Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Model name | Date | Type | Thrust (kg) / Power (eshp) | Fitted to |
| --- | --- | --- | --- | --- |
| RTD-1/VDR-2 | 1938 | Two-stage centrifugal compressor Turbojet | 500 kg estimated | Test-bed only |
| S-18/VDR-3 | 1945 | Axial flow compressor Turbojet | 1,250 kg | Gu-VRD project |
| TR-1 | 1946 | 8-stage Axial flow compressor Turbojet | 1,300 kg | Alekseyev I-21, Ilyushin Il-22, Sukhoi Su-10, Sukhoi Su-11 |
| TR-1A | 1947 | 8-stage Axial flow compressor Turbojet | 1,500 kg |  |
| TR-2 | 1947 |  |  | projected growth version of TR-1 |
| TR-3 and AL-5 | 1949 | 7-stage Axial-flow Turbojet | 4,600 kg (at qualification in 1950) | Il-30, Il-46, Lavochkin Aircraft 190, Tu-86, Yak-1000, Su-17(1949), "Aircraft 150" |
| TR-7 | 1950s | supersonic compressor prototype of the AL-7 |  | Prototype for AL-7 |
| AL-7 | 1954 | 9-stage supersonic compressor Turbojet | 6,500 kg | Il-54, Su-7B, Tu-98, Su |
| AL-21 | 1961 | 14-stage Axial compressor with variable stator blades | 11,000 kg | Yak-38, Tu-28/Tu-128, Su-17, Su-24 |
| AL-31 | 1976 | Twin-spool Turbofan 0.6 bypass ratio. | 13,300 kg | Su-27, Su-30, Su-34, Su-35, Su-47 Berkut |
Question:
what is the difference in thrust between the al-7 and al-21?
Answer:
4,500 kg