinates plans for the offensive. Significantly, even though the Russians lost large amounts of armor on the battlefield they could repair them since they held the battlefield after the offensive failed. Soviet defences varied considerably by unit deployment with Model's axis facing significantly better defences and better troops manning the first and second lines. At the end of the offensive the Germans had lost too many panzergrenadiers and infantry that they could ill afford to make up for in recruitment. The soviets by contrast were learning far faster than the germans were capable of dealing with and 800k casualties while significant were not enough to slow down the Soviet Army for long.
The military historian Robert Citino (of German Way of War fame) has written a book on 1943, The Wehrmacht Retreats.
His most interesting contention (looking at new sources) is that Kursk was not the "clash of armor" that it is often described as. Instead, it was the primarily the Soviets who had such horrific losses in their armor units... and Soviet generals after the fact, trying to justify their losses, "rewrote" Kursk as a titanic tank battle--and stalemate.
Nor was it an unmitigated German defeat. Instead, the goal of Germans was not really to "retake the offensive" (ala the previous summer, with the drive into the Caucasus), Citino argues, but rather, to throw the Soviets off-balance, and prevent a Soviet offensive. And the Germans called off the offensive once that had been accomplished.
I'm not a military historian per se, so I'm not really qualified to evaluate these arguments. And I think he's pushing these arguments too far (for the sake of having something "new" to say). But they're interesting.